CN116735688A - On-line double-gradient preconcentration detection method for soil heavy metal ions based on microchip electrophoresis non-contact conductivity detection - Google Patents
On-line double-gradient preconcentration detection method for soil heavy metal ions based on microchip electrophoresis non-contact conductivity detection Download PDFInfo
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Abstract
The invention discloses a microchip electrophoresis non-contact conductivity detection-based online double-gradient pre-concentration detection method for soil heavy metal ions, which comprises the steps of preparing an analyte sample to be detected, preparing a high-concentration buffer solution as a background buffer solution, preparing a microchip, performing microchip electrophoresis and the like. When tested, the background buffer contained 2- (N-morpholino) ethanesulfonic acid, L-histidine, and an electroosmotic flow inhibitor; the analyte sample to be tested comprises a sample matrix liquid and a sample leaching stock liquid. Wherein the sample matrix solution is a low concentration buffer solution, and glacial acetic acid is used for reducing the pH value. In the test, as the concentration gradient and the pH value gradient exist in the sample solution of the analyte to be detected and the background buffer solution, ions to be detected in the sample can be accumulated at the contact surface of the sample solution and the background buffer solution in a decelerating way in the migration process, so that the on-line pre-concentration is realized, and the detection sensitivity of the ions to be detected is improved. The invention greatly improves the separation degree of various heavy metal ions and effectively improves the detection sensitivity of the heavy metal ions.
Description
Technical Field
The invention belongs to the field of on-site rapid detection of soil heavy metals, and relates to an on-line double-gradient pre-concentration detection method of soil heavy metal ions based on microchip electrophoresis non-contact conductivity detection. The method is based on microchip electrophoresis, utilizes a field amplification sample stacking technology, and inhibits convection dispersion generated in the field amplification sample stacking process by adjusting the pH value of an analyte to be detected, thereby realizing on-line pre-concentration of heavy metal ions.
Background
With the development of industrial creative activities such as production and living of human workers and agriculture, heavy metal pollution becomes one of the main reasons for soil pollution. Currently, effective detection means for heavy metal ions mainly include atomic spectrometry, mass spectrometry, chromatography, electrochemical analysis and the like. The atomic spectrometry, mass spectrometry and chromatography have the defects of complex detection equipment, high operation difficulty, high cost, large volume and the like. The potentiometric analysis, polarography and capillary electrophoresis have the defects of long separation time, low sensitivity, difficult integration and the like in the electrochemical analysis method. For heavy metal ion detection, on-site rapid detection becomes a main development trend.
Microchip electrophoresis is an extension of capillary electrophoresis. In the 90s of the 20 th century, manz proposed the concept of a micro total analyzer, and the detection experiment was compressed on a chip, thus opening the research hot-line of microchip electrophoresis. In 2001, guiljt proposed a non-contact conductivity detection technique, and the life of a detection electrode is prolonged by adopting a design of a non-contact detection electrode. Although microchip electrophoresis technology combines with non-contact conductivity detection technology to realize the integration and onsite detection of heavy metal ions. But its detection sensitivity still suffers from a low disadvantage. Therefore, on-line pre-concentration of the sample becomes an important means for improving the detection sensitivity.
The on-line pre-concentration technology is mainly divided into isotachophoresis concentration technology, sample sweeping technology, field amplification sample stacking technology and dynamic pH node technology. The isotachophoresis technology is complex in operation, and the chip structure is complex and is not suitable for on-site rapid detection. The sample scanning technology has extremely high requirements on the types and the concentrations of the micelles, the concentration time is long, the specialization degree is high, and the method is also not suitable for on-site rapid detection. In the pre-concentration process, the field amplification samples are piled up, and analyte convection dispersion caused by local uneven electroosmotic flow exists, so that the bandwidth of a sample area is increased, and the pre-concentration effect is reduced. Dynamic pH junction technology improves the pre-concentration effect by changing the analyte surface charge to change the analyte migration rate. However, the dynamic pH junction technique does not change the surface charge of heavy metal ions. Therefore, in the field amplification sample stacking technology based on microchip electrophoresis, electroosmotic flow inhibitor is added into buffer solution, the pH value of an analyte to be detected is regulated, the electroosmotic flow in the electrophoresis process and convection dispersion generated in the field amplification sample stacking process are inhibited, and the concentration gradient and the pH value gradient between the sample solution and the buffer solution are utilized to enable the analyte to be stacked in a decelerating manner in the migration process, so that the migration time of the analyte is increased, and the online pre-concentration of heavy metal ions is realized. The invention mainly provides an online double-gradient pre-concentration detection method for heavy metal ions in soil based on microchip electrophoresis non-contact conductivity detection by adjusting the proportion of buffer solution and the pH value of sample matrix liquid based on a field amplification sample stacking technology.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides an online double-gradient pre-concentration detection method for soil heavy metal ions based on microchip electrophoresis non-contact conductivity detection, which utilizes concentration gradient generated by field amplification sample accumulation and pH gradient generated by adjusting the pH value of an analyte to be detected, and realizes online pre-concentration of the sample by inhibiting electroosmotic flow and convective dispersion in the field amplification sample accumulation process. The background buffer solution of the invention contains 2- (N-morpholinyl) ethanesulfonic acid (MES), L-histidine (His) and Cetyl Trimethyl Ammonium Bromide (CTAB), wherein the Cetyl Trimethyl Ammonium Bromide (CTAB) is used as an electroosmotic flow inhibitor to control the electroosmotic flow direction and size. The sample matrix solution contained low concentrations of 2- (N-morpholinyl) ethanesulfonic acid (MES), L-histidine (His), and cetyltrimethylammonium bromide (CTAB) and was pH adjusted using glacial acetic acid to form a concentration gradient and pH gradient with the background buffer. By optimizing a series of important experimental parameters such as CTAB concentration of a background buffer solution, MES/His concentration of a sample matrix solution, pH value and the like, the optimal condition is applied to detection of soil heavy metal ions, and the method is a technology with simpler operation, higher efficiency and higher sensitivity. The invention can greatly improve the detection sensitivity of microchip electrophoresis to heavy metal ions, and compared with the prior preconcentration method, the invention ensures higher enrichment effect and realizes simple and efficient operation.
In order to achieve the above purpose, the invention adopts the following technical scheme:
an online double-gradient preconcentration detection method for soil heavy metal ions based on microchip electrophoresis non-contact conductivity detection comprises the following steps:
(one) preparing a sample of the analyte to be detected:
the sample soil was ground and sieved and dried by a dryer for 30-45min. Then mixing the dried sample soil with a leaching agent for ultrasonic treatment for 30-60min, standing, taking supernatant, filtering by using filter paper, and filtering by using a 0.22 mu m water system filter to obtain a sample leaching stock solution; preferably, the leaching agent is ammonium acetate solution;
preparing a low-concentration buffer solution as a sample matrix solution, wherein the low-concentration buffer solution comprises 2- (N-morpholinyl) ethanesulfonic acid (MES), L-histidine (His) and an electroosmotic flow inhibitor; preferably, the concentration of 2- (N-morpholino) ethanesulfonic acid in the low concentration buffer is 1.25-10mM/L, L-histidine, the concentration of electroosmotic flow inhibitor is 0.0025-0.1mM/L; further preferably, the MES concentration is 2.5mM/L, the His concentration is 2.5mM/L, the electroosmotic flow inhibitor is cetyltrimethylammonium bromide (CTAB), and the CTAB concentration is 0.01mM/L;
and mixing the sample leaching stock solution and the sample matrix solution according to the volume ratio of 1:1, and regulating the pH value of the solution to 3-6 by using 4% (v/v) glacial acetic acid to obtain the analyte sample to be detected.
(II) preparation of background buffer:
preparing a mixed solution of 2- (N-morpholinyl) ethanesulfonic acid (MES), L-histidine (His) and cetyl trimethylammonium bromide (CTAB) as a background buffer; preferably, the MES concentration is 20mM/L, the His concentration is 20mM/L, and the CTAB concentration is 0.01mM/L.
(III) providing a microchip:
the microchip comprises a cross micro-channel, wherein the cross micro-channel comprises a first channel and a second channel which are mutually communicated, a sample pool and a first liquid storage pool are respectively formed at two ends of the first channel, and a second liquid storage pool and a third liquid storage pool are respectively formed at two ends of the second channel; the second channel is provided with a detection area at one end close to the third liquid storage tank, and the detection area consists of a transmitting electrode and a receiving electrode. The transmitting electrode transmits a high-frequency alternating current signal, the receiving electrode receives the same-frequency alternating current signal due to polarization, the amplitude of the alternating current signal is influenced by the conductance in the solution, when the analyte migrates to the detection area, the conductance in the detection area is changed, so that the amplitude of the current signal received by the receiving electrode is changed, an electrophoresis peak is obtained, and the information of the type, the concentration and the like of the analyte is contained in the electrophoresis peak;
and (IV) a microchip electrophoresis process, comprising a preparation stage, a sample injection stage, a separation stage and a detection stage, wherein:
the preparation phase process is as follows: after flushing the cross micro-channel by using a background buffer solution, filling the background buffer solution into the sample cell, the first liquid storage tank, the second liquid storage tank and the third liquid storage tank; when the micro-channel is used for the first time, the micro-channel is firstly washed for 15min by using 1mM NaOH solution, and is washed for 15min by distilled water; not for the first time, the microchannels were rinsed with 1mM NaOH solution for 5min and distilled water for 10min.
The sample injection stage process is as follows: replacing background buffer solution in the sample pool with an analyte sample to be detected, and respectively placing a high-voltage electrode, which is preferably a platinum electrode, in the sample pool, the first liquid storage pool, the second liquid storage pool and the third liquid storage pool; applying a voltage of 500-2000V for 14s to the sample cell, and simultaneously grounding the first liquid storage cell for 14s, and suspending the second liquid storage cell and the third liquid storage cell for 14s; in the process, because concentration gradients and pH value gradients exist at two sides of the interface between the analyte to be detected and the background buffer solution, heavy metal ions to be detected are accumulated at the interface in a decelerating way, and preconcentration in a sample injection stage is realized.
The separation stage process is as follows: and switching the voltage, applying 1000-2000V voltage for 80s to the second liquid storage tank, simultaneously grounding the third liquid storage tank for 80s, and suspending the sample tank and the first liquid storage tank for 80s. In the process, part of samples reaching the cross intersection are pushed into the second channel, different types of heavy metal ions to be detected are divided into different sample zones due to different migration speeds, and the concentration gradient and the pH value gradient exist on two contact surfaces of the different heavy metal ion zones to be detected and the background buffer solution, so that the heavy metal ion zones to be detected are continuously shortened in the migration process, and are accumulated at the front contact surface in a decelerating manner, so that the pre-concentration in the separation stage is realized.
The detection phase process is as follows: when the sample zone reaches the detection area, an electrophoresis pattern is drawn by collecting amplitude change caused by conductivity change.
The invention has the advantages that:
the invention is based on the field amplification sample accumulation technology, adjusts the pH value of the analyte to be detected, inhibits the convective dispersion in the pre-concentration process, establishes an online double-gradient pre-concentration detection method of heavy metal ions in soil based on microchip electrophoresis by utilizing the concentration gradient and the pH value gradient of the analyte to be detected and a background buffer solution, and pre-concentrates the heavy metal ion sample by adopting the method; the method is successfully applied to pre-concentration and detection of heavy metal ions in the true soil leaching solution, and the sensitivity of heavy metal lead and cadmium is respectively improved by 26.40 times and 43.73 times; the separation degree of heavy metals lead and cadmium is improved from 0.41 to 1.38. The method has the advantages of simplicity, high efficiency, low cost and the like, and is suitable for a portable microchip electrophoresis system.
Drawings
FIG. 1 is a schematic structural view of a microchip.
FIG. 2 is a diagram of a mechanism of pre-concentration.
FIG. 3 is an electrophoretogram obtained from testing under different electroosmotic flow inhibitor (CTAB) concentrations.
FIG. 4 is a graph showing the effect of different sample matrix fluid concentrations on the pre-concentration effect.
FIG. 5 is an electrophoretogram of a sample matrix fluid at different pH values.
FIG. 6 shows the effect of pH of the sample matrix liquid on heavy metal ions to be detected.
FIG. 7 is a comparison of conventional microchip electrophoresis and the pre-concentration method according to the present invention.
Reference numerals: the device comprises a 1-cross micro-channel, a 101-sample introduction channel, a 102-separation channel, a 2-sample cell, a 3-first liquid storage cell, a 4-second liquid storage cell, a 5-third liquid storage cell, a 6-transmitting electrode and a 7-receiving electrode.
Detailed Description
The present invention will be further described with reference to examples, which are not intended to be limiting, so that those skilled in the art will better understand the present invention and practice it.
In addition, the preparation processes in the following examples are conventional means in the art unless specifically described, and therefore, will not be described in detail;
an online double-gradient preconcentration detection method for soil heavy metal ions based on microchip electrophoresis non-contact conductivity detection comprises the following steps:
(one) preparing a sample of the analyte to be detected:
the sample soil was ground and sieved and dried by a dryer for 30-45min. Then mixing the dried sample soil with a leaching agent for ultrasonic treatment for 30-60min, standing, taking supernatant, filtering by using filter paper, and filtering by using a 0.22 mu m water system filter to obtain a sample leaching stock solution; preferably, the leaching agent is an ammonium acetate solution;
preparing a low-concentration buffer solution as a sample matrix solution, wherein the low-concentration buffer solution comprises 2- (N-morpholinyl) ethanesulfonic acid (MES), L-histidine (His) and an electroosmotic flow inhibitor; preferably, the concentration of 2- (N-morpholino) ethanesulfonic acid in the low concentration buffer is 1.25-10mM/L, L-histidine, the concentration of electroosmotic flow inhibitor is 0.0025-0.1M/L; further preferably, the MES concentration is 2.5mM/L, the His concentration is 2.5mM/L, and the CTAB concentration is 0.01mM/L;
and mixing the sample leaching stock solution and the sample matrix solution according to the volume ratio of 1:1, and regulating the pH value of the solution to 3-6 by using 4% (v/v) glacial acetic acid to obtain the analyte sample to be detected.
(II) preparation of background buffer:
preparing a mixed solution of 2- (N-morpholinyl) ethanesulfonic acid (MES), L-histidine (His) and Cetyl Trimethyl Ammonium Bromide (CTAB) as a background buffer solution; preferably, the MES concentration is 20mM/L, the His concentration is 20mM/L, and the CTAB concentration is 0.01mM/L.
(III) preparation of microchip:
cutting a polymethyl methacrylate (PMMA) bottom plate with the thickness of 5mm and a polymethyl methacrylate (PMMA) upper plate with the thickness of 0.2mm into a rectangle with the thickness of 30 multiplied by 70 by using a laser cutting machine to obtain a microchip substrate and a microchip bonding cover plate;
clamping a microchip substrate on a precise micro milling machine, and processing the microchip substrate at a high rotating speed of 10000rpm by using a milling cutter with the diameter of 0.1mm to obtain a cross micro-channel 1, wherein the cross micro-channel 1 comprises a first channel 101 and a second channel 102 which are mutually communicated; the cutting fluid is opened in the whole processing process, and the cross section of the micro channel is 100 mu m wide and 100 mu m deep.
Changing a precise micro milling machine tool, selecting a 2mm milling cutter, and processing four liquid storage tanks at the four end points of the cross micro channel at 4500rpm to obtain an open microchip, wherein the four liquid storage tanks are respectively a sample tank 2, a first liquid storage tank 3, a second liquid storage tank 4 and a third liquid storage tank 5;
taking an open microchip and a microchip bonding cover plate, cleaning for 15min by using ultrasonic waves, then treating for 5min by using plasma, attaching the treated surfaces of the microchip and the microchip bonding cover plate, and placing the microchip bonding cover plate into a hot press to perform hot pressing for 15min under the conditions of 63kg pressure and 101 ℃ by using a hot pressing technology to obtain the microchip; the second channel in the microchip is provided with a detection area at one end near the third reservoir, said detection area being composed of a transmitting electrode 6 and a receiving electrode 7. The transmitting electrode 6 transmits a high-frequency alternating current signal, and the receiving electrode 7 receives an alternating current signal with the same frequency due to polarization, the amplitude of the alternating current signal is influenced by the conductance in the solution, when the analyte migrates to the detection area, the conductance in the detection area changes, so that the amplitude of the current signal received by the receiving electrode changes, an electrophoresis peak is obtained, and information such as the type and concentration of the analyte is contained in the electrophoresis peak.
(IV) microchip electrophoresis process, including preparation phase, sample stage, separation phase and detection phase, figure 1 mainly illustrates the method of pre-concentration principle, wherein:
the preparation phase is as shown in fig. 2A: after flushing the cross micro-channel 1 by using background buffer solution, filling the sample cell 2, the first liquid storage cell 3, the second liquid storage cell 4 and the third liquid storage cell 5 with the background buffer solution;
the sample injection stage is as shown in fig. 2B: at this stage we first removed the background buffer in the sample cell using a syringe and then added 50 μl of analyte to be measured to the sample cell using a pipette. And then, four high-voltage platinum electrodes are placed in the sample tank and the three liquid storage tanks, the voltage condition is set to apply 500V voltage to the sample tank, the first liquid storage tank is grounded, the second liquid storage tank and the third liquid storage tank are suspended, and the duration is 14s. At this stage, heavy metal cations in the analyte to be detected move toward the first reservoir due to the high pressure of the sample. In addition, because the concentration gradient and the pH value gradient exist at the two sides of the contact surface of the analyte to be detected and the background buffer solution, heavy metal cations in the analyte to be detected can be accumulated at the contact surface in a decelerating way in the migration process, and the pre-concentration in the sample injection stage is realized. The principle of the deceleration accumulation of heavy metal cations on the contact surface is as follows: first, an amount of electroosmotic flow inhibitor is added to the background buffer and the sample matrix solution, so that the electroosmotic flow changes direction, but the size of the electroosmotic flow inhibitor cannot change the moving direction of heavy metal cations to the first liquid storage tank. Second, there is a concentration gradient between the background buffer and the analyte to be measured, so that the conductivities of the two are different, and the migration rate of heavy metal cations in the two is also different, and the migration rate in the analyte to be measured is greater than that in the background buffer. Therefore, when the heavy metal cations pass through the contact surface, the migration velocity decreases, and thus the heavy metal cations accumulate at a reduced speed. Furthermore, both the background buffer and the analyte to be measured have a pH gradient, and the pH is an important factor affecting electroosmotic flow, which are in positive correlation. At this time, due to the presence of the electroosmotic flow inhibitor, the direction of the electroosmotic flow is directed from the first reservoir to the sample cell, and the pH value of the analyte to be measured is lower than that of the background buffer, and the electroosmotic flow in the analyte to be measured is smaller than that in the background buffer. Therefore, when the heavy metal cations pass through the contact surface, the migration rate of the heavy metal cations becomes small due to the increase of the electroosmotic flow, and the heavy metal cations are further decelerated and accumulated at the contact surface.
The separation stage is as in fig. 2C: and switching the voltage, applying 1000V voltage to the second liquid storage tank, and simultaneously grounding the third liquid storage tank, suspending the sample tank and the first liquid storage tank, wherein the duration is 80s. At this stage, the analyte to be measured is pushed into the second channel at the cross and different kinds of heavy metal cations in the second channel gradually separate to form different zones due to the different migration rates so as to pass through the detection zone. In the process, two contact surfaces are formed between the front and rear of the sample zone and the background buffer solution, the contact surfaces in the migration direction are the same as the sample injection stage, the deceleration accumulation is continued, and heavy metal cations on the rear contact surface can migrate to the contact surfaces in the migration direction rapidly due to concentration gradient and pH value gradient, so that the sample zone can be continuously narrowed, and the pre-concentration effect of the heavy metal cations is further improved. .
The detection phase is as shown in fig. 2D: the sample zone is pre-concentrated until reaching the detection zone, and after reaching the detection zone, the different kinds of heavy metal cations are pre-concentrated and separated. And drawing an electrophoresis chart by collecting amplitude change caused by conductivity change in the detection area.
Example 1
This example investigated the effect of different electroosmotic flow inhibitor (CTAB) concentrations (0.0025, 0.01, 0.025, 0.05, 0.1mM/L on electroosmosis and heavy metal cations) in background buffers cadmium chloride was dissolved in distilled water and mixed 1:1 with 20mM sample base fluid to prepare standard samples for analysis.
The sample matrix liquid contains 20 mM/L2- (N-morpholinyl) ethanesulfonic acid (MES), 20mM/L L-histidine (His) and electroosmotic flow inhibitor with corresponding concentration, and the pH value is 6.1. The background buffer contained 20mM 2- (N-morpholino) ethanesulfonic acid (MES), 20mM L-histidine (His) and electroosmotic flow inhibitor at corresponding concentrations, pH 6.1.
The specific steps of analysis of the concentration of the electroosmotic flow inhibitor are:
1) Flushing microchannels
When the micro-channel is used for the first time, the micro-channel is firstly washed for 15min by using 1mM NaOH solution, and is washed for 15min by distilled water; not for the first time, the microchannels were rinsed with 1mM NaOH solution for 5min and distilled water for 10min.
2) The micro-channels were background buffer rinsed for 10-15min before use.
3) 50 μl of the sample was injected into the sample cell, and a high voltage platinum electrode was inserted.
4) In the sample injection stage, 500V voltage is applied to the sample pool, the first liquid storage pool is grounded, the second liquid storage pool and the third liquid storage pool are suspended, and the duration is 14s.
5) In the separation stage, 1000V is applied to the second liquid storage tank, the third liquid storage tank is grounded, and the sample tank and the first liquid storage tank are suspended for 80 seconds.
The results obtained are shown in FIG. 3. From the graph, the peak value of heavy metal cadmium becomes smaller and the migration time becomes longer with the increase of the concentration of the electroosmotic flow inhibitor. This is associated with a suppressed electroosmotic flow. When the concentration of the electroosmotic flow inhibitor reaches 0.05mM/L, the electrophoresis peak of the heavy metal cadmium completely disappears, which indicates that the electroosmotic flow is reversed and the migration direction of ions is changed at the moment. When the concentration of the electroosmotic flow inhibitor is between 0.0025 and 0.025mM/L, the electrophoresis peak value of heavy metal cadmium becomes smaller, and the migration time increases, which indicates that the electroosmotic flow is reversed but the ion migration direction is unchanged. In summary, the present invention selects 0.01mM/L of electroosmotic flow inhibitor for subsequent studies.
Example 2
This example investigated the effect of different concentrations of sample matrix fluid (1.25, 2.5, 10, 15, 20 mM/L) on the pre-concentration effect. Cadmium chloride is dissolved in distilled water and mixed with sample matrix liquid with different concentrations in a ratio of 1:1 to prepare a standard sample for analysis.
The sample matrix solution contained different concentrations (1.25, 2.5, 10, 15, 20 mM/L) of 2- (N-morpholino) ethanesulfonic acid (MES), L-histidine (His) and 0.01mM/L of electroosmotic flow inhibitor, and the pH value was 6.1. The background buffer contained 20mM 2- (N-morpholino) ethanesulfonic acid (MES), 20mM L-histidine (His) and 0.01mM/L electroosmotic flow inhibitor, pH 6.1; the other conditions were the same as in example 1.
The specific steps of the analysis of the sample matrix liquid are as follows:
1) Flushing microchannels
When the micro-channel is used for the first time, the micro-channel is firstly washed for 15min by using 1mM NaOH solution, and is washed for 15min by distilled water; not for the first time, the microchannels were rinsed with 1mM NaOH solution for 5min and distilled water for 10min.
2) The micro-channels were background buffer rinsed for 10-15min before use.
3) 50 μl of the sample was injected into the sample cell, and a high voltage platinum electrode was inserted.
4) In the sample injection stage, 500V voltage is applied to the sample pool, the first liquid storage pool is grounded, the second liquid storage pool and the third liquid storage pool are suspended, and the duration is 14s.
5) In the separation stage, 1000V is applied to the second liquid storage tank, the third liquid storage tank is grounded, and the sample tank and the first liquid storage tank are suspended for 80 seconds.
The results obtained are shown in FIG. 4. As can be seen from the figure, the electrophoresis peak height and sensitivity increase with decreasing concentration of the sample matrix liquid. However, when the concentration of the sample matrix liquid was reduced from 2.5mM/L to 1.25mM/L, the peak height and sensitivity of the electrophoresis were reduced from 24.10mV and 48.20mV/mM to 20.63mV and 42.26mV/mM. This is because, when the concentration of the sample matrix liquid is too small, the concentration difference between the sample matrix liquid and the background buffer liquid (BGE) is too large, and the electroosmotic flow at the interface between the two is disturbed, so that the concentration effect is deteriorated. Thus, the present invention selects 2.5mM/L as the optimal concentration of the sample matrix fluid for subsequent investigation.
Example 3
This example investigated the effect of sample matrix liquid pH values (3, 3.5, 4, 4.5, 6) on the effect of pre-concentration. Cadmium chloride was dissolved in distilled water and mixed 1:1 with 2.5mM/L of sample base solution, and pH was adjusted using 4% (v/v) glacial acetic acid as a standard sample for analysis.
The sample matrix solution contained 2.5 mM/L2- (N-morpholinyl) ethanesulfonic acid (MES), 2.5mM/L L-histidine (His) and 0.01mM/L electroosmotic flow inhibitor. The background buffer contained 20 mM/L2- (N-morpholinyl) ethanesulfonic acid (MES), 20mM/L L-histidine (His) and 0.01mM/L electroosmotic flow inhibitor, pH 6.1.
The specific steps of the analysis of the sample matrix liquid are as follows:
1) Flushing microchannels
When the micro-channel is used for the first time, the micro-channel is firstly washed for 15min by using 1mM NaOH solution, and is washed for 15min by distilled water; not for the first time, the microchannels were rinsed with 1mM NaOH solution for 5min and distilled water for 10min.
2) The micro-channels were background buffer rinsed for 10-15min before use.
3) 50 μl of the sample was injected into the sample cell, and a high voltage platinum electrode was inserted.
4) In the sample injection stage, 500V voltage is applied to the sample pool, the first liquid storage pool is grounded, the second liquid storage pool and the third liquid storage pool are suspended, and the duration is 14s.
5) In the separation stage, 1000V is applied to the second liquid storage tank, the third liquid storage tank is grounded, and the sample tank and the first liquid storage tank are suspended for 80 seconds.
The results obtained are shown in fig. 5 and 6, and as the pH value decreases, the corresponding electrophoresis pattern ion peak height is higher, the peak width is narrower, the electrophoresis peak is about sharp, the peak height reaches 202mV at ph=3, and the Relative Standard Deviation (RSD) of the peak height under the same experimental conditions is 8.71%. In addition, migration time increases with decreasing pH. The increase in migration time may be due to the reduced pH value resulting in reduced electroosmotic flow and reduced mobility of the ions to be tested. At the same time, the increase in migration time also increases the time for sample zones to accumulate at the interface. The decrease in peak width is due to the fact that as the pH value decreases, the concentration effect of the ions to be measured increases, and the shorter the time for the ions to pass through the detection region, the smaller the peak width. In summary, the invention selects the sample matrix liquid with the pH value of 3 as the optimal pH value for subsequent researches.
Example 4
The actual pre-concentration effect of the present invention was observed. The conventional microchip electrophoresis method is compared with the pre-concentration method proposed by the invention. The lead-cadmium contaminated soil extract was mixed 1:1 with 2.5mM/L of sample matrix solution and the pH was adjusted to 3 using 4% (v/v) glacial acetic acid as the sample for analysis.
The sample matrix solution contained 2.5 mM/L2- (N-morpholinyl) ethanesulfonic acid (MES), 2.5mM/L L-histidine (His) and 0.01mM/L electroosmotic flow inhibitor, and had a pH of 3.0. The background buffer contained 20mM 2- (N-morpholino) ethanesulfonic acid (MES), 20mM L-histidine (His) and 0.01mM/L electroosmotic flow inhibitor, pH 6.1.
The specific steps of the analysis of the sample matrix liquid are as follows:
1) Flushing microchannels
When the micro-channel is used for the first time, the micro-channel is firstly washed for 15min by using 1mM NaOH solution, and is washed for 15min by distilled water; not for the first time, the microchannels were rinsed with 1mM NaOH solution for 5min and distilled water for 10min.
2) The micro-channels were background buffer rinsed for 10-15min before use.
3) 50 μl of the sample was injected into the sample cell, and a high voltage platinum electrode was inserted.
4) In the sample injection stage, 500V voltage is applied to the sample pool, the first liquid storage pool is grounded, the second liquid storage pool and the third liquid storage pool are suspended, and the duration is 14s.
5) In the separation stage, 1000V is applied to the second liquid storage tank, the third liquid storage tank is grounded, and the sample tank and the first liquid storage tank are suspended for 80 seconds.
The result is shown in FIG. 7, and in the microchip electrophoresis non-contact conductivity detection method not using the dual gradient pre-concentration method proposed by the present invention, separated lead and cadmium electrophoresis peaks were observed, but the signal intensity was weak and the degree of separation was poor, as shown in FIG. 7 (a). After the online double-gradient pre-concentration detection method for the heavy metal ions in the soil based on microchip electrophoresis non-contact conductivity detection is provided, lead and cadmium are well concentrated, and strong enhancement peaks appear at 42s and 55s respectively, as shown in fig. 7 (b). In addition, the pre-concentration method of the invention allows the separation degree (R) of lead and cadmium to be improved visually. The increase in separation is related to the aggregation effect of the on-line dual gradient pre-concentration method on ions in the sample zone. Compared with the traditional microchip electrophoresis method, the pre-concentration method provided by the invention improves the sensitivity of heavy metal lead and cadmium by 26.4 times and 43.73 times respectively. In addition, the pre-concentration method provided by the invention is adopted to finish the pre-concentration and separation detection of heavy metal ions within 90 seconds. Therefore, the experiment proves that the invention can obtain a microchip with larger preconcentration multiple, simplicity and low cost in extremely short experiment time, and the preconcentration and the separation detection of heavy metal ions can be completed by simple operation.
The above examples are provided for illustrating the preferred embodiments of the present invention and are not intended to limit the scope of the present invention, and any person skilled in the art should make various improvements and modifications within the scope of the present invention defined by the appended claims without departing from the spirit and scope of the present invention.
Claims (9)
1. A microchip electrophoresis non-contact conductivity detection-based soil heavy metal ion online double-gradient preconcentration detection method is characterized by comprising the following steps of: the method comprises the following steps:
preparing a sample of the analyte to be measured: immersing sample soil in an extracting agent to obtain a sample extracting stock solution; mixing the sample leaching stock solution with a low-concentration buffer solution, and then adding acid to reduce the pH value of the mixture to obtain an analyte sample to be detected;
preparing a high-concentration buffer solution as a background buffer solution, wherein the concentration of the high-concentration buffer solution is higher than that of the low-concentration buffer solution;
providing a microchip: the microchip comprises a cross micro-channel, wherein the cross micro-channel comprises a first channel and a second channel which are mutually communicated, a sample pool and a first liquid storage pool are respectively formed at two ends of the first channel, and a second liquid storage pool and a third liquid storage pool are respectively formed at two ends of the second channel; a detection area is arranged at one end of the second channel, which is close to the third liquid storage tank, and the detection area consists of a transmitting electrode and a receiving electrode;
the microchip electrophoresis process comprises a preparation stage, a sample injection stage, a separation stage and a detection stage, wherein:
the preparation phase process is as follows: after flushing the cross micro-channel by using a background buffer solution, filling the background buffer solution into the sample cell, the first liquid storage tank, the second liquid storage tank and the third liquid storage tank;
the sample injection stage process is as follows: replacing background buffer solution in the sample pool with an analyte sample to be detected, and respectively placing a high-voltage electrode in the sample pool, the first liquid storage pool, the second liquid storage pool and the third liquid storage pool; applying voltage to the sample pool, and simultaneously grounding the first liquid storage pool, suspending the second liquid storage pool and the third liquid storage pool; at this time, the analyte to be detected is pre-concentrated and moves to the intersection of the cross micro-channel;
the separation stage process is as follows: applying voltage to the second liquid storage tank, simultaneously grounding the third liquid storage tank, suspending the sample tank and the first liquid storage tank, pushing the analyte to be detected into the second channel, and gradually separating different types of heavy metal cations in the second channel to form different sample zones and passing through the detection area due to different migration rates;
the detection phase process is as follows: when the sample zone reaches the detection area, an electrophoresis pattern is drawn by collecting amplitude change caused by conductivity change.
2. The on-line dual-gradient preconcentration detection method for soil heavy metal ions based on microchip electrophoresis non-contact conductivity detection as set forth in claim 1, wherein the method comprises the following steps: the leaching agent is ammonium acetate solution.
3. The on-line dual-gradient preconcentration detection method for soil heavy metal ions based on microchip electrophoresis non-contact conductivity detection as set forth in claim 1, wherein the method comprises the following steps: the low-concentration buffer solution and the high-concentration buffer solution have the same components and comprise 2- (N-morpholinyl) ethanesulfonic acid, L-histidine and electroosmotic flow inhibitor; the concentration of 2- (N-morpholino) ethanesulfonic acid in the low concentration buffer solution is 1.25-10mM/L, L-histidine, the concentration of electroosmotic flow inhibitor is 0.0025-0.1M/L; the concentration of 2- (N-morpholino) ethanesulfonic acid in the high concentration buffer solution is 20mM/L, L-histidine, the concentration of the electroosmotic flow inhibitor is 0.0025-0.1M/L.
4. The online double-gradient preconcentration detection method for soil heavy metal ions based on microchip electrophoresis non-contact conductivity detection according to claim 3, wherein the method comprises the following steps of: the concentration of the electroosmotic flow inhibitor in the low concentration buffer solution and the high concentration buffer solution is 0.01mM/L.
5. The on-line dual-gradient preconcentration detection method for soil heavy metal ions based on microchip electrophoresis non-contact conductivity detection as set forth in claim 3 or 4, wherein the method comprises the following steps: the electroosmotic flow inhibitor is cetyl trimethyl ammonium bromide.
6. The online double-gradient preconcentration detection method for soil heavy metal ions based on microchip electrophoresis non-contact conductivity detection according to claim 3, wherein the method comprises the following steps of: the concentration of 2- (N-morpholino) ethanesulfonic acid in the low concentration buffer is 2.5mM/L, L-histidine and the concentration is 2.5mM/L.
7. The on-line dual-gradient preconcentration detection method for soil heavy metal ions based on microchip electrophoresis non-contact conductivity detection as set forth in claim 1, wherein the method comprises the following steps: the acid is glacial acetic acid, hydrochloric acid or nitric acid; the pH value of the analyte sample to be detected is 3.0-6.0.
8. The on-line dual-gradient preconcentration detection method for soil heavy metal ions based on microchip electrophoresis non-contact conductivity detection as set forth in claim 1, wherein the method comprises the following steps: the voltage applied to the sample cell is 500-2000V; the voltage applied to the second reservoir is 1000-2000V.
9. The on-line dual-gradient preconcentration detection method for soil heavy metal ions based on microchip electrophoresis non-contact conductivity detection as set forth in claim 1, wherein the method comprises the following steps: the high-voltage electrode is a platinum metal electrode.
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