JPH0696727A - Improved application method of ion-trap mass spectrometer - Google Patents

Abstract

PURPOSE: To efficiently remove sample ions generated in an ion trap without changing RF trapping field between ionization process and reaction process of reagent gas, by impressing broadband auxiliary AC voltage to the ion trap. CONSTITUTION: An ion trap 10 comprises upper and lower end-cap electrodes 30, 35 and ring electrodes 20 which are arranged coaxially. Sample gas from a gas chromatograph 40 and reagent gas 50 for examining chemical ionization are introduced into the trap 10 and ionized by an electronic beam from a filament 60. The trapping field is created by applying AC voltage which has frequency and width necessary for ions with desired range of mass/charge on trap stably from a RF generator 80 through the electrodes 20. An auxiliary wave-form generator 100 creates auxiliary AC field via a transformer 110 and the electrodes 30, 35, and releases sample ions from the trap 10.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of using an ion trap mass spectrometer (hereinafter referred to as an ion trap device) by applying a supplemental voltage to the ion trap device. The present invention relates to a method of operating and performing a multiple mass spectrometry experiment (referred to as MS).

[0002]

BACKGROUND OF THE INVENTION Quadrupole ion traps, often referred to as ion storage or ion trap detectors, are well known devices for performing mass spectrometry. The ion trap consists of a ring electrode and two coaxial end-cap electrodes that define an inner trapping volume. Each of the electrodes preferably has a hyperboloid,
A quadrupole trapping field is created when the appropriate AC and DC voltages (typically labeled "V" and "U", respectively) are applied to the electrodes. This is accomplished simply by applying a fixed frequency (usually designated "f") between the ring electrode and the end cap electrode. The use of additional DC voltage is optional.

Ion traps are typically operated by introducing sample molecules into an ionizing ion trap. Depending on the operating trapping parameters, ions can be stably contained within the trap for a relatively long period of time. Under certain trap conditions, a wide range of masses are held simultaneously in the trap. Various means for detecting the ions thus trapped are known. One known method scans one or more trap parameters, resulting in a continuous instability of the ions, which can be detected using an electron multiplier or equivalent detector. And exit the trap where it can be detected. Another method is the use of a resonant emission technique in which a series of masses of ions are continuously scanned and detected from a trap.

The mathematical description of trapping fields is complex but well developed. The stability envelope familiar to ion trap users is shown in FIG. Whether or not the mass to charge number (m / e) of an ion is trapped for an ion trap with a certain value of U, V and f at a certain radius r ₀ depends on the solution for the following two equations. .

A _z = (− 8 eU) / (mr ₀ ² ω ² ) q _z = (4 eV) / (mr ₀ ² ω ² ), where ω is equal to 2πf.

By solving these equations, a and q are given for a certain value m / e. For a given ion, when the point (a, q) is within the stability envelope of FIG. 1, the ion is trapped by the quadrupole field. When the point (a, q) is outside the stability envelope of FIG. 1, the ion is not trapped in the ion trap and any ions generated in the ion trap will quickly exit. U, V
Alternatively, by changing the value of f, it is possible to control whether or not an ion having a specific mass can be trapped in the quadrupole field. It is known in the art that the terms mass and mass / charge number can be used interchangeably. But to be exact, the mass /
The use of the term charge number is appropriate.

In the absence of a DC voltage, the above equation is virtually related to the stability in the z-axis, ie in the axial direction of the electrode. Ions become unstable in this direction before they become unstable in the r direction, ie, radial to their axis.
Therefore, stability problems are usually limited to z-direction stability. Differential in stability
Results from the fact that unstable ions exit the ion trap in the z-axis, or axial direction.

In a commercially available ion trap device, D
The C voltage, U, is set to zero. First of the above formulas
As can be seen from the equation (1), when U = 0, a _z = 0 for all mass values. As can be seen from the second of the above equations, the value of q _z is inversely proportional to the mass of the ionic particle. That is, as the mass value increases, q _z
The value of becomes low. Furthermore, the higher the value of V, the higher the value of q _z . As can be seen from the stability envelope of FIG. 1, for U = 0, all masses above a certain cutoff value for a certain value of v are trapped in the quadrupole field. Although all masses above the cutoff value are stable in such trapping fields,
The amount of ions of a particular mass that are trapped is limited by space charge effects. As discussed below, the limitation on such quantity is also a function of the magnitude of V.

Various methods are known for ionizing sample molecules in an ion trap. Perhaps the most common method is to expose the sample to an electron beam.
When an electron collides with a sample molecule, the sample molecule is ionized. This method is commonly referred to as electron impact ionization or "EI".

A commonly used method of ionizing sample molecules in an ion trap is chemical ionization or "CI". Chemical ionization uses a reagent gas that is typically ionized by EI in an ion trap and can react with sample molecules to form sample ions. Commonly used reagent gases include ethane, isobutane and ammonia. Chemical ionization is considered a "more flexible" ionization technique. For many samples, the CI technique produces less ions than the EI technique, which simplifies the mass spectrometer. Chemical ionization is a well-known technique routinely used with most conventional types of mass spectrometers, such as quadrupole mass filters, as well as quadrupole ion traps.

Other, more specialized ionization methods can also be used in the mass spectrometer. For example, photoionization is a well-known technique similar to electron impact ionization that affects all molecules contained within an ion trap.

Many ion trap mass spectrometer systems in use today include a gas chromatograph ("GC") such as a sample separator and introducer. When using the GC for this purpose, the sample extracted from the GC continuously flows into the mass spectrometer, which is set to perform periodic mass analysis. Such analysis can be accomplished with a scan frequency of typically about 1 degree per second. This frequency is acceptable because the peaks are typically extracted from modern high resolution GCs for a few seconds to tens of seconds. When performing CI experiments with this device, a continuous flow of reagent gas is maintained. As a practical matter, blocking the flow of sample gas from the GC to the ion trap is undesirable. Similarly, it is not desirable to prevent the flow of reagent gas into the ion trap when performing both CI and EI experiments on the sample stream.

When performing CI, it is necessary to ionize the reagent gas, which chemically reacts with and ionizes the sample gas. In particular, electron impact ionization in an ion trap is a desirable method of ionizing a reagent gas. However, when the sample is present in the ion trap when the electron beam is injected to ionize the reagent gas, the sample is also subject to EI. As mentioned above, impeding the flow of sample gas is not practical when chromatography is used to separate the sample before it is introduced into the ion trap. Therefore, it is a realistic method to ionize the reagent gas without ionizing the sample. Therefore, unless relaxation treatment is performed, the sample ions are CI and E.
Formed by both I, leading to potentially confusing results.

A conventional solution to this problem is US Pat. No. 4,686,367 (Lauris et al., Entitled "How to Operate a Quadrupole Ion Trap Chemical Ionization Mass Spectrometer").
Issued on August 11, 1987). This method of US Pat. No. 4,686,367 minimizes the effect of EI on the sample by minimizing the number of sample ions trapped by the ion trap while the reagent gas is ionized. It's like a thing. The method taught to do this is to apply a low voltage V to the ion trap during the EI process, resulting in the trapping of low mass reagent gas, but a small number of high mass ions. In the words of this patent, "at a sufficiently low RF value (ie, the value of V), high molecular weight ions are not well trapped, so at low RF voltage only low mass ions are stored." (Column 5,
As described above, lines 33-36), operating using the RF only method (the method preferred in the above patent and used in what is known as the commercial embodiment of the ion trap): The ion trap essentially traps all masses above the cutoff mass set by the value of the RF trapping voltage.
In order to trap low mass ions, whether they are reagent ions or sample ions, it is necessary to set V to a sufficiently low value. Setting V low enough makes the ion trap essentially inefficient when trapping high mass ions due to space charge effects. From a theoretical consideration of this, the internal volume of an ion trap that stores a specific mass of ions is proportional to the value of V and inversely proportional to its mass. Thus, for a given value of V, the smaller volume of the ion trap will be available to store higher mass ions than lower mass ions. When its volume is very small, the number of ions that can be stored is reduced by the space charge effect.

It should be noted that when V is set to a low value, all high mass ions exit the ion trap. Such ions continue to have the values of a and q delineated within the stability envelope. U.S. Pat. No. 4,684
All that can be done according to the technology of 6,367 is EI
The number of high mass ions in the ion trap is reduced during the process. In this respect, the statement in this patent that "only low mass ions are stored at low RF voltage" appears erroneous. In an experiment done using the method of US Pat. No. 4,686,367, as described below,
The results of the experiment show that there is a detectable amount of high mass ions produced by EI. Moreover, the number of high mass ions that are trapped subsequently is mass dependent, resulting in the trapping of a substantial number of sample ions whose mass is close to but higher than the reagent ions.

Reagent molecules form different ions with different masses. Ionization at an RF voltage substantially lower than the voltage required only to trap the lowest reagent ions (which is the voltage required to remove the higher mass ions) causes the number of trapped reagent ions to be higher. Decrease with sample ions. This effect is mass-related, so that higher mass reagent ions are disproportionately lost from the ion trap.

There are problems associated with conducting both EI and CI experiments on the flow of one sample in an ion trap. As mentioned above, it is not desirable to stop the flow of reagent gas to the ion trap for practical reasons. However, when an EI experiment is performed, if a reagent gas is present, the reagent gas is ionized and, unless they are removed from the ion trap before the reaction occurs, reagent gas ions are generated and the CI (chemical Ionization) can be performed. This problem does not occur when EI experiments are performed on the sample stream. The reason is that the reagent gas streams can simply be separated during this experiment.

However, applying the method of lowering the trapping voltage does not solve this problem because reagent ions of low mass have not been removed from the ion trap. As taught by US Pat. No. 4,686,367, one solution used to solve this problem is to increase the RF trapping voltage so as not to store low mass reagent ions. However, this has the undesired effect of changing the trapping conditions from those normally used. For example, if the trapping voltage is set to store ions of mass 20 and above, the average ionization energy of the electrons entering the ion trap is 70 eV. The increase in trapping voltage to store only ions of mass 45 and above, such as removing methane mass ions of mass 43, is twice the average electron energy. Such an increase changes the mass spectrum of many compositions and reduces trapping efficiency for sample ions.

In the CI process, it is desirable to optimize the number of product ions undergoing mass spectrometry. Very few product ions make the mass analysis noisy, and too many product ions result in loss of resolution and linearity. The formation of product ions depends on the number of reagent ions in the ion trap, the number of sample molecules in the ion trap, the reaction rate between the reagent and sample ions,
And is a function of the reaction time that the reagent ions can react with the sample molecule. Increasing the EI ionization time, ie, maintaining the electron beam for a longer time, can increase the number of reagent ions present in the ion trap. Similarly, increasing the reaction time can increase the number of sample ions formed in the ion trap.

A conventional method directed to this problem is US Pat. No. 4,771,172, entitled "Method for increasing dynamic range and sensitivity of operation of a quadrupole ion trap mass spectrometer in chemical ionization mode". No. (Weber-Grabow et al., Issued September 13, 1988). This patent uses CI to optimize results.
Mode adjust method for adjusting parameters used in an ion trap. In order to optimize the parameters, this patent describes US Pat. No. 4,686,86 prior to each mass spec.
CI "prescan made according to the method of 367 (presca
n) ”is accomplished. This prescan is a complete CI scan cycle, in which the ionization and reaction times are fixed to values shorter than those used in conventional analytical scans and the product ions Is scanned from the ion trap faster than in a normal analytical scan. During the pre-scan, the product ions that emerge from the ion trap are not mass analyzed and the ion signal is given to give the total product ion signal. During the prescan, the total number of product ions in the ion trap is measured and the parameter, i.e. the ionization time and / or the reaction time, is adjusted for the subsequent mass spectrometry scan.

This patent therefore consists of first making a "pre-scan" of the content in the ion trap in order to obtain a determined total number of product ions in the ion trap.
Stage process followed by US Pat. No. 4,686,3
A mass spectrometric scan of the type taught in No. 67 is made and the mass spectrometric parameters are adjusted based on the data collected during the prescan. The disadvantage of the conventional method of using prescan to extend the dynamic range to estimate the amount of sample in the ion trap is that it requires additional time to perform the prescan, and therefore an analytical scan that can be done at the same time. Is that there will be less. Not only is each prescan time consuming, but each one produces non-valued data independent of its use when the parameters are adjusted for the mass spectrometry scan. However, adjustments to the mass spectrometry scan parameters are only necessary when changing conditions. No adjustments need to be made for each scan, so in addition to being time consuming, prescans often serve no useful purpose.
Therefore, to scan within the dynamic range,
There is a need for improved methods of conditioning ion traps during chemical ionization experiments.

There is a desire to use an ion trap mass spectrometer when performing so-called MN ⁿ experiments. MS ⁿ
In the experiment, a single mass of ions is separated by an ion trap and dissociated into small pieces. The debris produced directly from the sample species is conventionally known as the daughter ion and the sample is referred to as the parent ion. The daughter ions are also separated to produce granddaughter ions. The value of n refers to the number of generated ions. Therefore, in MS ² or MS / MS experiments, only daughter ions are formed and analyzed.

A conventional method for conducting an MS ⁿ experiment is "MS /
U.S. Pat. No. 4,736,101 entitled "Method of Scanning an Ion Trap in MS Mode" (Saika et al., Issued Apr. 5, 1988). Resonantly excited by one auxiliary AC frequency generating means tuned to the resonant frequency of the ions, the amplitude of the auxiliary frequencies allowing the ions to gain energy such that the emission of the ions is greater in the ion trap, However, the ions are set to a level that is not large enough to jump out of the ion trap, as they oscillate in the ion trap, they collide with molecules of the dampening gas in the ion trap, causing collision-induced dissociation. Upon receipt, daughter ions are formed.
By applying the resonance frequency related to the mass / charge number of the daughter's ions, they can be similarly broken into pieces.

The difficulty with the method of US Pat. No. 4,736,101 is that the exact resonant frequency of the ion cannot be determined a priori and must be determined a posteriori. The resonant frequency of an ion (also called the secular frequency) varies with the mass / charge number, the number of ions in the ion trap, and other parameters that cannot simply be determined accurately. Therefore, the exact resonant frequency of the ionic species must be empirically determined. Empirical determinations can be accomplished without extra difficulty when static samples are introduced into the ion trap, but when dynamic samples such as the output of the GC are used,
It will be very difficult.

The conventional approach to addressing the aforementioned problems of the sample ion of interest is to use broadband excitation centered around the calculated frequency. For example,
Such a broadband excitation may have a band around 10 kHz. Another method is to perform a frequency pre-scan, ie sweep the auxiliary field over the frequency range of the region and empirically observe the resonant frequency. However, none of these solutions are satisfactory.

The object of the present invention is therefore more efficient than previously known methods, in the ion trap during the ionization of the reagent gas without the need to change the RF trapping field during the ionization and reaction process. It is an object of the present invention to provide a novel method for removing the generated sample ions (both samples).

Another object of the present invention is to carry out electron impact ionization experiments in an ion trap in the presence of a reagent gas, whereby the reagent gases formed in the ion trap are allowed to react before they react with sample molecules. It is to provide a method of removing from an ion trap.

It is a further object of the invention to provide a method of optimizing the experimental parameters utilized in an ion trap to operate within the dynamic range of the ion trap.

Yet another object of the present invention is to provide a simple and efficient method for conducting MS ⁿ experiments in an ion trap that does not require empirical determination of the resonant frequency of the sample species within the ion trap. It is to be.

Yet another object of the invention is to provide another method of scanning an ion trap to obtain a mass spectrum.

[0031]

These and other objects that will be apparent to one of ordinary skill in the art upon reading the following description, claims and drawings are by applying an auxiliary field to the ion trap. It can be carried out by a novel method.
In one embodiment, the present invention provides for stable trapping of ions with mass charge numbers in the desired range.
It consists of applying a supplemental AC voltage to the ion trap to adjust the trapping field parameters of the ion trap, ionize the contents of the ion trap, and eject sample ions rather than reagent ions from the ion trap. The auxiliary AC voltage is a wide band voltage having a frequency component corresponding to the resonance frequency of a high mass sample ion, or a low voltage with a selected amplitude to cause only masses above the selected cutoff mass to escape from the ion trap. Either of the frequency voltages may be used.

In another embodiment of the invention, an auxiliary AC field is used to resonantly eject and remove reagent ions rather than sample ions formed during electron ionization of the contents of the ion trap. To be done. As a result, EI
Experiments can be conducted in the presence of a reagent gas stream without readjusting the trapping field.

In another embodiment of the invention, the spectral data of the mass associated with the highest peak measured during one scan of the ion trap is used for the adjustment and, if necessary, the ion trap is in the dynamic range. Experimental parameters are utilized during subsequent scans, as scanned at.

In another embodiment of the present invention, a low frequency auxiliary bipolar voltage is applied to the ion trap and used to subdivide the ions within the ion trap and to scan the contents of the ion trap. obtain.

[0035]

DETAILED DESCRIPTION OF THE INVENTION An apparatus for practicing the present invention is shown in FIG. A cross section of the ion trap 10 is shown. The ion trap 10 comprises a ring electrode 20 coaxially aligned with an upper end cap electrode 30 and a lower end cap electrode 35. Preferably, the electrodes of these traps have a hyperbolic inner surface, but can form a trapping field even if the electrodes have, for example, a circular arc in cross section. The design and construction of ion trap mass spectrometers are well known to those skilled in the art and need not be described in detail. A commercial model of an ion trap of the type described herein is sold by the assignee under the Model Designated Saturn.

A sample gas, for example a gas from the gas chromatograph 40, is introduced into the ion trap 10. Since GCs (gas chromatographs) typically operate at atmospheric pressure, a means of pressure reduction (not shown) is required while the ion trap operates at greatly reduced pressure. This means of reducing pressure is conventional,
It is well known to those skilled in the art. Although the present invention is described using GC as a sample source, this sample source is not considered part of the present invention and is not a limitation on using a gas chromatograph. Other sample sources may also be used, such as a liquid chromatograph with a special interface.

A source of reagent gas 50 for performing the chemical ionization test is also connected to the ion trap. The gas introduced into the sample and the inside of the ion trap 10 can be ionized by electron bombardment as described later. The electron beam from the thermionic filament 60, which is powered by the filament power supply 65, is controlled by the gate electrode 70. A hole is formed in the center of the upper end cap electrode 30 so that the electron beam generated by the filament 60 and the gate electrode 70 can enter the inside of the trap. The electron beam impinges on the sample and reagent molecules in the trap, thereby ionizing them. Electron impact ionization of sample and reagent gases is a well known process that does not require detailed explanation.

The trapping field is formed by the application of an AC (alternating current) voltage having the desired frequency and amplitude such that ions of mass and charge number within the desired range are stably trapped. An RF generator 80 is used to create this field and is applied to the ring electrode. It is well known that a DC (direct current) voltage may be applied to modify the trapping field and operate in different parts of the stability diagram of FIG. 1, but in practice, commercially available ion traps All work using only AC trapping fields.

Various methods are known for determining the mass-to-charge number of ions, which are trapped in an ion trap to obtain a mass spectrum of the sample. One known method is to scan the trap so that ions of consecutive mass and charge number are ejected in sequence. The first known method of trap scanning is to measure the magnitude of the AC voltage such that the ions become progressively unstable and exit the trap where they are detected using, for example, electron multiplier means 90. Scanning one of the trapping parameters such as

Another known trap scanning method involves an auxiliary AC dipole voltage applied across the upper and lower endcaps 30 and 35 of the ion trap 10. Such a voltage is generated by the auxiliary waveform generator 100 which is coupled to the upper and lower endcap electrodes by the transformer 110. The auxiliary AC field is used to resonantly eject the ions into the trap. Each ion in the trap has a resonance frequency, which is a function of its mass-charge number and a parameter of the trapping field. When an ion is excited by an auxiliary RF field at its resonant frequency, it derives energy from the field, and if effective energy is connected to the ion, its forced oscillation crosses the trap boundary region. That is, it jumps out of the trap. The ions thus ejected are also detected by the electron multiplier 90 or an equivalent detector. When using the resonance radial scanning technique, the contents of the trap are
It is sequentially scanned by scanning one of the frequencies of the auxiliary RF field or one of the trapping parameters such as the magnitude of the AC trapping voltage, V.
It is preferred to scan the magnitude of the AC voltage. Further, the method of scanning the ion trap is described below.

In one embodiment of the present invention, the auxiliary RF generator 100 can be used to scan a trap as previously described, formed by the EI during the time the reagent gas is being ionized. It is possible to generate a broadband RF field used to resonately eject the sample ions of interest. FIG. 3A shows the electron beam gate used to ionize the reagent gas. The electron gate 70 is turned on starting at t1 and ending at t2, and the electron beam can enter the trap to form reagent ions from the neutral reagent gas. As shown in FIG. 3 (b), which corresponds to an electron gate that allows electrons to enter the trap, the auxiliary waveform generator 100 moves the trap above and below the time interval starting at t1 and ending at t2. A broadband signal is applied to the end caps 30 and 35. As shown, the broadband excitation exceeds the gate time. Alternatively, the application of the auxiliary wideband signal is
If it is later than t1 or later than t2, that is,
It can start after the ionization of the electrons is complete. Similarly, the auxiliary signal can start at a time before t1. An auxiliary field for removal of unwanted sample ions,
It is important to be kept "on" during the extended time interval after the end of the extended time interval in which ions are formed.

Broadband A applied to upper and lower end caps
The C voltage can either be uncoordinated (dipole excitation) or co-ordinated (quadrupole excitation). Another way to obtain quadrupole excitation is to apply an auxiliary waveform to the ring electrode as shown in FIG. 11, rather than to the upper and lower end caps.

The auxiliary waveform includes a range of frequencies of sufficient amplitude to cause unwanted sample ions of mass greater than the maximum mass reagent ions to be ejected due to absorption of the resonant power by the trapped ions. Each of the sample ions resonates with the periodic component of the auxiliary waveform. Accordingly, they absorb power from the auxiliary field and leave the trapping field. After this auxiliary field ejects unwanted ions, it turns off and CI reagent ions
I React with sample atoms to produce sample ions. These ions are then scanned from the trap for detection in conventional fashion, as explained above.

The auxiliary waveform described above has a wide band,
It has the first frequency component corresponding to the lowest mass jumped out and the last frequency component corresponding to the highest mass jumped out. Between the first frequency and the last frequency is a series of discrete frequency components that are evenly or unequally spaced and have an arbitrary or fixed functional relationship. The amplitude of this frequency component can be uniform, or they can be compared to the distribution of q-values to compensate for the frequency dependence of the hardware or due to the distribution of the mass stored in the trap. Can be tailored to a functional shape to compensate. The broadband waveform has a sufficient number of frequency components so that all ions having a resonant frequency between the first and last components of the waveform are resonantly ejected by this auxiliary field.
Therefore, all sample ions formed during the EI are removed from the trap prior to the mass analysis scan and there are no gaps in the affected mass range.

In practice, the reagent gas used in the CI test has a molecular mass such that the reagent ions formed in the EI of the contents of the trap have a lower mass-charge number than the sample ions in most cases. All low. In rare cases, where sample ions of lower mass than the reagent gas are formed, a particular frequency may be added to the broad band excitation to cause ones of a particular mass to jump out with others.

An advantage of the invention over the prior art is the ability to remove unwanted sample ions formed by EI during the ionization of CI reagent gas. The ability to eject these ions allows the use of larger emission currents and longer ionization times. Therefore, the sensitivity of CI is increased.

FIG. 4 shows the residual EI spectrum of a sample of tetrachloroethane using the scanning conditions used in the prior art. FIG. 5 shows the removal of sample ions formed during the ionization process using a broadband waveform. FIG. 6 shows PFTBA and trichloroethane with a reagent gas of methane present in the trap using the prior art method.
2 shows the residual EI spectrum of the sample of FIG. 7 illustrates the removal of sample ions formed during the ionization process using the broadband waveform of the present invention. It can be seen that the reagent ions remain at mass 43 even though the sample ions, which have a mass slightly above them, were removed. FIG. 8 shows a spectrum under the same conditions as FIG. 7, except that the auxiliary waveform is off. FIG. 9 shows the spectrum of hexachlorobenzene using the prior art method. Mixed fragments of EI ions are observed at masses 282, 284, 286, 288 and 290. In addition, the ions from the sample with protons added (from CI) have masses of 283, 285
Observed at 287, 289 and 291. Figure 1
0 indicates the spectrum using the method described here. It can be seen that the unwanted ions from the EI process are almost completely removed.

In another aspect of the invention, the data obtained from one scan is used, if necessary, to adjust the parameters of subsequent scans to ensure that the trap operates in the dynamic range. Preferably, the strongest ion amplitude (base peak) of the scan is used to adjust the ionization and / or reaction time of the next scan. The size of the base peak is used to adjust the ionization and reaction time of subsequent scans so that a substantially constant number of ions in the base peak is maintained. It is a good indication of the total amount of charge from the sample in the trap, as much of the charge that is ejected from the trap during the scan is due to the base peaks. By maintaining a nearly constant total sample charge within the trap, the dynamic range of the sample can be increased. Alternatively, with mass spectral information from one scan, it is possible to adjust the parameters of the subsequent mass spectrometry scan, for example to focus only on the sample ions of interest, ie to optimize a particular type. is there. Preferably, when adjusting the parameters of the scan based on the previous scan, both the reaction time and the ionization time change at the set ratio. This makes it easier to standardize the results from one scan to the next.

An advantage of the method of the present invention is that it reduces the scan time for wide dynamic range samples. This is accomplished by using the intensity of the base peak from the previous scan as a measure of the amount of sample in the trap. Thus, the need for the time-consuming prescan used in the prior art is eliminated.

Broad band auxiliary fields are also used to remove reagent ions from the trap when EI tests are performed. Ion trap users can
It may be desired to perform both I and CI tests. Under these circumstances, stopping the flow of reagent gas into the trap during the EI is not desirable and the presence of reagent ions would likely confuse the analytical data. . By using assisted RF broadband excitation, all reagent ions formed during electron impact ionization of the sample are resonantly ejected from the trap as soon as they are formed. The same timing sequence shown in FIG. 3 can be used to implement this aspect of the invention. In this embodiment of the invention, the broadband RF excitation is done according to the previously described variant, except that the frequency range should be set to remove only low mass reagent ions.

The waveform generator 100 of FIG. 2 performs a CI test to obtain a mass spectrum, surrounds an MS ⁿ test, and applies a low frequency non-resonant field to scan the contents of the trap. used. A low frequency auxiliary voltage from the waveform generator 100 is applied as a dipole field to the upper and lower end caps 30 and 35 of the ion trap 10. The frequency of this dipole field is independent of the resonance frequency of all ions (sample or reagent ions) stored in the trap. This waveform is preferably a square wave,
It may be almost any waveform including a sine waveform, a sawtooth waveform or a triangular waveform. Especially, the frequency of the auxiliary voltage is 1
It is relatively low, such as between 00 Hz and thousands of Hz. Testing shows that the present invention can be performed at frequencies below about 10,000 Hz, which is near the beginning of the resonant frequency range of the sample ions. However, preferably this frequency should be in the range of a few hundred Hz.

The auxiliary square wave dipole field is said to be deformably transferred to different positions along the z-axis in the center of the pseudopotential well of the trapping field. The centers of the pseudopotential wells of the trapping field are each moved, and the trapped ions extract the converted energy from the trapping field and start oscillating around the new center. Therefore, this movement of the center of vibration increases the magnitude of the vibration. In general, when the ions lose energy to the background gas, they move toward a new center. If the center of the pseudopotential field moves, as when changing polarities at a square wave, this process repeats. It will be appreciated that the frequency of the auxiliary dipole field should be low so that the ions can move towards the new center before the field is changed.

As mentioned above, when the center of the pseudopotential well is moved, the ions begin to oscillate around a new point in space and become more energized. The energy added to the ions is sufficient to cause a large number of ions to dissociate by colliding with the damping gas. Thereby, the daughter ions are formed. When this process is repeated, more ions dissociate. Another advantage of this method is that it imparts more energy than the resonant excitation to the ions, and thus in some cases can result in more ion fragmentation.

Since the method described above does not rely on the resonant frequency of the ions in the ion trap, it operates on all the ions in the trap simultaneously. Thus, using this method, it is possible to form products of various ion fragments without having to apply the resonant frequency associated with each ion fragment. If desired, the ion of interest is first separated in the trap.

Using this method it is possible to obtain a complete "fingerprint" of the component which facilitates the component identification. However, the mass-charge number alone cannot be used to unambiguously identify parent ions. However, knowledge of the mass of all ion fragments as well as the mass-charge number of the parent ion can be used to unambiguously identify its parent.

It has been found that applying a low frequency voltage to the ion trap can be used as a mechanism for removing ions having a mass above a certain cutoff mass from the ion trap. The mass of this cutoff is a function of the auxiliary low frequency voltage value. One model of how ion traps work is that ions are essentially in a potential well with a potential well "depth" that is a function of the mass-to-charge number with other materials. Is to be trapped. The greater the mass, the shallower the well. The phenomenon of ion removal of a larger mass is often observed in the application of a low-frequency auxiliary field, which is said to be related to the relatively shallow depth of the potential well associated with the large mass. In particular, it is believed that the central transition of the pseudopotential well causes the ions of large mass to gain enough energy to overcome the well barrier and exit the ion trap.

This phenomenon can be exploited to favor both chemical ionization testing and ion trap scanning. As explained above, when performing a chemical ionization test, it is necessary to remove the large mass of sample ions formed in the EI of the reagent gas. This variant of sample ion removal is, as explained above, an effective value of the low-frequency auxiliary field to remove all sample ions from the trap, while the reagent ions exit unaffected. Is to apply. The timing sequence in applying this supplemental low frequency field may be any of the modified timing sequences as depicted in FIG. 3 or described above in connection therewith. It should be noted that the ionization period in FIG. 3 (a), which has an interval of one thousandth of a second or less, is shorter than the half cycle of the low frequency auxiliary voltage. Therefore, as shown in FIG. 3B, the application interval of the auxiliary voltage is much longer and is not shown in FIG.

The application of a low frequency auxiliary voltage can also be used as a mechanism to scan the ion trap so that a mass spectrum is obtained. This is done by the auxiliary low frequency voltage value. If this auxiliary voltage is low at the beginning and has a further upward slope, most will jump out of the trap in decreasing order. Alternatively, the low frequency auxiliary voltage is kept constant and one of the trapping parameters is scanned for the same effect.

FIG. 12 shows 1, 1, 1-obtained by the conventional method.
2 is a mass spectrum of trichloroethane of. Mass 97
The peak at corresponds to CH ₃ CCl ₂ +. In contrast,
FIG. 13 shows the mass of 1,1,1-trichloroethane obtained using the same test parameters as FIG. 12, except that a low frequency auxiliary square wave voltage (100 Hz, 42 volts) was applied for 20 ms. It is a spectrum. It can be seen from FIG. 13 that the peak intensity at mass 97 is reduced and the mass 61 (CH ₂ CCL +) is abundant. As a result of non-resonant excitation, the mass 97 ion absorbed energy and was dissociated to form the mass 61 ion.

FIGS. 14 and 15 use the same parameters used to obtain the results of FIGS. 12 and 13 except that the frequencies of the auxiliary square waves were set to 300 and 600 Hz, respectively. Obtained 1, 1,
1 shows the spectrum of 1-trichloroethane. Figure 1
The spectral similarity of 3, 14 and 15 indicates that the dissociation is largely independent of the auxiliary field frequency over a wide range. Finally, FIG. 16 shows a non-resonant sine wave of 139.6 KHz (resonance frequency of the z-axis of the ion mass 97) using the prior art method, ie, not using the non-resonant low frequency square wave. 2 at the level of 800 mV
The mass spectrum of 1,1,1-trichloroethane obtained by applying for 0 ms is shown. It can be seen that the amounts of daughter ions produced by both methods are almost the same.

17, 18 and 19 show mass spectra of PFTBA under various conditions to show how the method of the present invention was used to remove large mass ions from an ion trap. Is shown. FIG. 17 shows a complete mass spectrum that includes both parent and fragment ions. FIG.
Shows that all ions with masses above 131 were removed from the trap when the auxiliary square wave voltage increased to 20V. FIG. 19 shows that increasing the voltage to 33V removes all ions with a mass of 100 or more from the trap.

FIGS. 20, 21 and 22 show an application of a chemical ionization test in which a large mass of ions can be removed from an ion trap by using a low frequency auxiliary field. 20, 21 and 22 are respectively FIG.
It is shown to be the same CI test as 7 and 10. However, rather than using a broadband resonant jump to remove unwanted sample ions from the trap, a low frequency auxiliary waveform was used. It can be seen that the results are substantially the same with either method. Figure 20
21 is obtained using an auxiliary field having a frequency of 600 Hz, the result of FIG. 21 is obtained using an auxiliary field of frequency of 300 Hz, and the result of FIG.
Obtained using an auxiliary field with a frequency of Hz. In each case, the magnitude of the auxiliary voltage is 20 and 40V
It was between.

Although the present invention has been described in connection with the preferred embodiment, it is not intended that the matter thus described be limiting, as other variations and equivalents will occur to those skilled in the art. It will be done easily.
From this, the scope of the present invention should be determined only by reference to the appended claims. For example,
Although the present invention has been partially described in connection with performing a chemical ionization test prior to the step of electron impact ionization, the method may also be performed using photoionization instead of electron impact ionization. it can.

[Brief description of drawings]

FIG. 1 shows a stability diagram associated with an ion trap.

FIG. 2 shows a partial schematic view of the apparatus used to carry out the method of the invention.

FIG. 3 is a graph showing control of an auxiliary broadband AC field related to the gating of an electron beam used for electron impact ionization in accordance with the present invention.

FIG. 4 is a mass spectrum of a sample comparing the present invention with the prior art.

FIG. 5 is a mass spectrum of a sample comparing the present invention with the prior art.

FIG. 6 is a mass spectrum of a sample comparing the present invention with the prior art.

FIG. 7 is a mass spectrum of a sample comparing the present invention with the prior art.

FIG. 8 is a mass spectrum of a sample comparing the present invention with the prior art.

FIG. 9 is a mass spectrum of a sample comparing the present invention with the prior art.

FIG. 10 is a mass spectrum of a sample comparing the present invention with the prior art.

11 shows a modified arrangement of the device of FIG. 2 used to carry out the invention.

FIG. 12 is a mass spectrum of a sample showing how the application of a supplemental low frequency field used to remove large mass ions from an ion trap is made.

FIG. 13 is a mass spectrum of a sample showing how the application of a supplemental low frequency field used to remove large mass ions from an ion trap is made.

FIG. 14 is a mass spectrum of a sample showing how the application of a supplemental low frequency field used to remove large mass ions from an ion trap is made.

FIG. 15 is a mass spectrum of a sample showing how the application of a supplemental low frequency field used to remove large mass ions from an ion trap is made.

FIG. 16 is a mass spectrum of a sample showing how the application of a supplemental low frequency field used to remove large mass ions from an ion trap is made.

FIG. 17 is a mass spectrum showing how the application of a supplemental low frequency field used to remove large mass ions from an ion trap is made.

FIG. 18 is a mass spectrum showing how the application of a supplemental low frequency field used to remove high mass ions from an ion trap is made.

FIG. 19 is a mass spectrum showing how the application of a supplemental low frequency field used to remove large mass ions from an ion trap is made.

FIG. 20 is a mass spectrum showing how the supplemental low frequency field used in conducting a chemical ionization test has been applied.

FIG. 21 is a mass spectrum showing how the supplemental low frequency field used in conducting a chemical ionization test is applied.

FIG. 22 is a mass spectrum showing how the supplemental low frequency field used in conducting a chemical ionization test is applied.

[Explanation of symbols]

 10. ． ． Ion trap 20. ． ． Ring electrode 30. ． ． Upper end cap electrode 35. ． ． Lower end cap electrode 40. ． ． Gas chromatograph 50. ． ． Reagent gas 60. ． ． Thermionic filament 65. ． ． Filament power supply 70. ． ． Gate electrode 80. ． ． RF generator for trapping field 90. ． ． Electron multiplier 100. ． ． Auxiliary waveform generator 110. ． ． Transformer

Claims (39)

[Claims] 1. A method of using an ion trap mass spectrometer in chemical ionization mode, wherein ions having a mass charge number within a desired range are stably trapped within the ion trap mass spectrometer. Adjusting the trapping field parameters of the ion trap mass spectrometer; introducing ions into the ion trap mass spectrometer; introducing reagent gas into the ion trap mass spectrometer; mass within the desired range Ionizing the sample and reagent gases in an ion trap mass spectrometer such that ions of the sample and reagent having a charge number are formed in the ion trap mass spectrometer; and sample ions formed during the ionization step. The auxiliary AC field to eject the ion trap mass spectrometer. A step of applying to the mass spectrometer,
A method consisting of. 2. The method of claim 1, wherein the ionizing step exposes the contents of the ion trap mass spectrometer to an electron beam such that the sample and reagent ions are formed by electron impact ionization. A method consisting of. 3. The method of claim 1, wherein the ionizing step comprises exposing the contents of the ion trap mass spectrometer to light such that the sample and reagent ions are formed by photoionization. Method. 4. The method of claim 1, wherein the auxiliary AC field is applied to an ion trap mass spectrometer during the ionizing step. 5. The method of claim 1, wherein the auxiliary AC field is applied to an ion trap mass spectrometer for a time interval after the ionization step is complete. 6. The method of claim 1, wherein applying the auxiliary AC field to the ion trap mass spectrometer begins no later than when the ionizing step begins and the ionizing step is complete. A method that lasts for some later time interval. 7. The method of claim 1, wherein the auxiliary AC field is a quadrupole field. 8. The method according to claim 1, wherein the auxiliary AC field is a dipole field. 9. The method according to claim 1, wherein the auxiliary AC field is a monopolar field. 10. The method according to claim 1, wherein the auxiliary AC field is applied to an end cap of an ion trap mass spectrometer. 11. The method according to claim 1, wherein the auxiliary AC field is applied to a ring electrode of an ion trap mass spectrometer. 12. The method of claim 1, wherein the auxiliary AC field is broadband excitation for resonantly ejecting the sample ions from an ion trap mass spectrometer. So that only the sample ions are ejected from the ion trap mass spectrometer.
The method, wherein the highest frequency component contained in the C field is below the frequency required to drive the reagent ions out of the ion trap mass spectrometer. 13. The method of claim 12, wherein the auxiliary AC field has a highest frequency and a highest frequency corresponding to ejecting the sample ions with the lowest mass-to-charge number out of the ion trap mass spectrometer. The method, wherein the method has a lowest frequency corresponding to ejecting mass-charge number sample ions from the ion trap mass spectrometer. 14. The method of claim 12, wherein the auxiliary AC field is at the maximum and minimum so that substantially all sample ions in an ion trap mass spectrometer are ejected by the auxiliary AC field. Between frequency components,
A method that consists of a series of discrete frequency components. 15. The method according to claim 14, wherein the discrete frequency components are regularly separated. 16. The method of claim 14, wherein the discrete frequency components are indefinitely separated. 17. The method of claim 14, wherein the discrete frequency components are separated by any phase. 18. The method of claim 14, wherein the discrete frequency components have phases with a fixed functional relationship. 19. The method of claim 14, wherein the discrete frequency components have a constant amplitude. 20. The method of claim 14, wherein the discrete frequency components have amplitudes formed into a selected functional form. 21. The method of claim 1, wherein the sample ions formed during the ionization step are removed from the ion trap mass spectrometer so that the sample ions are formed by chemical ionization. The method further comprising allowing the sample to react with the reagent ions for a selected reaction period. 22. The method of claim 21, wherein the trapping field is kept constant during the ionization and reaction steps. 23. The method of claim 21, wherein the sample ions are formed by chemical ionization such that successive mass-charge number sample ions emerge from the ion trap mass spectrometer and are detected in sequence. The method further comprising the step of scanning the ion trap mass spectrometer after scanning. 24. The method of claim 23, wherein the step of claim 21 is repeated after adjusting the reaction period based on the magnitude of the highest peak detected during the scanning step. By the way. 25. The method of claim 23, wherein the ionization time is adjusted during the scanning step based on the magnitude of the highest peak detected.
This method is repeated. 26. The method of claim 23, wherein both the duration of the ionization step and the reaction period are adjusted during the scanning step based on the size of the highest peak detected. The method wherein the step of item 21 is repeated. 27. The method of claim 23, wherein the reaction period is such that the total amount of charge in the ion trap mass spectrometer is maintained substantially constant from one scan to another. How to be adjusted. 28. The method of claim 1, wherein the auxiliary AC field is a low frequency dipole field such that sample ions are removed from the ion trap mass spectrometer by non-resonant ejection. the method of. 29. The method of claim 28, wherein the low frequency auxiliary field has a frequency in the range of 100-10,000 Hz. 30. The method of claim 28, wherein the auxiliary AC field has a square wave waveform. 31. A method of using an ion trap mass spectrometer in electron impact ionization mode during continuous feeding of a reagent gas to the ion trap mass spectrometer, wherein the mass charge number is within a desired range. Adjusting the parameters of the trapping field of the ion trap mass spectrometer so that the ions contained therein are stably trapped in the ion trap mass spectrometer; and the sample is an ion trap mass spectrometer having an inflow flow of a reagent gas. The sample and reagent in the ion trap mass spectrometer such that ions of the sample and reagent having a mass charge number in the desired range are formed by electron impact ionization in the ion trap mass spectrometer. Exposing the gas to an electron beam and the reagent ions formed during the electron impact ionization. Applying a broadband auxiliary AC field to the ion trap mass spectrometer to cause it to resonate out of the ion trap mass spectrometer and maintain only the sample ions within the ion trap mass spectrometer by the broadband auxiliary AC field. Scanning the ion trap mass spectrometer such that a series of mass-charge number sample ions are ejected from the ion trap mass spectrometer and detected sequentially. 32. A method for adjusting the dynamic range of an ion trap mass spectrometer used in a chemical ionization multiple scan mode, comprising: (a) stably trapping ions within a desired mass to charge number range. In the ion trap mass spectrometer,
Applying a trapping field, (b) introducing the sample and reagent gas into an ion trap mass spectrometer, (c) ionizing the sample and reagent gas during an ionization period, and (d) Removing sample ions formed during the ionization period from the ion trap mass spectrometer, and (e) allowing reaction of sample molecules with the reagent ions during a chemical ionization period to form sample ions. And (f) scanning the ion trap mass spectrometer to cause the sample ions of consecutive mass and charge number to sequentially exit the ion trap mass spectrometer, and (g) the sample ions are ions Detecting sample ions as they exit the trap mass spectrometer; (h) identifying sample ions present at the highest concentration, and Repeating the steps (a) to (g) using the concentration information to detect the concentration of the sample ions, and (i) adjust the ionization period, the chemical ionization period, or both. And a method comprising steps. 33. A method of subdividing parent ions in an ion trap mass spectrometer, the method comprising: forming and trapping parent ions in the ion trap mass spectrometer; and collision of the parent ions with background gas. Applying to the ion trap mass spectrometer a low frequency auxiliary AC dipole field having a frequency different from the resonant frequency of the parent ion to undergo induced dissociation, and obtaining a mass spectrum of the contents of the ion trap mass spectrometer. And a method comprising steps. 34. The method of claim 33, wherein the low frequency auxiliary AC dipole field is 100-10,000 Hz.
A method having a frequency in the range of. 35. The method of claim 33, wherein the low frequency auxiliary AC dipole field is for a period interval long enough to multiplex ion debris from the parent ions. Applied to an ion trap mass spectrometer,
By the way. 36. The method of claim 35, further comprising utilizing a mass spectrum of the contents of the ion trap mass spectrometer to unambiguously identify parent ions. 37. A method of scanning an ion trap mass spectrometer to obtain a mass spectrum of the contents of an ion trap mass spectrometer, wherein ions having a mass charge number within a desired range are ion trap mass spectrometers. Adjusting the trapping field parameters of the ion trap mass spectrometer for stable trapping in the meter, introducing sample ions into the ion trap mass spectrometer, and applying a low frequency auxiliary AC dipole field to the ions. Applying at least one of the trapping field parameters and / or the amplitude of the low frequency AC dipole field so that ions of consecutive mass and charge number are sequentially ejected out of resonance from the ion trap mass spectrometer. One consisting of the step of scanning and the step of detecting the ions ejected from the ion trap mass spectrometer . 38. The method of claim 37, wherein the scanning step comprises increasing the amplitude of the low frequency auxiliary AC dipole field. 39. The method of claim 37, wherein the low frequency auxiliary AC dipole field is 100-10,000H.
A method, having a frequency in the z range.