CN112198219B - High-resolution high-sensitivity time-of-flight mass spectrometry equipment and method for measuring VOCs content - Google Patents
High-resolution high-sensitivity time-of-flight mass spectrometry equipment and method for measuring VOCs content Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/62—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating the ionisation of gases, e.g. aerosols; by investigating electric discharges, e.g. emission of cathode
- G01N27/68—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating the ionisation of gases, e.g. aerosols; by investigating electric discharges, e.g. emission of cathode using electric discharge to ionise a gas
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
- H01J49/00—Particle spectrometers or separator tubes
- H01J49/26—Mass spectrometers or separator tubes
- H01J49/34—Dynamic spectrometers
- H01J49/40—Time-of-flight spectrometers
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Abstract
The invention relates to an analysis and detection instrument, in particular to a time-of-flight mass spectrometry device and a method for measuring the content of VOCs with high resolution and high sensitivity. The ion source glow discharge device comprises a reaction chamber, wherein the reaction chamber is respectively connected with water vapor generating equipment and other reaction gas inlets, the reaction chamber comprises an ion source glow discharge region, an ion source drift region, a first differential vacuum cavity and a reaction tube which are sequentially adjacent, the rear part of the reaction chamber is sequentially connected with a second differential vacuum cavity, an ion transmission region and a TOF detection region, a single lens is arranged in the ion transmission region, and the single lens consists of three groups of electrodes which are arranged left and right; the accelerating electric field region at the upper part of the TOF detection region is communicated with the reflecting region through a field-free drift region, an XY deflection plate is arranged in the field-free drift region, and three groups of reflectors and detectors are arranged in the reflecting region. The invention has the advantages of wide variety of volatile organic compounds, high resolution and sensitivity, less interference factors in the detection process after the detectable mass-nuclear ratio reaches decimal places, less fragment generation and easy analysis.
Description
Technical Field
The invention relates to an analysis and detection instrument, in particular to a time-of-flight mass spectrometry device and a method for measuring the content of VOCs with high resolution and high sensitivity.
Background
With the release of 'ten gas', the measurement of volatile organic compounds in air and waste gas becomes the key content of atmosphere monitoring, and in order to acquire the VOCs emission condition of enterprises or the content of VOCs in emergency environment air in real time, an online monitoring method is widely applied. In the current stage, more VOCs on-line monitoring methods are mainly detected by adopting gas chromatography tandem mass spectrometry or directly adopting FID detectors, and the detection methods can realize the analysis of the ambient air and the gas collected by the fixed pollution sources, but have a plurality of defects: the detection process needs to be preprocessed and separated by gas chromatography and then analyzed by a mass spectrometer, the process consumes long time, the time interval of the obtained VOCs data is large, the data lacks continuity, and the continuous monitoring of the concentration change trend is almost impossible; and high cost, the analysis process requires continuous supply of carrier gas; in addition, the analysis instrument adopts a destructive hard ionization EI ion source, the spectrogram obtained in the measurement is ion fragment information, and the spectrogram is complex and difficult to analyze; in addition, the monitoring instrument has higher requirements on the monitored gas sample, the moisture has larger influence on the measurement result, and particularly when the complex sample of the components is measured, the content of the components such as water, carbon dioxide and the like is high, and the measurement result has larger interference.
Based on this, a proton transfer reaction mass spectrometer (PTR-MS) capable of rapidly determining VOCs in air has been used. More recently developed PTR-MS mainly utilizes: after proton transfer reaction is carried out on sample molecules to be detected in the gas and the reaction gas, the sample molecules are screened by using a quadrupole mass filter and then detected by a detector. The device and the method have high detection speed, but have a plurality of problems: at present, most PTR reaction gases use water vapor as the reaction gas, but other reaction gases are not involved, the water vapor cannot react with organic molecules with proton affinity smaller than that of water, so that the detection range is limited, and different reaction gases cannot be well switched. In addition, the traditional electrostatic four-level rod focusing separation is adopted, charged ions are easy to lose, the mass discrimination phenomenon exists, the measurement molecular weight range is narrow, the resolution ratio and the sensitivity are low, and the detection limit is high.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to provide the time-of-flight mass spectrometry equipment for measuring the content of VOCs with high resolution and high sensitivity, the types of volatile organic compounds measured are wide, the resolution and the sensitivity are high, the detection limit is low, the interference factors in the detection process are less after the detectable mass-to-nuclear ratio reaches four decimal places, the fragments are less, the spectrogram is simple and the analysis is easy, and the invention also provides a detection method adopting the time-of-flight mass spectrometry equipment.
The invention relates to a high-resolution high-sensitivity time-of-flight mass spectrometry device for measuring the content of VOCs, which comprises a reaction chamber, wherein a reaction gas inlet is arranged on the reaction chamber, the reaction gas inlet of the reaction chamber is connected with an outlet of a first tee joint, two inlets of the first tee joint are respectively connected with a vapor generation device and other reaction gas inlets, the reaction chamber comprises an ion source glow discharge region, an ion source drift region, a first differential vacuum cavity and a reaction tube which are adjacent in sequence, the rear part of the reaction chamber is sequentially connected with a second differential vacuum cavity, an ion transmission region and a TOF detection region, the first differential vacuum cavity is communicated with the reaction tube through a first small hole, the reaction tube is communicated with the second differential vacuum cavity through a second small hole, the second differential vacuum cavity is communicated with the ion transmission region through a third small hole, the ion transmission region is communicated with the detection region through a differential guide hole, the outsides of the ion source drift region, the first differential vacuum cavity and the reaction region of the reaction tube are respectively provided with equivalent voltage dividing resistors, the tail ends of the first differential vacuum cavity are connected with a first mechanical pump, the tail ends of the second differential vacuum cavity are connected with a second mechanical pump, the tail ends of the second differential vacuum cavity are connected with the second vacuum cavity, the single-electrode Cs are arranged on the wall of the reaction tube, the first differential vacuum cavity is close to the first differential vacuum cavity, a single-electrode set is arranged at the position of the first differential vacuum cavity, and a single-electrode is arranged in a single-electrode-phase electrode Cs arranged in the left vacuum lens, and a left-air cell, and a single-electrode set is placed in the vacuum cell, and a left electrode is formed by a left electrode, and a position electrode is placed; the accelerating electric field region at the upper part of the TOF detection region is communicated with the reflecting region through a field-free drift region, an XY deflection plate is arranged in the field-free drift region, and three groups of reflectors and detectors are arranged in the reflecting region.
Wherein:
the ion source drift region, the first differential vacuum cavity and the reaction region of the reaction tube are respectively provided with an equivalent voltage dividing resistor, direct current voltage is applied, and the electric field direction is from left to right along the axial direction of the drift tube; the reaction tube focusing area is provided with high-frequency alternating voltage, and the electric field direction is the radial direction of the drift tube.
Through the isolation effect of the first differential vacuum cavity, the influence of the pressure increase of the reaction tube on the ion source is reduced, and the air inflow of the reaction tube is increased to improve the sensitivity of the instrument.
The accelerating electric field region above the TOF detection region is communicated with the reflection region through a field-free drift region.
The ion transmission area is provided with a first turbomolecular pump, and the TOF detection area is provided with a second turbomolecular pump.
The inner part of the reaction tube is divided into a reaction zone and a focusing zone which are not blocked in space.
The water vapor generating device comprises a water storage bottle, wherein a thermocouple and a heating wire are arranged in the water storage bottle, a heat insulation layer is arranged outside the water storage bottle, the top of the water storage bottle is connected with one inlet of a first tee joint through a pipeline, and a water switch is arranged on the pipeline between the water storage bottle and the first tee joint; the other reaction gas inlets are connected with one of the inlets of the first tee through a pipeline, and other reaction gas regulating switches are arranged on the pipeline between the other reaction gas inlets and the first tee; and a flow meter and a flow regulating switch are arranged on a pipeline between the first tee outlet and the reaction chamber.
A reaction gas inlet is arranged at a position, opposite to the first differential vacuum cavity, on the ion source drift region, and a flow regulating switch is connected with the reaction gas inlet; the sixteen-way switching valve is provided with a gas inlet pipe to be tested.
The single lens is composed of three groups of electrodes which are arranged left and right, each group of electrodes is composed of two electrodes which are opposite up and down, wherein the electrodes at the two ends are extraction lens electrodes, the electrode at the middle is a transmission lens electrode, and the voltages applied by the three groups of electrodes are respectively as follows: the voltage range of the extraction lens electrode is-6V-0V, and the voltage range of the transmission lens electrode is-80V-110V.
The position close to the differential deflector hole in the TOF detection area is provided with a Faraday cup, four polar plates are arranged in the TOF detection area, the four polar plates are a bottom plate, an extraction polar plate, a middle polar plate and an acceleration polar plate from top to bottom in sequence, and the voltage range of each polar plate is as follows: the voltage of the bottom plate is minus 100V to minus 200V, pulse voltage is applied to the extraction polar plate, ions to be detected enter the polar plate ion introduction area, the voltage value of the extraction polar plate is the same as the voltage value of the bottom plate, when the ions with the largest mass-to-charge ratio enter the ion introduction area after one sample injection, the voltage value of the extraction polar plate is increased to minus 400V to minus 450V, the voltage of the middle polar plate is minus 500V to minus 650V, and the voltage of the acceleration polar plate is minus 1800V to minus 2200V.
The anode voltage in the glow discharge zone of the ion source is 400-550V, and the glow discharge supply causes the cathode voltage to float down to about 750V, below the anode set point, before the glow discharge turns on but the plasma begins. Thus, if the anode voltage is 450V, the cathode voltage will drop to about-300V. After the plasma starts, the cathode voltage starts to change. At this time, the discharge potential between the anode and the cathode is maintained between 350V and 400V. The pressure of the glow discharge area of the ion source is 10-100Pa, and the discharge current is 4-6mA; the pressure in the ion transport zone is of the order of 10 -4 mbar; the pressure in the TOF detection zone was of the order of 10 -5-10-7 mbar.
The outside of the reaction chamber is coated with a heat-insulating layer, a heating wire, a fan and a temperature controller are arranged in the interior; the gas flow rate at the inlet of the reaction chamber is 3-14mL/min; the inner diameter of the first small hole is 0.05-0.5cm; the diameters of the second small hole and the third small hole are 0.1-1.0cm, and the inner diameter of the differential diversion hole is 1-1.5mm.
According to the invention, through the isolation effect of the first differential vacuum cavity, the influence of the pressure increase of the reaction tube on the ion source is reduced, and the air inflow of the reaction tube is increased so as to improve the sensitivity of the instrument. Because the content of VOCs in the ambient air sample is low, in order to improve the accuracy of sample measurement, the pressure in the reaction tube is set to be 5-10mbar, the first mechanical pump is used for pumping vacuum, the direct current voltage is applied to the two ends of the reaction zone of the reaction tube to be 1.5-3KV, and the sample gas injection amount is 200-300ml/min; and simultaneously, high-frequency alternating voltage is applied to a focusing area of the reaction tube, the frequency of the alternating voltage is 0.4-0.6MHz, and the voltage amplitude is 40-60V. The length of the reaction tube is 90-120cm.
Three groups of reflectors are arranged in the reflecting area, each group of reflectors consists of a reflecting plate, a sleeve and annular polar plates, the reflecting plate is circular, the sleeve is perpendicular to the reflecting plate, two groups of annular polar plates are arranged on the inner wall of the sleeve, the two groups of annular polar plates are distributed up and down, the voltage of one group of annular polar plates close to the opening of the sleeve is-1800V to-2200V, the voltage of one group of annular polar plates close to the reflecting plate is-1700V to-1900V, and the electromotive force is higher than that of the first group of annular polar plates; the voltage of the reflecting plate is 1800V-2000V; the three groups of reflectors are respectively a first reflector, a second reflector and a third reflector, the sleeve openings of the first reflector and the third reflector are upward, the sleeve opening of the second reflector is downward, the first reflector and the third reflector are positioned on two sides of the second reflector and have the same height, and the height of the second reflector is higher than that of the first reflector and the third reflector. The first reflector is located below the XY deflector plate.
The VOCs gas inlet is connected with a standard gas outlet of the dynamic gas diluter and a zero gas inlet pipe through pipelines, the sixteen switching valves are connected with the gas inlet pipe to be tested, the gas inlet pipe to be tested can be an ambient air inlet pipe or a Suma tank of each path, the standard gas steel bottle and the nitrogen steel bottle are respectively connected with the dynamic gas diluter through pipelines, a first mass flowmeter and a first pressure reducing valve are arranged on the pipelines between the standard gas steel bottle and the dynamic gas diluter, a second mass flowmeter and a second pressure reducing valve are arranged on the pipelines between the nitrogen steel bottle and the dynamic gas diluter, the zero gas inlet pipe is connected with the sixteen switching valves after being connected with the zero gas generator, the zero gas inlet pipe is sequentially connected with an oil-free pump, a pre-cooling dehydrator and a molecular sieve which are arranged inside the zero gas generator, the VOCs gas outlet is arranged on the second differential vacuum cavity, the VOCs gas outlet is sequentially connected with a first interface of an air pump and a second three-way through pipelines, the second three-way second interface is connected with a dynamic gas diluter waste gas outlet, and the second three-way waste gas outlet pipe is connected with a third interface of the two-way waste gas evacuation pipe through the exhaust pipe.
The detector is a double-microchannel plate detector, the double-microchannel plate detector is connected with a 4GTDC time digital conversion module, the 4GTDC time digital conversion module is connected with an industrial personal computer, and the industrial personal computer is simultaneously connected with a controller of the sixteen-channel switching valve and an electronic weather station and is remotely connected with a mobile phone or a computer terminal.
Wherein:
The ion source comprises an ion source glow discharge region and an ion source drift region, and the interior of the reaction tube is divided into a reaction region and a focusing region which are not blocked in space; the ion source is provided with a reaction gas inlet, the reaction gas inlet can be simultaneously connected with a water vapor inlet and other reaction gas inlets through a first tee joint, the water vapor generator and the adjusting control part thereof are connected with other reaction gas switches and the adjusting control part thereof through tee joint ball valves, and the quick switching of various reaction gases can be realized only by switching the reaction gas inlet without changing the ion source.
The reaction gas can be water vapor, or O 2, NO, etc., and the gas flow rate at the inlet of the reaction chamber is 3-14mL/min, so that the ion abundance of positive ions (H 3O]+、[O2]+ or NO +) of the reaction gas reaches more than 1.6X10 5, and the volatile organic molecules R are smoothly converted into VOCs positive ions (RH + or R +).
The first differential vacuum chamber serves a dual purpose, one, to further purify the reactant gas ions. The first differential vacuum cavity is externally applied with voltage and connected with equivalent resistance voltage division, and a uniform electric field is formed inside the first differential vacuum cavity. Under the action of electric field force, positive ions move towards the reaction tube, and neutral molecules do not have obvious orientation because of being free from the action of the electric field force, and are pumped away by the molecular pump, so that the aim of continuously extracting the reaction gas ions from the reaction gas in the process of moving towards the reaction tube is fulfilled. Secondly, the pressure in the reaction tube can have a certain influence on the ion source, so that the pressure in the reaction tube which can be born by the ion source is limited, the pressure of the traditional drift tube can only reach 1-2mbar, the ion source and the reaction tube can be isolated to a greater extent by means of the first differential vacuum cavity, the pressure in the reaction tube can reach 5-10mbar, the ion number density in the reaction tube can be improved, the sample injection amount of sample gas is increased to obviously enhance the response value of each VOCs substance to be measured, the sensitivity of the instrument is improved, and the accuracy of a measurement result is ensured. The ion flow is monitored by a Faraday cup arranged at the tail end of the reaction tube through an ion motion simulation experiment, and the ion flow intensity is 500nA when the first differential vacuum cavity is not arranged, and is increased to 700nA when the low-concentration sample is measured by the action of the first differential vacuum cavity. The increase in ion current intensity versus instrument sensitivity is embodied here in a significant decrease in detection limit: the lowest detection limit of the instrument can only reach a few ppt when the first differential vacuum chamber is not arranged, and the detection limit of each substance can reach below the ppt level by arranging the differential vacuum chamber. The inner diameter of the first small hole is 0.05-0.5cm, and the rear part of the first small hole is further pumped with the reaction gas by a molecular pump, so that the unprotonated reaction gas entering the reaction tube is furthest reduced.
The inner wall of the reaction tube is modified, and polydimethylsiloxane can be used as a modification material to reduce the adsorption of VOCs molecules.
An equivalent voltage dividing resistor is arranged outside the reaction area of the reaction tube to divide the direct-current voltage outside the reaction tube, so that a uniform axial electric field is formed in the reaction tube, and axial power is provided for ion movement; the VOCs react with the positive ion collisions of the reactant gas within the tube. The pressure in the tube is set by a computer, the molecular pump is used for vacuumizing, the air inlet needle valve is controlled by the valve motor to deliver sample gas, and the constant pressure in the reaction tube and the constant air inflow are ensured. When a sample is measured, the pressure in the reaction tube is 5-10mbar, the direct current voltage is applied to the two ends of the reaction tube and is 1.5-3KV, and the sample gas injection amount is 200-300ml/min; and simultaneously, high-frequency alternating voltage is applied to the focusing region, the frequency of the alternating voltage is 0.4-0.6MHz, the voltage amplitude is 40-60V, and the voltage provides radial constraint force for ion movement, so that ions are focused towards the center of the reaction tube, and the number of ions passing through the second differential vacuum small hole is increased. Through an ion motion simulation experiment, the Faraday cup is used for monitoring the ion flow, and the passing rate of ions can be improved by 10 times through the focusing effect of the high-frequency alternating voltage arranged at the ion flow. The length of the reaction tube is 90-120cm, and the increase of the length can increase the collision probability of VOCs and ions of the reactant, improve the reaction efficiency and improve the sensitivity of the instrument measurement.
The single lens is formed by arranging three groups of electrodes left and right, each group of electrodes is two pieces which are opposite up and down, wherein the electrodes at the two ends are extraction lens electrodes, the electrodes at the middle are transmission lens electrodes, and focusing of ion flow is completed by adjusting the voltage values of the electrodes. The extraction lens electrode voltage is high, the transmission lens electrode voltage is low, and the voltages applied by the three groups of electrodes are respectively as follows: the voltage range of the extraction lens electrode is-6V-0V, and the voltage range of the transmission lens electrode is-80V-110V. The ion transport zone pressure was maintained at a level of 10 -4 mbar with the first turbomolecular pump.
The ion transmission area is communicated with the TOF detection area through a differential diversion hole, the inner diameter of the differential diversion hole is 1-1.5mm, the pressure in the TOF detection area is 10 -5-10-7 mbar level, and the pressure is maintained by a second turbomolecular pump. And a Faraday cup is arranged in the TOF detection area and close to the position of the differential deflector hole to monitor ion flow, so that the ion utilization rate is improved. Four polar plates are arranged in the TOF detection area, the four polar plates are a bottom plate, an extraction polar plate, a middle polar plate and an acceleration polar plate from top to bottom in sequence, the space between the bottom plate and the extraction polar plate is an ion introduction area, the space between the extraction polar plate and the acceleration polar plate is an ion acceleration area, the acceleration area is accelerated by double fields, the space between the extraction polar plate and the middle polar plate is separated by the space between the extraction polar plate and the middle polar plate, and the space between the middle polar plate and the acceleration polar plate is a secondary acceleration area. Voltage range of each polar plate: the voltage of the bottom plate is minus 100V to minus 200V, pulse voltage is applied to the extraction polar plate, ions to be detected enter the polar plate ion introduction area, the voltage value of the extraction polar plate is the same as the voltage value of the bottom plate, when the ions with the largest mass-to-charge ratio (M/Z) enter the ion introduction area after one sample injection, the voltage value of the extraction polar plate is increased to minus 400V to minus 450V, the voltage of the middle polar plate is minus 500V to minus 650V, and the voltage of the acceleration polar plate is minus 1800V to minus 2200V.
The XY deflector is composed of an X deflector and a Y deflector, the X deflector is positioned above the Y deflector, the XY deflector is used for horizontally adjusting the ion beam entering the reflector and flying to the detector, the ion beam is guided to impinge on the detector to the maximum extent, and the detector detects the ions of each component.
The three groups of reflectors can improve the ion transmittance, ions reach the detector for detection after tertiary reflection, the ion flight path is increased, the ion focusing effect is enhanced, and the detection resolution is improved.
The detector is a double micro-channel plate detector, and the directly output signal is an analog signal and has a linear relation with the flight time and the ion quantity, so that the problem of large time and amplitude errors of ions caused by signal distortion is solved by adopting a high-frequency sampling and transmission mode. The double-microchannel plate detector is connected with the 4 GTDC-time digital conversion module, the 4 GTDC-time digital conversion module is connected with the industrial personal computer, and the industrial personal computer is connected with the sixteen-way switching valve controller so as to control the sixteen-way switching valve. The TDC time-digital conversion module samples the frequency of 4GHz and accurately measures the time interval between the start and stop of the pulse signal. The electronic pulse signal of the time-of-flight mass spectrum is converted and output into a fixed amplitude signal by a peak discriminator (CFD), finally, the signal is recorded by a time-to-digital converter (TDC), and after the data is processed by a computer, the formed time-of-flight mass spectrum has very narrow mass spectrum peak, the rising edge of the peak is less than 1 nanosecond, and the signal bandwidth is more than 1-2 GHz.
According to the invention, the generation and purification of positive ions of the reaction gas can be realized by the action of the electric field applied outside the first differential vacuum cavity, so that the purity of the reaction ions is improved; the reaction with VOCs molecules is completed in the modified reaction tube, the influence of pressure increase in the reaction tube on an ion source is reduced by means of the isolation effect of the first differential vacuum cavity, so that the air inflow of a sample is improved, the molecular density in the tube is increased, the sensitivity of an instrument can be remarkably improved, and the anti-adsorption effect of a modifier plays an important role in improving the sensitivity of the instrument; the direct current applied by the reaction area of the reaction tube and the high-frequency alternating current applied by the focusing area can screen and focus ions generated by reaction, so that the density of the ions entering the ion transmission area is increased, and the resolution of the instrument is improved. The focusing effect and high transmittance of the single lens also have important contribution to improving the sensitivity and resolution of the instrument; the ion flow of the focused ion beam is monitored by a Faraday cup, so that the ion utilization rate is improved; the ion flow is reflected by three reflectors after vertically entering the TOF detection area, so that the movement path of the ions is increased, and the influence on resolution caused by the speed direction and the non-uniform size (speed dispersion) and the different positions (space dispersion) of the ion flow when the ion flow enters is reduced; the accurate design of the voltage value of each polar plate of the TOF area detector, in particular to the adjustment of peak shape by the middle polar plate and the high-resolution detection of the double-microchannel plate detector, plays a vital role in improving the resolution of the instrument. The instrument has less ion fragments generated, low mass spectrum background noise and capability of realizing quick, high-sensitivity and high-resolution detection of VOCs.
The invention also provides a method for measuring the content of VOCs by adopting the time-of-flight mass spectrometry equipment, which comprises the following steps:
The method comprises the steps that reactive gas ions generated in an ion source enter a reaction tube after passing through a first differential vacuum cavity to collide with VOCs molecules R in a sample to be detected, positive ions are generated, a first differential vacuum cavity is arranged between an ion source drift region and the reaction tube, an equivalent voltage dividing resistor is arranged outside the first differential vacuum cavity, direct-current voltage is applied and connected with the equivalent voltage dividing resistor, a uniform electric field is formed inside the first differential vacuum cavity, the generated reactive gas ions move towards the reaction tube under the action of the electric field force, and reactive gas neutral molecules (H 2O、O2 and NO) do not have obvious orientation due to the fact that the reactive gas neutral molecules are not acted by the electric field force, the tail end of the first differential vacuum cavity is connected with a first mechanical pump, and the first mechanical pump pumps the reactive gas neutral molecules to prevent the neutral molecules from entering the reaction tube; positive ions [ RH ] + or [ R ] + to be detected generated by the reaction enter an ion transmission area through a second small hole under the action of a uniform electric field in a reaction tube, a single lens is arranged in the ion transmission area and is formed by arranging three groups of electrodes left and right, each group of electrodes is formed by two electrodes which are opposite up and down, wherein the electrodes at the two ends are extraction lens electrodes, the middle electrode is a transmission lens electrode, and focusing of ion flow is completed by adjusting the voltage value of the electrodes; the ions flow through the differential deflector hole after focusing and then horizontally enter the TOF detection area, a Faraday cup is arranged in the TOF detection area and close to the differential deflector hole, the ions focused by the lens horizontally enter the TOF detection area, the ion flow is monitored by the Faraday cup, then the ions enter the TOF detection area, and ions with different mass-to-charge ratios are distinguished in the continuous motion process; the accelerating electric field area at the upper part of the TOF detection area is communicated with the reflecting area through a field-free drift area, an XY deflection plate is arranged in the field-free drift area, the ion beam entering the reflector and flying to the detector is regulated, three groups of reflectors are arranged in the reflecting area, the ion beam is guided to impinge on the detector to the maximum extent, and the detector realizes the detection of each component ion.
When the reactive gas ion is [ H 3O]+ or [ O 2]+ ] or [ NO ] +, when the reactive gas ion is [ H 3O]+ ], water vapor generated by the water vapor generating device flows through the water switch and then enters the ion source glow discharge area of the reaction chamber through the first tee joint by the reactive gas inlet, the reactive gas ion [ H 3O]+ ] is generated by ionization of the hollow cathode lamp of the ion source, and when the reactive gas ion is [ O 2]+ or [ NO ] +, O 2 or NO enters the ion source glow discharge area of the reaction chamber through the other reactive gas inlets, and different kinds of reactive gas are ionized by the hollow cathode lamp of the ion source to generate the reactive gas ion [ O 2]+ or [ NO ] +.
The process of measuring the gas sample containing VOCs by the instrument is as follows:
The reaction gas is ionized to generate positive ions in the ion source in the reaction chamber. The reaction chamber comprises an ion source glow discharge region, an ion source drift region, a first differential vacuum cavity and a reaction tube which are sequentially adjacent. The rear part of the reaction chamber is sequentially connected with a second differential vacuum chamber, an ion transmission area and a TOF detection area. The outside cladding heat preservation of reaction chamber, inside set up heater strip, fan and temperature controller, the heater strip heats it, and the fan promotes inside temperature equilibrium to with the temperature controller accuse temperature. Since some of the volatile organic molecules cannot be measured by proton transfer reaction with hydronium ions, measurement by other reaction gases (O 2, NO, etc.) is required. The replacement of the reaction gas is often accompanied by the replacement and adjustment of the ion source, the operation is complex, and the requirements on technicians are high. The invention designs an ion source with simpler operation, wherein the ion source is provided with a reaction gas inlet, the reaction gas inlet is connected with an outlet of a first tee joint, two inlets of the first tee joint are respectively connected with a vapor generating device and other reaction gas inlets, the vapor generating device comprises a water storage bottle, a thermocouple and a heating wire are arranged in the water storage bottle, an insulating layer is arranged outside the water storage bottle, the top of the water storage bottle is connected with one inlet of the tee joints through a pipeline, and a water switch is arranged on the pipeline between the water storage bottle and the tee joint; the other reaction gas inlets are connected with one of the inlets of the first tee through a pipeline, and other reaction gas regulating switches are arranged on the pipeline between the other reaction gas inlets and the first tee; and a flow meter and a flow regulating switch are arranged on a pipeline between the first tee outlet and the reaction chamber. Thus, the rapid switching of the plurality of reaction gases can be realized by simply switching the gas inlet without changing the ion source.
The method uses the water vapor as the reaction gas, is suitable for monitoring most volatile organic molecules, and can detect VOCs molecules which cannot react with hydronium ions in a proton transfer way by using the reaction gases such as O 2, NO and the like. Taking water as a reactant for example, a water switch is turned on, and the water storage bottle is subjected to constant temperature control in order to ensure the stability of the generation rate of the reactive ions. The temperature of water in the water storage bottle is maintained at 40 ℃ under the heating of the heating wire and the heat preservation effect of the heat preservation groove, so that saturated water vapor with the constant pressure of 70mbar is continuously generated, and the gas flow is 3-14mL/min, so that the ion abundance of hydronium ions reaches more than 1.6X10 5. The generated water vapor flows through a water switch, then enters a reaction chamber ion source through a reaction gas inlet to be ionized to generate hydronium ions after passing through a first tee joint, a flowmeter and a flow regulating switch. To obtain better ionization efficiency of the reaction gas, the anode voltage of the glow discharge area of the ion source needs to be ensured to be 400-550V, and the cathode voltage is caused to float downwards to about 750V by the glow discharge supply before the glow discharge is switched on but the plasma starts; below the anode setpoint. Thus, if the anode voltage is 450V, the cathode voltage will drop to about-300V. After the plasma starts, the cathode voltage starts to change. At this time, a voltage difference of the sustain discharge between the anode and the cathode is maintained between 350V and 400V. The pressure of the glow discharge area of the ion source is about 10-100Pa, and the discharge current is 4-6mA. A large amount of [ H +、[H3O]+ ] is generated by utilizing cathode glow discharge, and further reacts in an ion source drift region to generate the [ H 3O]+ ] with higher purity. If other reaction gases are used, the gases enter the ion source after passing through the first tee joint, the flowmeter and the flow regulating switch after passing through other reaction gas inlets, and the ion source does not need to be replaced.
The reaction gas ion generated by the reaction enters a reaction tube to collide with VOCs molecule R, wherein R represents a molecule and reacts to generate a molecule [ RH ] + or [ R ] + to be detected. The ions of the reaction gas are brought into the reaction tube while they are brought into the water vapor to different extents. In order to reduce the water molecules entering the reaction tube as much as possible and improve the purity of hydronium ions, a first differential vacuum cavity is designed and arranged between an ion source drift region and the reaction tube, and neutral reaction gas (H 2 O or NO or O 2) is pumped by a first mechanical pump at the tail end of the first differential vacuum cavity.
The first differential vacuum cavity is applied with direct current voltage and connected with equivalent resistance voltage division, and a uniform electric field is formed inside the first differential vacuum cavity. Under the action of the electric field force, positive ions move towards the reaction tube, and neutral molecules do not have obvious orientation because of being free from the action of the electric field force, so that the aim of continuously extracting the reactive gas ions (H 3O+) from the reactive gas (H 2 O) in the process of moving towards the reaction tube is fulfilled. Meanwhile, the first differential vacuum chamber is internally pumped with a first mechanical pump to absorb neutral molecules (H 2 O) of the reaction gas, so that neutral water molecules are prevented from entering the reaction tube, the purity of hydronium ions entering the reaction tube is improved, and the sensitivity of the instrument is improved.
The first differential vacuum chamber plays a role in improving the sample measurement sensitivity, not only in improving the purity of the reaction gas ions, but also has another important role in: the ion source and the reaction tube can be isolated to a large extent while the reaction gas ions are not influenced to enter the reaction tube, so that the influence of the increase of the pressure in the reaction tube on the ion source is reduced. Therefore, when a sample is measured, the sample injection amount of the sample gas can be increased by properly increasing the molecular number density and the voltage of the reaction tube, so that the response value of each VOCs substance is obviously enhanced and the sensitivity of the instrument is improved. The specific operation is as follows: by means of the isolation function of the differential vacuum cavity, the air inlet needle valve is controlled by the valve motor to improve the air inflow of the delivered sample gas, the molecular number density in the reaction tube is increased, and the pressure in the reaction tube is regulated to be increased to 5-10mbar. By virtue of the effect of the first differential vacuum cavity on increasing the ion current intensity, the sensitivity of the instrument is obviously improved, and the method is particularly embodied in the aspect of obviously reducing the detection limit: the lowest detection limit of the instrument can only reach a few ppt when the first differential vacuum chamber is not arranged, and the detection limit of each substance in the differential vacuum chamber can reach below the ppt level.
The direct current voltage is applied to the two ends of the reaction zone of the reaction tube and is 1.5-3KV, and an equivalent voltage dividing resistor is arranged outside the reaction zone to divide the direct current voltage outside the reaction tube, so that a uniform electric field is formed in the reaction tube. E/n is maintained within the range of 170-180, and the sample gas injection amount is 200-300ml/min. And high-frequency alternating voltage is applied to the focusing region, the frequency of the alternating voltage is 0.4-0.6MHz, the voltage amplitude is 40-60V, the voltage provides radial constraint force for ion movement, so that ions are focused towards the center of the reaction tube, and the number of ions passing through the second differential vacuum small hole is increased. The material and length of the reaction tube have important influence on the sensitivity and resolution of the instrument measurement, and the stainless steel reaction tube can generate adsorption effect and memory effect, so that the inner wall of the reaction tube is modified by the polydimethylsiloxane coating, the adsorption effect and the memory effect are reduced, and the measurement sensitivity is improved. The length of the reaction tube is designed to be 90-120cm, so that the collision probability of VOCs and ions of the reaction reagent can be increased, the reaction efficiency is improved, and the measuring sensitivity of the instrument is improved.
The vacuum degree in the reaction tube is maintained by a second mechanical pump, and unreacted VOCs gas in a second differential vacuum cavity connected to the tail end of the reaction tube is pumped out by an air pump through a VOCs gas outlet through a flowmeter and is exhausted through a tee joint.
And the ions of the object to be detected generated by the reaction enter an ion transmission area through a second small hole under the action of an even electric field in the reaction tube, the diameter of the second small hole is 0.1-1.0cm, and the ion transmission area is vacuumized by a turbomolecular pump, so that the pressure of the ion transmission area is ensured to be 10 -4 mbar. Based on the fact that the ion current can be diverged to a certain extent in the ion movement process and has serious influence on the resolution of an instrument, the inventor comprehensively considers the factors of the ion passing rate and the ion beam focusing, the designed ion transmission area uses a single lens, the single lens is formed by arranging three groups of electrodes left and right, each group of electrodes is two pieces which are opposite up and down, the electrodes at the two ends are extraction lens electrodes, the electrode at the middle is a transmission lens electrode, and the focusing of the ion current is completed by adjusting the voltage value of the electrodes. The single lens has simple structure and high ion passing rate, and can achieve ideal focusing effect by precisely controlling the electrode voltage values of the three groups of lenses. The voltages applied by the three groups of electrodes are respectively as follows: the voltage range of the extraction lens electrode is-6V-0V, and the voltage range of the transmission lens electrode is-80V-110V. Therefore, a curve-shaped electric field is formed, positive ions or negative ions entering the electric field can be focused to form concentrated ion flow, the problem of ion divergence is prevented, the introduction efficiency of ions is ensured, and the method has important contribution to improving the resolution of an instrument. The ions flow through the differential diversion holes and horizontally enter the TOF detection area after focusing, the inner diameter of the differential diversion holes is 1-1.5mm, the ion passing rate can be ensured, the ion transmission area and the TOF detection area are effectively isolated, and the high vacuum degree of the TOF detection area is ensured. The design that TOF detection area is directly connected without using a quadrupole mass analyzer has obvious advantages compared with the design that a quadrupole mass analyzer is used, the traditional quadrupole mass analyzer mainly plays a role in ion screening, has low resolution and can detect molecular weight which is not more than three hundred; the design solves the problem of discrimination of the mass of the ions monitored by the four-stage rod mass analyzer, and is a better choice in the aspect of monitoring macromolecules of VOCs.
The ions focused by the lens horizontally enter the TOF detection area and then are applied with an electric field in the vertical direction, and the electric field forces applied by different ions in the vertical direction work the same, so that the ions with different mass-to-charge ratios (M/Z) have different movement speeds, the ions with large mass are slow in speed and the ions with small mass are fast in speed, and the ions with different mass-to-charge ratios (M/Z) can be continuously distinguished after moving for a certain distance. However, considering the practical situation of ion motion, the ions have velocity dispersion and space dispersion when entering the TOF detection region, which severely restricts the sensitivity and resolution of the detector. For this reason, the measures we take in the sensitivity and resolution of ion measurement for noise reduction in the TOF detection region design are: the pressure of the TOF detection area (high vacuum degree is guaranteed) is controlled to be 10 -5-10-7 mbar, and influence of stray ions on ion flight is reduced. And a Faraday cup is arranged in the TOF detection area and close to the position of the differential deflector hole to monitor ion flow, so that the ion utilization rate is improved. The ion accelerating area adopts double-field acceleration and is formed by four polar plates, the four polar plates sequentially comprise a bottom plate, an extraction polar plate, a middle polar plate and an accelerating polar plate from top to bottom, a space between the bottom plate and the extraction polar plate is an ion introducing area, the space between the extraction polar plate and the accelerating polar plate is an ion accelerating area, the accelerating area adopts double-field acceleration, the space between the extraction polar plate and the middle polar plate is divided by the space between the extraction polar plate and the middle polar plate, and a secondary accelerating area is arranged between the middle polar plate and the accelerating polar plate. The middle polar plate plays an important role in adjusting the ion peak shape and improving the resolution ratio: in the optimal voltage value range, when the voltage of the middle polar plate is smaller, the kinetic energy obtained by the ions between the extraction polar plate and the middle polar plate is small, the initial acceleration speed of the ions is small, the movement time is prolonged, the peak shape is widened, and the resolution is reduced; the resolution can be improved by properly increasing the voltage of the middle polar plate, increasing the initial acceleration speed of ions, and reducing the movement time to narrow the peak shape, but too high the voltage of the middle polar plate also reduces the resolution. Experiments show that when water is used as a reaction reagent, the optimal value of the voltage of the middle polar plate is-600V when benzene in the air is measured, and when the voltage of the middle polar plate is-490V, the peak height is 638, and the resolution is 3060; increasing the voltage to-550V with peak height 952 and resolution 4647; when the voltage is increased to-570V, the peak height is 974, and the resolution is 4669; when the voltage is increased to-600V, the peak height is 1016, the resolution is 5012, and the resolution is highest at the moment; when the voltage was increased to-620V, the peak shape was rather poor.
And an XY deflection plate is arranged at the position of the field-free drift region, which is close to the accelerating electrode plate, and the ion beam entering the reflector and flying to the detector is adjusted to be guided to impinge on the detector to the maximum extent. In order to compensate the space dispersion and the energy dispersion of the ions to a greater extent, three groups of reflectors are used, wherein the three groups of reflectors are respectively a first reflector, a second reflector and a third reflector, the sleeve openings of the first reflector and the third reflector are upward, the sleeve openings of the second reflector are downward, the first reflector and the third reflector are positioned on two sides of the second reflector and have the same height, the height of the second reflector is higher than that of the first reflector and the third reflector, that is, the positions of the three groups of reflectors are positioned at three endpoints of an inverted V shape, the first reflector is positioned below an XY deflector plate, the positions of the XY deflector plate, the three groups of reflectors and the detector are positioned at five endpoints of a W shape, and the design increases the movement path of the ions, and the increase of the movement path can effectively compensate the space dispersion; the reflector can make the depth of the ion with large initial energy entering the reflecting area deeper, the depth of the ion with small initial energy entering the reflector shallower, the fast one has more travel distance, the slow one has less travel distance, and the same ion almost reaches the detector at the same time after three reflections, thereby compensating the velocity dispersion of the ion and improving the resolution ratio. The three-stage reflectors are adopted, so that the compensation of ion space dispersion and initial energy dispersion is realized to a greater extent, and the resolution of the instrument is greatly improved; the double micro-channel plate detector is adopted to improve the ion detection multiple, and has an important effect on improving the resolution of the instrument.
A Faraday cup is arranged at the position, close to the differential deflector hole, inside the TOF detection area, of each polar plate voltage of the TOF detection area, wherein the voltages of a bottom plate, a middle polar plate and an accelerating polar plate maintain set optimal values, namely-100V-200V, -500V-650V, -1800V-2200V, respectively, only the extracting polar plate takes the form of pulse voltage, when the ion flow continuously enters the ion introduction areas of the bottom plate and the extracting polar plate, the voltages of the extracting polar plate are-100V-200V, the voltages of the extracting polar plate and the bottom plate are the same, no electric field exists between the extracting polar plate and the bottom plate, and ions transversely enter the ion introduction area; when ions with the maximum mass-to-charge ratio (M/Z) after one sample injection enter an ion introduction area, the voltage of an extraction polar plate is increased to-400 v to-450 v, and at the moment, the ion flow moves downwards under the action of an electric field, so that the ion flow can be modulated into a single ion group, the ion utilization rate is ensured, and the sensitivity is improved; the intermediate polar plate is used for preliminary acceleration, the introduction of the intermediate polar plate has important significance, the remarkable effect is that the peak shape of ions is regulated by regulating the voltage of the intermediate polar plate, the resolution is improved, when the voltage of the intermediate polar plate is smaller, the preliminary acceleration speed of the ions is small, the movement time is prolonged, the peak shape is widened, the resolution is reduced, and the preliminary acceleration speed of the ions is increased, the movement time is reduced, the peak shape is narrowed and the resolution is improved by properly increasing the voltage of the intermediate polar plate; ions primarily accelerated by the intermediate polar plate are then accelerated under the action of high voltage (-2000V) of the accelerating polar plate, so that larger kinetic energy is obtained. The charged ions are then deflected by X/Y to adjust the lateral position of the ion beam and direct the ion beam to impinge maximally upon the detector. The ions then move into the field-free drift region and are separated continuously during the movement. Ions enter the reflecting area after passing through the field-free drift area, the voltage of a group of annular polar plates close to the sleeve opening is-1800V to-2200V, the voltage of a group of annular polar plates close to the reflecting plate is-1700V to-1900V, the voltage of a group of annular polar plates close to the sleeve opening is the same as that of the accelerating polar plates, a field-free drift area is formed between the two polar plates, the voltage of the reflecting plate is 1800V to 2000V, and the double reflecting area provides a medium for effective reflection of the ions and has important significance for compensating speed dispersion. In this region, ions are accelerated in opposite directions into the next ion reflection region after being gradually decelerated by an acting force opposite to the movement direction, and after the multi-stage reflection is completed, the ions strike the double microchannel plate and are detected by the high-resolution double microchannel plate detector.
The signal measured by the detector passes through a 4GTDC time-digital conversion module, the sampling frequency is 4GHz, and the time interval between the start and stop of the pulse signal is accurately measured. The electronic pulse signal of the time-of-flight mass spectrum is converted and output into a fixed amplitude signal by a peak discriminator (CFD), finally, the signal is recorded by a time-to-digital converter (TDC), and after the data is processed by a computer, the formed time-of-flight mass spectrum has very narrow mass spectrum peak, the rising edge of the peak is less than 1 nanosecond, and the signal bandwidth is more than 1-2 GHz.
In summary, the invention has the following advantages:
(1) The organic species that can be detected by the invention are of a wide variety. The method can switch various reaction gases such as H 2O、O2, NO and the like instantaneously without barriers on the premise of not changing the ion source to react.
(2) The invention can obviously improve the accuracy of the measurement of the VOCs gas sample. The first differential vacuum cavity is added between the ion source drift region and the reaction tube, and the influence of the pressure increase in the reaction tube on the ion source is reduced through the isolation effect of the first differential vacuum cavity, so that the pressure in the reaction tube can be greatly increased, the sample air inflow is improved, the molecular number density in the tube is increased, and the detection accuracy is remarkably improved.
(3) The invention can obviously improve the sensitivity of VOCs measurement. The inner wall of the reaction tube used by the method is modified by a coating, so that the adsorption effect of VOCs molecules can be reduced; the reaction tube focusing area applies high-frequency alternating voltage and can focus ion flow; the transmissivity of ions entering the TOF detection area is further improved through secondary focusing of the ion transmission area, meanwhile, the ion flow is monitored by a Faraday cup before the ion beam enters the TOF detection area, and the ion utilization rate is improved; the instrument has the advantages of less generated ion fragments, low mass spectrum background noise, high sensitivity and low detection limit.
(4) The invention can improve the resolution of VOCs measurement. The three-stage reflectors are combined with the double-microchannel plate detector, so that the movement path of ions is increased, and the influence of speed direction, inconsistent size (speed dispersion) and different positions (space dispersion) on resolution when the ion flow enters is reduced; the accurate design of the voltage value of each polar plate of the TOF detection area detector, especially the effect of the middle polar plate on adjusting the ion peak shape and the high-resolution detection of the double-microchannel plate detector play a vital role in improving the resolution of the instrument.
(5) The invention can rapidly monitor the concentration of VOCs in the directly collected multi-pipeline ambient air sample and the ambient air collected in the Suma tank or the air collection bag. The sample injection system uses sixteen-way switching valves to control the nondestructive switching of sampling gas pipelines through a computer, so that the function of simultaneously measuring the air samples of the multi-pipeline environment air and the air samples of the sigma tank or the air sampling bag by the single reaction time-of-flight mass spectrometry equipment is realized. The data measurement speed is high, the concentration value of the VOCs component in the pollution source can be obtained within a millisecond range, and the method plays an important role in rapid emergency monitoring.
(6) The invention can select to use the reaction gas positive ions ([ H 3O]+ or [ O 2]+ or [ NO ] +) generated by the ionization of various reaction gases such as water vapor, O 2, NO and the like by a high-intensity hollow cathode lamp to react with VOCs molecules R in a modified reaction tube to generate charged ions [ RH ] + or [ R ] +, and then the charged ions enter a novel lens of a transmission region to focus without using the traditional electrostatic quaternary rod DCQ for focusing separation, thereby avoiding the loss of the charged ions, reducing the quality discrimination, increasing the range of the measured molecular weight, and finally directly measuring by a TOF detection region. The interference factors in the detection process are few; the ionization source is soft chemical ionization, has less fragment generation, simple spectrogram, easy analysis, wide molecular weight measurement range, high resolution and sensitivity and low detection limit.
Drawings
FIG. 1 is a schematic structural view of embodiment 1 of the present invention;
fig. 2 is a graph showing the resolution test of the device for different voltages of the intermediate plate according to embodiment 1 of the present invention.
In the figure: 1-heat preservation, 2-water storage bottle, 3-heating wire, 4-thermocouple, 5-water switch, 6-other reaction gas inlet, 7-other reaction gas regulating switch, 8-first tee, 9-flowmeter, 10-flow regulating switch, 11-ion source glow discharge region, 12-ion source drift region, 13-first differential vacuum chamber, 14-reaction tube, 15-second differential vacuum chamber, 16-ion transmission region, 17-Faraday cup, 18-TOF detection region, 19-bottom plate, 20-VOCs gas inlet, 21-first aperture, 22-first mechanical pump, 23-equivalent voltage dividing resistor, 24-second aperture, 25-electrode, 26-field-free drift region, 27-XY deflection plate, 28-reflection area, 29-detector, 30-reflector, 31-sleeve, 32-annular polar plate, 33-reflection plate, 34-4GTDC time digital conversion module, 35-second mechanical pump, 36-industrial computer, 37-sixteen path switching valve, 38-molecular sieve, 39-precooling dehydrator, 40-oil-free pump, 41-zero gas inlet pipe, 42-dynamic gas diluter, 43-first mass flowmeter, 44-first pressure reducing valve, 45-standard gas cylinder, 46-second mass flowmeter, 47-second pressure reducing valve, 48-nitrogen cylinder, 49-exhaust pipe, 50-second tee joint, 51-air pump, 52-third small hole, 53-differential diversion hole, 54-extraction polar plate, 55-middle polar plate, 56-accelerating electrode plate, 57-gas inlet pipe to be tested, 58-reaction gas inlet and 59-VOCs gas outlet.
Detailed Description
The invention is further illustrated below with reference to examples.
Example 1
The samples containing propylene, benzene, toluene, styrene and acetone were measured using water vapor as a reaction gas.
The adopted high-resolution high-sensitivity time-of-flight mass spectrometry equipment for measuring the content of VOCs comprises a reaction chamber, as shown in fig. 1, wherein the reaction chamber is provided with a reaction gas inlet 58, the reaction gas inlet 58 is connected with an outlet of a first tee joint 8, two inlets of the first tee joint 8 are respectively connected with a water vapor generating device and other reaction gas inlets 6, the reaction chamber comprises an ion source glow discharge region 11, an ion source drift region 12, a first differential vacuum chamber 13 and a reaction tube 14 which are sequentially adjacent, the rear part of the reaction chamber is sequentially connected with a second differential vacuum chamber 15, an ion transmission region 16 and a TOF detection region 18, the first differential vacuum chamber 13 is communicated with the reaction tube 14 through a first small hole 21, the reaction tube 14 is communicated with the second differential vacuum chamber 15 through a second small hole 24, the second differential vacuum chamber 15 is communicated with the ion transmission region 16 through a third small hole 52, the ion transmission region 16 is communicated with the TOF detection region 18 through a differential guide hole 53, the ion source drift region 12, the first differential vacuum chamber 13 and the outside the reaction region of the reaction tube 14 are respectively provided with an equivalent voltage dividing resistor 23, the first differential vacuum chamber 13 is connected with the second differential vacuum chamber 13, the second differential vacuum chamber 13 is connected with the second differential vacuum pump 13, the second differential vacuum pump 13 is arranged in the opposite to the first vacuum chamber 15, the second differential vacuum chamber is arranged in the single-vacuum chamber 15, the single-vacuum chamber is arranged in the vacuum chamber, the vacuum chamber is in the vacuum chamber; the upper part of the TOF detection region 18 communicates with a reflection region 28 via a field-free drift region 26, an XY deflection plate 27 being provided in the field-free drift region 26, and three sets of reflectors 30 and detectors 29 being provided in the reflection region 28.
The other reaction gas inlets 6 are connected with one inlet of the first tee joint 8 through a pipeline, and other reaction gas regulating switches 7 are arranged on the pipeline between the other reaction gas inlets 6 and the first tee joint 8; a flowmeter 9 and a flow regulating switch 10 are arranged on the pipeline between the outlet of the first tee joint 8 and the reaction chamber.
The VOCs gas inlet 20 is connected with a standard gas outlet of the dynamic gas diluter 42 and the zero gas inlet pipe 41 through pipelines, the sixteen switching valves 37 are connected with the gas inlet pipe 57 to be tested, the gas inlet pipe 57 to be tested can be an ambient air inlet pipe or a Suma tank, the standard gas steel bottle 45 and the nitrogen steel bottle 48 are respectively connected with the dynamic gas diluter 42 through pipelines, a first mass flowmeter 43 and a first pressure reducing valve 44 are arranged on the pipelines between the standard gas steel bottle 45 and the dynamic gas diluter 42, a second mass flowmeter 46 and a second pressure reducing valve 47 are arranged on the pipelines between the nitrogen steel bottle 45 and the dynamic gas diluter 42, the sample inlet pipe 41 is sequentially connected with the oil-free pump 40, the pre-cooling dehydrator 39 and the molecular sieve 38 which are arranged inside the zero gas generator, the sixteen switching valves 37 are connected with each other, the VOCs gas outlet 59 is arranged on the second differential vacuum chamber 15, the VOCs gas outlet 59 is sequentially connected with a first interface of the air extracting pump 51 and a second three-way 50 through pipelines, a second interface of the second three-way valve 50 is connected with a waste gas outlet of the dynamic gas diluter 42, a third interface of the second three-way 50 is connected with the waste gas outlet 49, and two ways of the waste gas is exhausted through the exhaust pipe 49.
The detector 29 is a double-micro-channel plate detector, the double-micro-channel plate detector is connected with the 4GTDC time digital conversion module 34,4GTDC time digital conversion module 34 and the industrial personal computer 36, and the industrial personal computer 36 is simultaneously connected with the controller of the sixteen-channel switching valve 37 and the electronic weather station and is remotely connected with a mobile phone or a computer terminal.
The ion transport region 16 is provided with a first turbomolecular pump and the TOF detection region 18 is provided with a second turbomolecular pump.
A reaction gas inlet 58 is arranged at a position, opposite to the first differential vacuum cavity, on the ion source drift region, and a flow regulating switch is connected with the reaction gas inlet 58; the sixteen-way switching valve 37 is provided with a gas inlet pipe 57 to be measured.
The standard gas containing 1ppm of propylene, benzene, toluene, styrene and acetone was prepared into standard gas having concentrations of 10ppb, 20ppb, 100ppb, 200ppb, 400ppb and 1000ppb, respectively, in terms of volume ratio. The standard gas is prepared from the standard gas in standard gas cylinder 45 and the nitrogen in nitrogen cylinder 48 in dynamic gas diluter 42. And sequentially sampling from low concentration to high concentration, detecting, establishing a calibration curve according to the relation between the concentration and the response value, and calculating the concentration of each component in the sample from the calibration curve by measuring the response value of each component in the sample.
During the measurement, the first tee 8 is connected with a steam inlet, and the other reaction gas inlets 6 are closed. The water switch 5 is turned on, and the water storage bottle 2 is subjected to constant temperature control in order to ensure the stability of the generation rate of the reactive ions. The temperature of water in the water storage bottle 2 is maintained at 40 ℃ under the heating of the heating wire 3 and the heat preservation effect of the heat preservation layer 1, saturated water vapor with the constant pressure of 70mbar is continuously generated, the flow rate of the water vapor is regulated to be 4mL/min, the generated water vapor flows through the water switch 5, then enters the ion source glow discharge zone 11 in the reaction chamber through the first tee joint 8, the flowmeter 9 and the flow regulating switch 10 through the reaction gas inlet, and is ionized to generate hydronium ions (H 3O]+) by utilizing the hollow cathode lamp of the ion source, and the ion abundance of the hydronium ions is ensured to reach 2.0x10 5.
For better ionization efficiency, the anode voltage of the ion source glow discharge region 11 is 450V, and the glow discharge supply causes the cathode voltage to drop to about-300V before the glow discharge is turned on but the plasma starts. After the plasma starts, the cathode voltage starts to change. At this time, the voltage difference of the sustain discharge between the anode and the cathode was maintained at 400V. The pressure of the ion source glow discharge area 11 is 100Pa, and the discharge current is 5mA; a large amount of [ H +、[H3O]+ ] is generated by utilizing cathode glow discharge, and further reacts in an ion source drift region to generate the [ H 3O]+ ] with higher purity. The reaction chamber is heated by the heating wire arranged in the reaction chamber before the measurement of the sample, the internal temperature balance is promoted by the fan, the temperature of the heat preservation layer coated outside is kept constant, and the temperature is strictly controlled by a temperature controller to be 100 ℃.
The [ H 3O]+ ] generated by the ion source enters the reaction tube 14 with the length of 96cm through the first differential vacuum cavity 13 to collide with VOCs molecules R for reaction, and at the moment, the first differential vacuum cavity 13 effectively reduces water molecules entering the reaction tube 14, improves the purity of hydronium ions [ H 3O]+ ], and plays an important role in improving the sensitivity of the instrument; the inner diameter of the first small hole 21 is 0.1cm, so that the reaction gas ions can effectively enter the reaction tube 14, and the ion source is isolated from the reaction tube 14 to a large extent. The reaction tube 14 is modified with polydimethylsiloxane to reduce adsorption of organic molecules.
When the gas samples containing propylene, benzene, toluene, styrene, acetone and the like are measured, the ion source and the reaction tube 14 can be isolated to a greater extent through the first differential vacuum cavity 13, the influence of the pressure increase in the reaction tube 14 on the ion source is reduced, the pressure in the reaction tube 14 is controlled to be increased to 10mbar through a computer, the voltage at the two ends is set to be 3KV, E/n is maintained at 175, the sample injection amount of the sample gas is 260ml/min, and therefore the ion number density in the reaction tube 14 is increased, and the response value of each substance to be measured is obviously enhanced. Meanwhile, high-frequency alternating voltage is applied to the focusing area, the frequency of the alternating voltage is 0.5MHz, the voltage amplitude is 55V, the voltage provides radial constraint force for ion movement, so that ions are focused towards the center of the reaction tube, the number of ions passing through the second differential vacuum small hole is improved, and the sensitivity of the instrument is improved. The method is particularly embodied in the aspect of obviously reducing the detection limit: the lowest detection limit of the instrument can only reach a few ppt when the first differential vacuum chamber 13 is not arranged, and the detection limit of each substance can reach below the ppt level by arranging the first differential vacuum chamber 13.
The protonated products of propylene, benzene, toluene, styrene and acetone [ RH ] + generated by the reaction enter the ion transmission area 16 after passing through the second small hole 24 and the third small hole 52 with the diameter of 0.5cm under the action of the uniform electric field in the reaction tube 14, the second small hole 24 and the third small hole 52 isolate the ion transmission area 16 from the reaction tube 14, and the pressure of the ion transmission area 16 is ensured to be 10 -4 mbar by the first turbomolecular pump for vacuum. The single lens is composed of three groups of electrodes 25 which are arranged left and right, each group of electrodes 25 is composed of two electrodes which are opposite up and down, wherein the electrodes 25 at two ends are extraction lens electrodes, the electrode 25 at the middle is a transmission lens electrode, and voltages applied by the three groups of electrodes 25 are respectively as follows: the extraction electrode voltage is-3V, the transmission electrode voltage is-100V, and the curve-shaped electric field formed by the extraction electrode voltage is used for focusing [ RH ] + entering the electric field to form concentrated ion flow, so that the problem of ion divergence is prevented, and the resolution of the instrument is improved. The focused ion flow horizontally enters the TOF detection area 18 after passing through the differential deflector hole 53, the inner diameter of the differential deflector hole 53 is 1.0mm, the ion passing rate is ensured, meanwhile, the ion transmission area and the TOF detection area are effectively isolated, the second turbomolecular pump is used for pumping vacuum, and the high vacuum degree of the TOF area is ensured to be 10 -7 mbar.
A faraday cup 17 is disposed inside the TOF detection region 18 near the differential deflector aperture 53, and ion flow is monitored by the faraday cup 17 as ions focused by the lens enter the TOF detection region 18 horizontally, and then ions of different mass to charge ratios (M/Z) are distinguished during continuous movement into the TOF detection region 18. The ion accelerating area at the upper part of the TOF detection area 18 is provided with four polar plates, namely a bottom plate 19, an extraction polar plate 54, a middle polar plate 55 and an accelerating polar plate 56 from top to bottom, wherein the space between the bottom plate 19 and the extraction polar plate 54 is an ion introducing area, the space between the extraction polar plate 54 and the accelerating polar plate 56 is an ion accelerating area, the accelerating area adopts double-field acceleration, the space between the extraction polar plate 54 and the middle polar plate 55 is separated by the space between the extraction polar plate 54 and the middle polar plate 55, and the space between the middle polar plate 55 and the accelerating polar plate 56 is a secondary accelerating area. Wherein, the optimal voltage values of the bottom plate 19, the middle polar plate 55 and the accelerating polar plate 56 are respectively-150V, -600V and-2000V, and the pulse voltage of the extracting polar plate 54 is as follows: when ion current continuously enters the ion introducing region of the bottom plate 19 and the extraction polar plate 54, the voltage of the extraction polar plate 54 is-150V, and when ions with the largest mass-to-charge ratio (M/Z) after one sample injection (protonated [ RH ] +) enter the ion introducing region, the voltage of the extraction polar plate is raised to-420V, and the ions acquire kinetic energy perpendicular to the incident direction. The optimum voltage of the middle polar plate is 600V, and the middle polar plate voltage plays a critical role in improving resolution. Experiments show that when the voltage of the middle polar plate is reduced, the peak shape of the middle polar plate is widened, and the resolution is reduced; resolution can be improved by increasing the middle plate voltage. For component benzene, peak height 638 and resolution 3060 when the intermediate plate voltage is set to-490V; increasing the voltage to-550V with peak height 952 and resolution 4647; when the voltage is increased to-570V, the peak height is 974, and the resolution is 4669; when the voltage is increased to-600V, the peak height is 1016, and the resolution is 5012; When the voltage of the middle polar plate is increased to 600V below zero, the resolution R is increased slowly, and when the voltage is increased to 650V below zero, the peak shape is poor. After passing through the four plates, the ions then adjust their direction of movement through the XY deflector 27. The light enters the reflecting area 28 after passing through the field-free drift area 26, three groups of reflectors 30 are arranged in the reflecting area, each group of reflectors 30 consists of a reflecting plate 33, a sleeve 31 and an annular polar plate 32, the reflecting plate 33 is round, the sleeve 31 is perpendicular to the reflecting plate 33, two groups of annular polar plates 32 are arranged on the inner wall of the sleeve 31, the two groups of annular polar plates 32 are distributed up and down, the voltage of one group of annular polar plates 32 close to the opening of the sleeve is minus 2000V, the voltage of one group of annular polar plates 32 close to the reflecting plate 33 is minus 1800V, and the voltage of the reflecting plate 33 is 2000V; The three sets of reflectors 30 are respectively a first reflector, a second reflector and a third reflector, the sleeve openings of the first reflector and the third reflector are upward, the sleeve openings of the second reflector are downward, the first reflector and the third reflector are positioned on two sides of the second reflector and have the same height, and the height of the second reflector is higher than that of the first reflector and the third reflector.
The detector 29 is a double-micro-channel plate detector, the double-micro-channel plate detector is connected with the 4GTDC time digital conversion module 34,4GTDC time digital conversion module 34 and the industrial personal computer 36, and the industrial personal computer 36 is connected with the sixteen-way switching valve controller 37; an oil-free pump 40, a pre-cooling dehydrator 39 and a molecular sieve 38 are sequentially arranged on a pipeline between the zero gas inlet pipe 41 and the sixteen-way switching valve 37.
The signal measured by the detector 29 passes through the 4GTDC time-digital conversion module 34, the sampling frequency is 4GHz, the time interval between the start and stop of the pulse signal is accurately measured, and the signal is calculated and stored by the industrial personal computer 36. The method comprises the following steps: the electronic pulse signal of the time-of-flight mass spectrum is converted and output into a fixed amplitude signal by a peak discriminator (CFD), and finally the signal is recorded by a time-to-digital converter (TDC), and the data is processed and stored by a computer.
(1) The device resolution was determined for different plate voltages based on the above instrument conditions, using a benzene standard gas of 1ppm as an example, and the results are shown in FIG. 2.
TABLE 1
| Intermediate plate voltage | M (benzene characteristic peak) | Half-peak width Δm | Resolution R |
| -490V | 79.05478 | 0.02583 | 3060 |
| -550V | 79.05478 | 0.01701 | 4647 |
| -570V | 79.05478 | 0.01693 | 4669 |
| -600V | 79.05478 | 0.01577 | 5012 |
Δm in table 1: half-width, the resolution values at different intermediate plate voltages are calculated using the calculation formula r=m/Δm for mass resolution R, and as can be seen from fig. 2 and table 1, as the intermediate plate voltage increases, the peak gradually becomes higher and narrower, and the resolution gradually increases. When the intermediate plate voltage is further increased to-650V after the intermediate plate voltage is increased to-600V, the resolution R increases slowly. The resolution can be improved by increasing the voltage of the middle polar plate, the optimal voltage of the middle polar plate is 600V, the resolution is up to 5012 at the moment, and the voltage of the middle polar plate plays an important role in improving the resolution.
(2) The influence of the first differential vacuum chamber on the detection limit was examined. The calculation results of the detection limits of the substances corresponding to the first differential vacuum chamber were not set, as shown in table 2 below:
TABLE 2
The detection limit comparison experiment of the VOCs component in the presence or absence of the differential vacuum cavity can show that the detection limit of the VOCs substances such as propylene, benzene, toluene, styrene, acetone and the like is reduced by 4.30-6.34 times by the existence of the first differential vacuum cavity, which indicates that the first differential vacuum cavity plays a vital role in improving the sensitivity of the instrument.
(3) Quantitative analysis of samples
And sequentially sampling from low concentration to high concentration to obtain mass spectrum response values of propylene, benzene, toluene, styrene and acetone with different concentrations, taking the concentration of each component as an abscissa and the response value of each substance as an ordinate, and establishing an external standard curve.
The correction equation between the response value Ai of each component and its concentration Ci is: y=kx+b.
Ci＝(Ai-b)/k
Where Ci is the concentration of each volatile organic component in the sample, ai is the response value of each component, k is the slope of the standard curve, and b is the intercept of the standard curve.
Linear fitting regression equations for propylene, benzene, toluene, styrene, acetone in standard gas and relevant fitting parameters are shown in table 3:
TABLE 3 Table 3
| Component name | Slope k | Intercept b | Linear equation | Linear correlation coefficient |
| Propylene | 969540 | 39885.1 | y＝969540x+39885.1 | 0.9989 |
| Benzene | 1000359 | 12300.2 | y＝1000359x+12300.2 | 0.9998 |
| Toluene (toluene) | 777481 | 6764.8 | y＝777481x-6764.8 | 0.9978 |
| Styrene | 908100 | 8915.5 | y＝908100x-8915.5 | 0.9967 |
| Acetone (acetone) | 894361 | 6893.6 | y＝894361x+6893.6 | 0.9979 |
The gas samples containing 0.4ppm of propylene, benzene, toluene, styrene and acetone were measured according to the same procedure, the mass spectrum response values of the components were obtained, the concentrations of the components were obtained by calculation through a linear regression equation, each substance was measured in parallel 7 times, the precision and accuracy were calculated respectively, and the results are summarized in table 4:
TABLE 4 Table 4
Through the experiment, the curve linear correlation of each component in the object to be detected is good, the recovery rate can reach more than 87.5% (compared with the traditional VOCs measuring method, the recovery rate is obviously improved by 60%), the precision is high, the analysis time is quick, and the concentration information of the VOCs in the gas sample can be obtained within 50 seconds.
Example 2
The measurement was carried out on a sample containing propylene, benzene, toluene, styrene, acetone and methyl tert-butyl ether using oxygen as a reaction gas.
The adopted high-resolution high-sensitivity reaction flight time mass spectrometry equipment for measuring the content of VOCs comprises a reaction chamber, as shown in fig. 1, wherein the reaction chamber comprises a reaction chamber, a reaction gas inlet 58 is arranged on the reaction chamber, the reaction gas inlet 58 is connected with an outlet of a first tee joint 8, two inlets of the first tee joint 8 are respectively connected with a water vapor generating device and other reaction gas inlets 6, the reaction chamber comprises an ion source glow discharge region 11, an ion source drift region 12, a first differential vacuum chamber 13 and a reaction tube 14 which are sequentially adjacent, the rear part of the reaction chamber is sequentially connected with a second differential vacuum chamber 15, an ion transmission region 16 and a TOF detection region 18, the first differential vacuum chamber 13 is communicated with the reaction tube 14 through a first small hole 21, the reaction tube 14 is communicated with the second differential vacuum chamber 15 through a second small hole 24, the second differential vacuum chamber 15 is communicated with the ion transmission region 16 through a third small hole 52, the ion transmission region 16 is communicated with the TOF detection region 18 through a differential guide hole 53, the ion source drift region 12, the first vacuum chamber 13 and the outside the reaction region of the reaction tube 14 are respectively provided with an equivalent resistor 23, the first differential vacuum chamber 13 is connected with the second differential vacuum chamber 13, the second differential vacuum chamber 13 is connected with the second differential vacuum pump 13, a single-electrode group is arranged in the right side of the vacuum chamber is formed by a single-channel electrode group 20, the second differential vacuum pump is arranged in the vacuum chamber is arranged at the position of the right side, the vacuum chamber is adjacent to the second differential vacuum chamber is arranged in the vacuum chamber 15, and the single-vacuum chamber is arranged in the vacuum chamber is opposite to the vacuum chamber 20, and the vacuum chamber is arranged in the vacuum chamber is opposite to the vacuum chamber; the upper part of the TOF detection region 18 communicates with a reflection region 28 via a field-free drift region 26, an XY deflection plate 27 being provided in the field-free drift region 26, and three sets of reflectors 30 and detectors 29 being provided in the reflection region 28.
The other reaction gas inlets 6 are connected with one inlet of the first tee joint 8 through a pipeline, and other reaction gas regulating switches 7 are arranged on the pipeline between the other reaction gas inlets 6 and the first tee joint 8; a flowmeter 9 and a flow regulating switch 10 are arranged on the pipeline between the outlet of the first tee joint 8 and the reaction chamber.
The VOCs gas inlet 20 is connected with a standard gas outlet of the dynamic gas diluter 42 and the zero gas inlet pipe 41 through pipelines, the sixteen switching valves 37 are connected with the gas inlet pipe 57 to be tested, the gas inlet pipe 57 to be tested can be an ambient air inlet pipe or a Suma tank of each path, the standard gas steel bottle 45 and the nitrogen steel bottle 48 are respectively connected with the dynamic gas diluter 42 through pipelines, a first mass flowmeter 43 and a first pressure reducing valve 44 are arranged on the pipelines between the standard gas steel bottle 45 and the dynamic gas diluter 42, a second mass flowmeter 46 and a second pressure reducing valve 47 are arranged on the pipelines between the nitrogen steel bottle 45 and the dynamic gas diluter 42, the zero gas inlet pipe 41 is sequentially connected with the oil-free pump 40, the pre-cooling dehydrator 39 and the molecular sieve 38 which are arranged inside the zero gas generator, the VOCs gas outlet 59 is arranged on the second differential vacuum chamber 15, the VOCs gas outlet 59 is sequentially connected with a first interface of the air extracting pump 51 and a second three-way 50 through pipelines, a second three-way second interface is connected with a waste gas outlet of the dynamic gas diluter 42, a third interface of the second three-way 50 is connected with the exhaust pipe 49, and two waste gas is exhausted through the exhaust pipe 49.
The detector 29 is a double-micro-channel plate detector, the double-micro-channel plate detector is connected with the 4GTDC time digital conversion module 34,4GTDC time digital conversion module 34 and the industrial personal computer 36, the industrial personal computer 36 is simultaneously connected with the sixteen-way switching valve 37 and the electronic weather station, and is remotely connected with a mobile phone or a computer terminal, and the industrial personal computer 36 is connected with a controller of the sixteen-way switching valve 37.
The ion transmission area is provided with a first turbomolecular pump, and the TOF detection area is provided with a second turbomolecular pump.
The mixed standard gas of propylene, benzene, toluene, styrene, acetone and methyl tertiary butyl ether with the concentration of 1ppm is diluted with high-purity nitrogen according to the volume ratio to prepare standard gas with the concentration of 10ppb, 20ppb, 100ppb, 200ppb, 400ppb and 1000ppb respectively. The standard gas is prepared from the standard gas in standard gas cylinder 45 and the nitrogen in nitrogen cylinder 48 in dynamic gas diluter 42. And sequentially sampling from low concentration to high concentration, detecting, establishing a calibration curve according to the relation between the concentration and the response value, and calculating the concentration of each component in the sample from the calibration curve by measuring the response value of each component in the sample.
In the measurement process, the first tee joint 8 is communicated with the other reaction gas inlets 6, the port of the first tee joint 8 connected with the water storage bottle 2 is closed, water vapor does not enter the instrument any more, oxygen enters the ion source glow discharge area 11 in the reaction chamber through the reaction gas inlets after passing through the first tee joint 8, the flowmeter 9 and the flow regulating switch 10 after passing through the other reaction gas regulating switch 7 to be ionized into [ O 2]+ ], and the flow of oxygen is regulated by regulating the flowmeter and the flow regulating switch, so that the ion abundance of [ O 2]+ is ensured to reach 2.0x10 5.
For better ionization efficiency, the anode voltage of the ion source glow discharge region 11 is 400V, and the glow discharge supply causes the cathode voltage to drop to about-350V before the glow discharge is turned on but the plasma starts. After the plasma starts, the cathode voltage starts to change. At this time, the voltage difference of the sustain discharge between the anode and the cathode was maintained at 400V. The pressure of the ion source glow discharge area 11 is 80Pa, and the discharge current is 3.2mA; a large amount of [ O 2]+ and the like are generated by utilizing cathode glow discharge, and further react in an ion source drift region to generate the [ O 2]+ ] with higher purity. The reaction chamber is heated by the heating wire arranged in the reaction chamber before the measurement of the sample, the internal temperature balance is promoted by the fan, the temperature of the heat preservation layer coated outside is kept constant, and the temperature is strictly controlled by a temperature controller to be 100 ℃.
The [ O 2]+ ] generated by the ion source enters the reaction tube 14 with the length of 96cm through the first differential vacuum cavity 13 to collide with VOCs molecules R for reaction, and at the moment, the first differential vacuum cavity 13 effectively reduces oxygen molecules entering the reaction tube 14, improves the purity of [ O 2]+, and plays an important role in improving the sensitivity of the instrument; the inner diameter of the first small hole 21 is 0.12cm, so that the reaction gas ions can effectively enter the reaction tube 14, and the ion source is isolated from the reaction tube 14 to a large extent. The reaction tube 14 is modified with polydimethylsiloxane to reduce adsorption.
When a sample containing propylene, benzene, toluene, styrene, acetone, methyl tertiary butyl ether and the like is measured, the ion source and the reaction tube 14 can be isolated to a greater extent through the first differential vacuum cavity 13, the influence of the pressure increase in the reaction tube 14 on the ion source is reduced, the pressure increase in the reaction tube 14 is controlled to 8mbar through a computer, the voltage at two ends is set to 2.4KV, E/n is maintained at 170, the sample injection amount of the sample gas is increased to 250ml/min, the ion number density in the reaction tube 14 is increased, the response value of each substance to be measured is obviously enhanced, meanwhile, high-frequency alternating voltage is applied to a focusing area, the alternating voltage frequency is 0.45MHz, the voltage amplitude is 52V, the voltage provides radial constraint force for ion movement, and the ions are focused towards the center of the reaction tube, so that the ion number passing through a second differential vacuum pore is increased, and the sensitivity of the instrument is improved.
Charged positive ion products [ R ] + of propylene, benzene, toluene, styrene, acetone and methyl tertiary butyl ether generated by the reaction enter the ion transmission area 16 through a second small hole 24 and a third small hole 52 with the diameters of 0.42cm under the action of a uniform electric field in the reaction tube 14, wherein the second small hole 24 and the third small hole 52 isolate the ion transmission area 16 from the reaction tube 14 and are vacuumized through a first turbomolecular pump, so that the pressure of the ion transmission area 16 is ensured to be 10 -4 mbar. The single lens is composed of three groups of electrodes 25 which are arranged left and right, each group of electrodes 25 is composed of two electrodes which are opposite up and down, wherein the electrodes 25 at two ends are extraction lens electrodes, the electrode 25 at the middle is a transmission lens electrode, and voltages applied by the three groups of electrodes 25 are respectively as follows: the extraction electrode voltage is-5V, the transmission electrode voltage is-110V, and the curve-shaped electric field formed by the extraction electrode voltage focuses [ RH ] + entering the electric field to form concentrated ion flow, so that the problem of ion divergence is prevented, and the resolution of the instrument is improved. The focused ion flow horizontally enters the TOF detection area 18 after passing through the differential deflector hole 53, the inner diameter of the differential deflector hole 53 is 1.2mm, the ion passing rate is ensured, meanwhile, the ion transmission area and the TOF detection area are effectively isolated, and the high vacuum degree of the TOF area is ensured to be 10 -7 mbar by the pumping of a second turbomolecular pump.
A faraday cup 17 is disposed inside the TOF detection region 18 near the differential deflector aperture 53, and ion flow is monitored by the faraday cup 17 as ions focused by the lens enter the TOF detection region 18 horizontally, and then ions of different mass to charge ratios (M/Z) are distinguished during continuous movement into the TOF detection region 18. Four polar plates are arranged in the ion acceleration area at the upper part of the TOF detection area 18, the four polar plates are a bottom plate 19, an extraction polar plate 54, a middle polar plate 55 and an acceleration polar plate 56 from top to bottom, the optimal voltage values of the bottom plate 19, the middle polar plate 55 and the acceleration polar plate 56 are respectively-120V, -570V and-1900V, and the pulse voltage of the extraction polar plate 54 is as follows: when ion flow continuously enters an ion introduction region of the bottom plate 19 and the extraction polar plate 54, the voltage of the extraction polar plate 54 is 120V, and when ions (R +) with the largest mass-to-charge ratio (M/Z) enter the ion introduction region after one sample injection, the voltage of the extraction polar plate is raised to 400V, the ions obtain kinetic energy perpendicular to the incident direction, and the ions with different mass-to-charge ratios (M/Z) are distinguished in the continuous movement process. The ion movement direction is then adjusted via the XY deflector 27. Ions enter the reflecting area 28 after passing through the field-free drift area 26, three groups of reflectors 30 are arranged in the reflecting area, each group of reflectors 30 consists of a reflecting plate 33, a sleeve 31 and an annular polar plate 32, the reflecting plate 33 is round, the sleeve 31 is perpendicular to the reflecting plate 33, two groups of annular polar plates 32 are arranged on the inner wall of the sleeve 31, the two groups of annular polar plates 32 are distributed up and down, the voltage of one group of annular polar plates 32 close to the opening of the sleeve is-1900V, the voltage of one group of annular polar plates 32 close to the reflecting plate 33 is-1800V, and the voltage of the reflecting plate 33 is 2000V; the three sets of reflectors 30 are respectively a first reflector, a second reflector and a third reflector, the sleeve openings of the first reflector and the third reflector are upward, the sleeve openings of the second reflector are downward, the first reflector and the third reflector are positioned on two sides of the second reflector and have the same height, and the height of the second reflector is higher than that of the first reflector and the third reflector.
The detector 29 is a double-micro-channel plate detector, the double-micro-channel plate detector is connected with the 4GTDC time digital conversion module 34,4GTDC time digital conversion module 34 and the industrial personal computer 36, and the industrial personal computer 36 is simultaneously connected with the controller of the sixteen-way switching valve 37 and the electronic weather station and is remotely connected with a mobile phone or a computer terminal; an oil-free pump 40, a pre-cooling dehydrator 39 and a molecular sieve 38 are sequentially arranged on a pipeline between the zero gas inlet pipe 41 and the sixteen-way switching valve 37.
The signal measured by the detector 29 passes through the 4GTDC time-digital conversion module 34, the sampling frequency is 4GHz, the time interval between the start and stop of the pulse signal is accurately measured, and the signal is calculated and stored by the industrial personal computer 36. The method comprises the following steps: the electronic pulse signal of the time-of-flight mass spectrum is converted and output into a fixed amplitude signal by a peak discriminator (CFD), and finally the signal is recorded by a time-to-digital converter (TDC), and the data is processed and stored by a computer.
The gas samples containing 0.2ppm of propylene, benzene, toluene, styrene, acetone and methyl tert-butyl ether were measured according to the same procedure, the concentrations of the components were calculated by a linear regression equation, each substance was measured in parallel 7 times, the precision and accuracy were calculated, and the results are summarized in Table 5:
TABLE 5
It was found through the above experiments that the reaction gas can be replaced by simply switching the reaction gas inlet without replacing the ion source. The curve linearity correlation of each component in the object to be detected is good, the recovery rate can reach 100% -109% (compared with the 60% recovery rate of the traditional VOCs measuring method, the precision is high, the analysis time is short, and the concentration information of the VOCs in the gas sample can be obtained within 50 seconds.
Example 3
The measurement was performed on a propylene, benzene, toluene, and styrene sample using nitric oxide as a reaction gas.
The adopted high-resolution high-sensitivity time-of-flight mass spectrometry equipment for measuring the content of VOCs comprises a reaction chamber, as shown in figure 1, wherein the reaction chamber is provided with a reaction gas inlet 58, the reaction gas inlet 58 is connected with an outlet of a first tee joint 8, two inlets of the first tee joint 8 are respectively connected with a water vapor generating device and other reaction gas inlets 6, the reaction chamber comprises an ion source glow discharge region 11, an ion source drift region 12, a first differential vacuum chamber 13 and a reaction tube 14 which are adjacent in sequence, the rear part of the reaction chamber is sequentially connected with a second differential vacuum chamber 15, an ion transmission region 16 and a TOF detection region 18, the first differential vacuum chamber 13 is communicated with the reaction tube 14 through a first small hole 21, the reaction tube 14 is communicated with the second differential vacuum chamber 15 through a second small hole 24, the second differential vacuum chamber 15 is communicated with the ion transmission area 16 through a third small hole 52, the ion transmission area 16 is communicated with the TOF detection area 18 through a differential deflector hole 53, equivalent divider resistors 23 are arranged outside the ion source drift area 12, the first differential vacuum chamber 13 and the reaction area of the reaction tube 14, the tail end of the first differential vacuum chamber 13 is connected with a first mechanical pump 22, the tail end of the second differential vacuum chamber 15 is connected with a second mechanical pump 35, a VOCs gas inlet 20 is arranged on the wall of the reaction tube 14 at a position close to the first differential vacuum chamber 13, a VOCs gas outlet 59 is arranged on the second differential vacuum chamber, a single lens is arranged in the ion transmission area 16, the single lens consists of three groups of left and right electrodes 25, and each group of electrodes 25 consists of two electrodes which are opposite up and down; the upper part of the TOF detection region 18 communicates with a reflection region 28 via a field-free drift region 26, an XY deflection plate 27 being provided in the field-free drift region 26, and three sets of reflectors 30 and detectors 29 being provided in the reflection region 28.
The water vapor generating device comprises a water storage bottle 2, wherein a thermocouple 4 and a heating wire 3 are arranged in the water storage bottle 2, a heat preservation layer 1 is arranged outside the water storage bottle 2, the top of the water storage bottle 2 is connected with one of inlets of a first tee joint 8 through a pipeline, and a water switch 5 is arranged on the pipeline between the water storage bottle 2 and the first tee joint 8; the other reaction gas inlets 6 are connected with one inlet of the first tee joint 8 through a pipeline, and other reaction gas regulating switches 7 are arranged on the pipeline between the other reaction gas inlets 6 and the first tee joint 8; a flowmeter 9 and a flow regulating switch 10 are arranged on the pipeline between the outlet of the first tee joint 8 and the reaction chamber.
The VOCs gas inlet 20 is connected with a standard gas outlet of the dynamic gas diluter 42 and the zero gas inlet pipe 41 through pipelines, the sixteen switching valves 37 are connected with the gas inlet pipe 57 to be tested, the gas inlet pipe 57 to be tested can be an ambient air inlet pipe or a Suma tank of each path, the standard gas steel bottle 45 and the nitrogen steel bottle 48 are respectively connected with the dynamic gas diluter 42 through pipelines, a first mass flowmeter 43 and a first pressure reducing valve 44 are arranged on the pipelines between the standard gas steel bottle 45 and the dynamic gas diluter 42, a second mass flowmeter 46 and a second pressure reducing valve 47 are arranged on the pipelines between the nitrogen steel bottle 45 and the dynamic gas diluter 42, the zero gas inlet pipe 41 is sequentially connected with the oil-free pump 40, the pre-cooling dehydrator 39 and the molecular sieve 38 which are arranged inside the zero gas generator, the VOCs gas outlet 59 is arranged on the second differential vacuum chamber 15, the VOCs gas outlet 59 is sequentially connected with a first interface of the air extracting pump 51 and a second three-way 50 through pipelines, a second three-way second interface is connected with a waste gas outlet of the dynamic gas diluter 42, a third interface of the second three-way 50 is connected with the exhaust pipe 49, and two waste gas is exhausted through the exhaust pipe 49.
The detector 29 is a double-micro-channel plate detector, the double-micro-channel plate detector is connected with the 4GTDC time digital conversion module 34,4GTDC time digital conversion module 34 and the industrial personal computer 36, the industrial personal computer 36 is simultaneously connected with the sixteen-way switching valve 37 and the electronic weather station, and is remotely connected with a mobile phone or a computer terminal, and the industrial personal computer 36 is connected with a controller of the sixteen-way switching valve 37.
The ion transmission area is provided with a first turbomolecular pump, and the TOF detection area is provided with a second turbomolecular pump.
The mixed standard gas (concentration of 1 ppm) of propylene, benzene, toluene and styrene was diluted with high-purity nitrogen gas in a volume ratio to prepare standard gases having concentrations of 10ppb, 20ppb, 100ppb, 200ppb, 400ppb and 1000ppb, respectively. The standard gas is prepared from the standard gas in standard gas cylinder 45 and the nitrogen in nitrogen cylinder 48 in dynamic gas diluter 42. And sequentially sampling from low concentration to high concentration, detecting, establishing a calibration curve according to the relation between the concentration and the response value, and calculating the concentration of each component in the sample from the calibration curve by measuring the response value of each component in the sample.
In the measurement process, the first tee joint 8 is communicated with the other reaction gas inlets 6, the port of the first tee joint 8 connected with the water storage bottle 2 is closed, water vapor does not enter the instrument any more, nitric oxide enters the ion source glow discharge zone 11 in the reaction chamber through the reaction gas inlets to generate [ NO ] + after passing through the first tee joint 8, the flowmeter 9 and the flow regulating switch 10 after passing through the other reaction gas regulating switch 7, and the ion abundance of the reaction gas ions is ensured to reach 2.0x10 5.
For better ionization efficiency, the anode voltage of the ion source glow discharge region 11 is 450V, and the glow discharge supply causes the cathode voltage to drop to about-300V before the glow discharge is turned on but the plasma starts. After the plasma starts, the cathode voltage starts to change. At this time, the voltage difference of the sustain discharge between the anode and the cathode was maintained at 400V. The pressure of the ion source glow discharge area 11 is 100Pa, and the discharge current is 4.2mA; a large amount of NO + is generated by utilizing cathode glow discharge, and further reacts in an ion source drift region to generate NO + with higher purity. The reaction chamber is heated by the heating wire arranged in the reaction chamber before the measurement of the sample, the internal temperature balance is promoted by the fan, the temperature of the heat preservation layer coated outside is kept constant, and the temperature is strictly controlled by a temperature controller to be 100 ℃.
The NO + generated by the ion source enters the reaction tube 14 with the length of 96cm through the first differential vacuum cavity 13 to collide with VOCs molecules R to generate the R +, at this time, the first differential vacuum cavity 13 effectively reduces nitric oxide molecules entering the reaction tube 14, improves the purity of NO +, and plays an important role in improving the sensitivity of the instrument; the inner diameter of the first small hole 21 is 0.10cm, so that the reaction gas ions can effectively enter the reaction tube 14, and the ion source is isolated from the reaction tube 14 to a large extent. The reaction tube 14 is modified with polydimethylsiloxane to reduce adsorption.
When the propylene, benzene, toluene and styrene samples are measured, the ion source and the reaction tube 14 can be isolated to a greater extent through the first differential vacuum cavity 13, the influence of the pressure increase in the reaction tube 14 on the ion source is reduced, the pressure increase in the reaction tube 14 is controlled to 10mbar through a computer, the voltage at two ends is set to 2.8KV, E/n is maintained at 175, the sample injection amount of the increased sample gas is increased to 260ml/min, the ion number density in the reaction tube 14 is increased, the response value of each substance to be measured is remarkably enhanced, meanwhile, high-frequency alternating voltage is applied to a focusing area, the alternating voltage frequency is 0.50MHz, the voltage amplitude is 55V, the voltage provides radial constraint force for ion movement, ions are focused towards the center of the reaction tube, the ion number passing through the second differential vacuum pore is increased, and the instrument sensitivity is improved.
The ionized product [ R ] + of the gas to be detected generated by the reaction enters the ion transmission area 16 through the second small hole 24 and the third small hole 52 with the diameters of 0.50cm under the action of the uniform electric field in the reaction tube 14, the second small hole 24 and the third small hole 52 isolate the ion transmission area 16 from the reaction tube 14, and the pressure of the ion transmission area 16 is ensured to be 10 -4 mbar by the vacuum pumping of the first turbomolecular pump. The single lens is composed of three groups of electrodes 25 which are arranged left and right, each group of electrodes 25 is composed of two electrodes which are opposite up and down, wherein the electrodes 25 at two ends are extraction lens electrodes, the electrode 25 at the middle is a transmission lens electrode, and voltages applied by the three groups of electrodes 25 are respectively as follows: the extraction electrode voltage is-3V, the transmission electrode voltage is-115V, and the curve-shaped electric field formed by the extraction electrode voltage focuses [ R ] + entering the electric field to form concentrated ion flow, so that the problem of ion divergence is prevented, and the resolution of the instrument is improved. The focused ion flow horizontally enters the TOF detection area 18 after passing through the differential deflector hole 53, the inner diameter of the differential deflector hole 53 is 1.0mm, the ion passing rate is ensured, meanwhile, the ion transmission area and the TOF detection area are effectively isolated, and the high vacuum degree of the TOF area is ensured to be 10 -7 mbar by the pumping of a second turbomolecular pump.
A faraday cup 17 is disposed inside the TOF detection region 18 near the differential deflector aperture 53, and ion flow is monitored by the faraday cup 17 as ions focused by the lens enter the TOF detection region 18 horizontally, and then ions of different mass to charge ratios (M/Z) are distinguished during continuous movement into the TOF detection region 18. Four polar plates are arranged in the ion acceleration area at the upper part of the TOF detection area 18, the four polar plates are a bottom plate 19, an extraction polar plate 54, a middle polar plate 55 and an acceleration polar plate 56 from top to bottom, the optimal voltage values of the bottom plate 19, the middle polar plate 55 and the acceleration polar plate 56 are respectively-145V, -570V and-1900V, and the pulse voltage of the extraction polar plate 54 is as follows: when ion flow continuously enters an ion introduction region of the bottom plate 19 and the extraction polar plate 54, the voltage of the extraction polar plate 54 is 145V, and when ions (R +) with the largest mass-to-charge ratio (M/Z) enter the ion introduction region after one sample injection, the voltage of the extraction polar plate is raised to 410V, the ions obtain kinetic energy perpendicular to the incident direction, and the ions with different mass-to-charge ratios (M/Z) are distinguished in the continuous movement process. The ion movement direction is then adjusted via the XY deflector 27. Ions enter the reflecting area 28 after passing through the field-free drift area 26, three groups of reflectors 30 are arranged in the reflecting area, each group of reflectors 30 consists of a reflecting plate 33, a sleeve 31 and an annular polar plate 32, the reflecting plate 33 is round, the sleeve 31 is perpendicular to the reflecting plate 33, two groups of annular polar plates 32 are arranged on the inner wall of the sleeve 31, the two groups of annular polar plates 32 are distributed up and down, the voltage of one group of annular polar plates 32 close to the opening of the sleeve is-1900V, the voltage of one group of annular polar plates 32 close to the reflecting plate 33 is-1800V, and the voltage of the reflecting plate 33 is 2000V; the three sets of reflectors 30 are respectively a first reflector, a second reflector and a third reflector, the sleeve openings of the first reflector and the third reflector are upward, the sleeve openings of the second reflector are downward, the first reflector and the third reflector are positioned on two sides of the second reflector and have the same height, and the height of the second reflector is higher than that of the first reflector and the third reflector.
The detector 29 is a double-micro-channel plate detector, the double-micro-channel plate detector is connected with the 4GTDC time digital conversion module 34,4GTDC time digital conversion module 34 and the industrial personal computer 36, and the industrial personal computer 36 is simultaneously connected with the controller of the sixteen-way switching valve 37 and the electronic weather station and is remotely connected with a mobile phone or a computer terminal; an oil-free pump 40, a pre-cooling dehydrator 39 and a molecular sieve 38 are sequentially arranged on a pipeline between the zero gas inlet pipe 41 and the sixteen-way switching valve 37.
The signal measured by the detector 29 passes through the 4GTDC time-digital conversion module 34, the sampling frequency is 4GHz, the time interval between the start and stop of the pulse signal is accurately measured, and the signal is calculated and stored by the industrial personal computer 36. The method comprises the following steps: the electronic pulse signal of the time-of-flight mass spectrum is converted and output into a fixed amplitude signal by a peak discriminator (CFD), and finally the signal is recorded by a time-to-digital converter (TDC), and the data is processed and stored by a computer.
The concentrations of the components were calculated by linear regression equation for the samples containing 0.5ppm of propylene, benzene, toluene and styrene, each of which was measured in parallel 7 times, and the precision and accuracy were calculated, and the results are summarized in Table 6:
TABLE 6
It was found through the above experiments that the reaction gas can be replaced by simply switching the reaction gas inlet without replacing the ion source. The curve linear correlation of each component in the object to be detected is good, the recovery rate can reach 91.8% -108%, compared with the traditional method, 60%, the precision is high, the analysis time is short, and the concentration information of the VOCs in the gas sample can be obtained within 50 seconds.
Claims (6)
1. A high-resolution high-sensitivity time-of-flight mass spectrometry device for measuring the content of VOCs is characterized in that: comprises a reaction chamber, wherein a reaction gas inlet (58) is arranged on the reaction chamber, the reaction gas inlet (58) is connected with an outlet of a first tee joint (8), two inlets of the first tee joint (8) are respectively connected with water vapor generating equipment and other reaction gas inlets (6), the reaction chamber comprises an ion source glow discharge zone (11), an ion source drift zone (12), a first differential vacuum cavity (13) and a reaction tube (14) which are adjacent in sequence, the rear part of the reaction chamber is sequentially connected with a second differential vacuum cavity (15), an ion transmission zone (16) and a TOF detection zone (18), the first differential vacuum cavity (13) is communicated with the reaction tube (14) through a first small hole (21), the reaction tube (14) is communicated with the second differential vacuum cavity (15) through a second small hole (24), the second differential vacuum cavity (15) is communicated with the ion transmission zone (16) through a third small hole (52), the ion source drift zone (12), the first differential vacuum cavity (13) is communicated with the TOF detection zone (18) through a differential guide hole (53), a second differential vacuum pump (23) is arranged at the end of the second differential vacuum cavity (13) which is connected with the reaction tube (14), a VOCs gas inlet (20) is arranged on the wall of the reaction tube (14) near the first differential vacuum cavity (13), a VOCs gas outlet (59) is arranged on the second differential vacuum cavity (15), a single lens is arranged in the ion transmission area (16), the single lens is composed of three groups of electrodes (25) which are arranged left and right, and each group of electrodes (25) is composed of two electrodes which are opposite up and down; an accelerating electric field region at the upper part of the TOF detection region (18) is communicated with a reflecting region (28) through a field-free drift region (26), an XY deflection plate (27) is arranged in the field-free drift region (26), and three groups of reflectors (30) and detectors (29) are arranged in the reflecting region (28);
the single lens is composed of three groups of electrodes (25) which are arranged left and right, each group of electrodes (25) is composed of two electrodes which are opposite up and down, wherein the two electrodes (25) are extraction lens electrodes, the middle electrode (25) is a transmission lens electrode, and the applied voltages are respectively as follows: the voltage range of the extraction lens electrode is-6V-0V, and the voltage range of the transmission lens electrode is-80V-110V;
The position, close to the differential diversion hole (53), in the TOF detection area (18) is provided with a Faraday cup (17), four polar plates are arranged in the TOF detection area (18), and the four polar plates sequentially comprise a bottom plate (19), an extraction polar plate (54), a middle polar plate (55) and an acceleration polar plate (56) from top to bottom, wherein the voltage range of each polar plate is as follows: the voltage of the bottom plate (19) is-100V to-200V, pulse voltage is applied to the extraction polar plate (54), ions to be detected enter the polar plate ion introduction area, the voltage value of the extraction polar plate (54) is the same as the voltage value of the bottom plate (19), when the ions with the largest mass-to-charge ratio enter the ion introduction area after one sample injection, the voltage value of the extraction polar plate (54) is increased to-400V to-450V, the voltage of the middle polar plate (55) is-500V to-650V, and the voltage of the acceleration polar plate (56) is-1800V to-2200V;
The pressure in the reaction tube (14) is 5-10mbar, the voltage applied to the two ends of the reaction tube (14) is 1.5-3KV, and the sample gas injection amount is 200-300ml/min; the length of the reaction tube (14) is 90-120cm;
Three groups of reflectors (30) are arranged in the reflecting area, each group of reflectors (30) consists of a reflecting plate (33), a sleeve (31) and annular polar plates (32), the reflecting plate (33) is round, the sleeve (31) is perpendicular to the reflecting plate (33), two groups of annular polar plates (32) are arranged on the inner wall of the sleeve (31), the two groups of annular polar plates (32) are distributed up and down, the voltage of one group of annular polar plates (32) close to the opening of the sleeve is-1800V to-2200V, the voltage of one group of annular polar plates (32) close to the reflecting plate (33) is-1700V to-1900V, and the voltage of the reflecting plate (33) is 1800V to 2000V; the three groups of reflectors (30) are respectively a first reflector, a second reflector and a third reflector, the sleeve openings of the first reflector and the third reflector are upward, the sleeve openings of the second reflector are downward, the first reflector and the third reflector are positioned on two sides of the second reflector and have the same height, and the height of the second reflector is higher than that of the first reflector and the third reflector;
The VOCs gas inlet (20) is connected with a sixteen-way switching valve (37) through a pipeline, the sixteen-way switching valve (37) is connected with a dynamic gas diluter (42) standard gas outlet and a zero gas inlet pipe (41), the standard gas steel bottle (45) and the nitrogen steel bottle (48) are connected with the dynamic gas diluter (42) through pipelines respectively, a first mass flowmeter (43) and a first pressure reducing valve (44) are arranged on the pipeline between the standard gas steel bottle (45) and the dynamic gas diluter (42), a second mass flowmeter (46) and a second pressure reducing valve (47) are arranged on the pipeline between the nitrogen steel bottle (48) and the dynamic gas diluter (42), the zero gas inlet pipe (41) is connected with the sixteen-way switching valve after being connected with the zero gas generator, the VOCs gas outlet (59) is arranged on a second differential vacuum cavity (15), the VOCs gas outlet (59) is sequentially connected with a first interface of an extraction pump (51) and a second three-way (50), a second interface is connected with an exhaust gas outlet of the dynamic gas diluter (42) through the pipeline, and a third interface is connected with a third interface (49).
2. The high resolution high sensitivity time-of-flight mass spectrometry apparatus for determining VOCs content according to claim 1, wherein: the water vapor generating device comprises a water storage bottle (2), wherein a thermocouple (4) and a heating wire (3) are arranged inside the water storage bottle (2), a heat preservation layer (1) is arranged outside the water storage bottle (2), the top of the water storage bottle (2) is connected with one inlet of a first tee joint (8) through a pipeline, and a water switch (5) is arranged on the pipeline between the water storage bottle (2) and the first tee joint (8); the other reaction gas inlets (6) are connected with one inlet of the first tee joint (8) through pipelines, and other reaction gas regulating switches (7) are arranged on the pipelines between the other reaction gas inlets (6) and the first tee joint (8); a flowmeter (9) and a flow regulating switch (10) are arranged on the pipeline between the outlet of the first tee joint (8) and the reaction chamber.
3. The high resolution high sensitivity time-of-flight mass spectrometry apparatus for determining VOCs content according to claim 1, wherein: the anode voltage of the ion source glow discharge area (11) is 400-550V, the discharge potential between the anode and the cathode is maintained between 350V and 400V, the pressure of the ion source glow discharge area (11) is 10-100Pa, and the discharge current is 4-6mA; the pressure in the ion transfer zone (16) is of the order of 10 -4 mbar; the pressure in the TOF detection zone (18) is of the order of 10 -5-10-7 mbar.
4. The high resolution high sensitivity time-of-flight mass spectrometry apparatus for determining VOCs content according to claim 1, wherein: the gas flow rate at the inlet of the reaction chamber is 3-14mL/min; the inner diameter of the first small hole (21) is 0.05-0.5cm; the diameters of the second small hole (24) and the third small hole (52) are 0.1-1.0 cm, and the inner diameter of the differential diversion hole (53) is 1-1.5mm.
5. The high resolution high sensitivity time-of-flight mass spectrometry apparatus for determining VOCs content according to claim 1, wherein: the detector (29) is a double-micro-channel plate detector, the double-micro-channel plate detector is connected with the 4GTDC time digital conversion module (34), the 4GTDC time digital conversion module (34) is connected with the industrial personal computer (36), and the industrial personal computer (36) is simultaneously connected with a controller of the sixteen-channel switching valve (37) and an electronic weather station and is remotely connected with a mobile phone or a computer terminal.
6. A method for determining VOCs content using the high resolution high sensitivity time-of-flight mass spectrometry apparatus of any one of claims 1-5, characterized by: the method comprises the following steps:
The method comprises the steps that reactive gas ions generated in an ion source enter a reaction tube after passing through a first differential vacuum cavity to collide with VOCs molecules R in a sample to be detected, positive ions are generated, a first differential vacuum cavity is arranged between an ion source drift region and the reaction tube, an equivalent voltage dividing resistor is arranged outside the first differential vacuum cavity, direct-current voltage is applied and connected with the equivalent voltage dividing resistor, a uniform electric field is formed inside the first differential vacuum cavity, the generated reactive gas ions move towards the reaction tube under the action of the electric field force, and the reactive gas neutral molecules do not have obvious orientation due to the fact that the reactive gas neutral molecules are not acted by the electric field force, the tail end of the first differential vacuum cavity is connected with a first mechanical pump, and the first mechanical pump pumps the reactive gas neutral molecules to prevent the neutral molecules from entering the reaction tube; positive ions to be detected generated by the reaction enter an ion transmission area through a second small hole under the action of a uniform electric field in a reaction tube, a single lens is arranged in the ion transmission area and is formed by arranging three groups of electrodes left and right, each group of electrodes consists of two electrodes which are opposite up and down, wherein the electrodes at the two ends are extraction lens electrodes, the electrode at the middle is a transmission lens electrode, and focusing of ion flow is completed by adjusting the voltage value of the electrodes; the ions flow through the differential deflector hole after focusing and then horizontally enter the TOF detection area, a Faraday cup is arranged in the TOF detection area and close to the differential deflector hole, the ions focused by the lens horizontally enter the TOF detection area, the ion flow is monitored by the Faraday cup, then the ions enter the TOF detection area, and ions with different mass-to-charge ratios are distinguished in the continuous motion process; the accelerating electric field area on the upper part of the TOF detection area is communicated with the reflecting area through a field-free drift area, an XY deflection plate is arranged in the field-free drift area, the ion beam entering the reflector and flying to the detector is adjusted, three groups of reflectors are arranged in the reflecting area, the ion beam is guided to strike the detector to the maximum extent, and the detector realizes the detection of the ions of each VOCs component.
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