CN112201561A - Time-of-flight mass spectrometer reaction chamber - Google Patents
Time-of-flight mass spectrometer reaction chamber Download PDFInfo
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- 238000006243 chemical reaction Methods 0.000 title claims abstract description 164
- 230000005540 biological transmission Effects 0.000 claims abstract description 45
- 239000012855 volatile organic compound Substances 0.000 claims abstract description 37
- 239000012495 reaction gas Substances 0.000 claims description 34
- 239000007789 gas Substances 0.000 claims description 30
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Chemical compound O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims description 19
- 238000000605 extraction Methods 0.000 claims description 13
- 230000037427 ion transport Effects 0.000 claims description 4
- 238000001514 detection method Methods 0.000 abstract description 34
- 230000035945 sensitivity Effects 0.000 abstract description 23
- 238000000034 method Methods 0.000 abstract description 8
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- UHOVQNZJYSORNB-UHFFFAOYSA-N Benzene Chemical compound C1=CC=CC=C1 UHOVQNZJYSORNB-UHFFFAOYSA-N 0.000 description 12
- YXFVVABEGXRONW-UHFFFAOYSA-N Toluene Chemical compound CC1=CC=CC=C1 YXFVVABEGXRONW-UHFFFAOYSA-N 0.000 description 12
- 230000000694 effects Effects 0.000 description 11
- -1 hydronium ions Chemical class 0.000 description 10
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- 125000004805 propylene group Chemical group [H]C([H])([H])C([H])([*:1])C([H])([H])[*:2] 0.000 description 4
- 238000001184 proton transfer reaction mass spectrometry Methods 0.000 description 4
- 239000004205 dimethyl polysiloxane Substances 0.000 description 3
- 229920000435 poly(dimethylsiloxane) Polymers 0.000 description 3
- 238000005086 pumping Methods 0.000 description 3
- 238000001196 time-of-flight mass spectrum Methods 0.000 description 3
- 230000008859 change Effects 0.000 description 2
- 239000003153 chemical reaction reagent Substances 0.000 description 2
- 239000011248 coating agent Substances 0.000 description 2
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- 238000004088 simulation Methods 0.000 description 2
- 238000002834 transmittance Methods 0.000 description 2
- 238000004458 analytical method Methods 0.000 description 1
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- 229910052739 hydrogen Inorganic materials 0.000 description 1
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- 229910001220 stainless steel Inorganic materials 0.000 description 1
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- H01J49/00—Particle spectrometers or separator tubes
- H01J49/26—Mass spectrometers or separator tubes
- H01J49/34—Dynamic spectrometers
- H01J49/40—Time-of-flight spectrometers
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- G—PHYSICS
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- G01N27/62—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating the ionisation of gases, e.g. aerosols; by investigating electric discharges, e.g. emission of cathode
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
- H01J49/00—Particle spectrometers or separator tubes
- H01J49/02—Details
- H01J49/04—Arrangements for introducing or extracting samples to be analysed, e.g. vacuum locks; Arrangements for external adjustment of electron- or ion-optical components
- H01J49/0422—Arrangements for introducing or extracting samples to be analysed, e.g. vacuum locks; Arrangements for external adjustment of electron- or ion-optical components for gaseous samples
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Abstract
The invention relates to an analytical detection instrument, in particular to a reaction chamber of a time-of-flight mass spectrometer. The first differential vacuum cavity is communicated with the reaction tube through a first small hole, the reaction tube is communicated with the second differential vacuum cavity through a second small hole, the second differential vacuum cavity is communicated with the ion transmission area through a third small hole, the ion transmission area is communicated with the detection area through a differential flow guide hole, a single lens is arranged in the ion transmission area, the single lens is composed of three groups of electrodes which are arranged left and right, and each group of electrodes is composed of two vertically opposite electrodes. The method can obviously improve the accuracy and sensitivity of the determination of the VOCs gas sample; after passing through the reaction chamber, the ion fragments are few, the mass spectrum background noise is low, the sensitivity is high, and the measuring range is wide.
Description
Technical Field
The invention belongs to the field of analytical detection instruments, and particularly relates to a reaction chamber of a time-of-flight mass spectrometer.
Background
With the promulgation of the national ten-best, the determination of volatile organic compounds in air and waste gas becomes the key content of atmospheric monitoring, and in order to obtain the content of VOCs in the air of enterprises or in the environment of an emergency in real time, proton transfer reaction mass spectrometry (PTR-MS) capable of rapidly determining the content of VOCs in air is more and more widely applied. At present, more PTR-MS is developed, the PTR-MS is usually suitable for measuring VOCs gas samples with higher concentration, and sample molecules and reaction gas are subjected to proton transfer reaction and then are screened by a quadrupole mass analyzer and then are detected by a detector.
In addition, the substance to be detected is screened by using a four-stage rod mass filter after being protonated, the resolution ratio is low, the quality discrimination effect is obvious, the method is often used for measuring VOCs molecules with small molecular weight, and the application range is small.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to provide a reaction chamber of a time-of-flight mass spectrometer, which can improve the reaction pressure of a reaction tube, increase the gas inflow and obviously improve the accuracy and the sensitivity of the determination of a VOCs gas sample; after passing through the reaction chamber, the ion fragments are few, the mass spectrum background noise is low, the sensitivity is high, the single-lens focusing application range is wide, the focusing effect is good, and the improvement of the determination sensitivity can be realized.
The invention relates to a reaction chamber of a time-of-flight mass spectrometer, which comprises an ion source glow discharge region, an ion source drift region, a first differential vacuum chamber and a reaction tube which are sequentially adjacent, wherein the reaction tube is sequentially connected with a second differential vacuum chamber and an ion transmission region, the first differential vacuum chamber is communicated with the reaction tube through a first small hole, the reaction tube is communicated with the second differential vacuum chamber through a second small hole, the second differential vacuum chamber is communicated with the ion transmission region through a third small hole, a differential diversion hole is arranged on the wall of one side of the ion transmission region, which is far away from the second differential vacuum chamber, a single lens is arranged in the ion transmission region, the single lens is composed of three groups of left and right electrodes, and each group of electrodes is composed of two vertically opposite electrodes.
Wherein:
equivalent voltage-dividing resistors are arranged outside the ion source drift region, the first differential vacuum cavity and the reaction region of the reaction tube.
The end of the first differential vacuum cavity is connected with the first mechanical pump, and the end of the second differential vacuum cavity is connected with the second mechanical pump. The vacuum degree in the reaction tube is maintained by a second mechanical pump.
And a VOCs gas inlet is arranged at a position, close to the first differential vacuum cavity, on the wall of the reaction tube, and a VOCs gas outlet is arranged on the second differential vacuum cavity.
The single lens is formed by three groups of electrodes which are placed left and right, each group of electrodes is formed by two opposite electrodes from top to bottom, wherein two ends of the electrodes are used as extraction lens electrodes, the middle electrode is used as a transmission lens electrode, and the applied voltage is respectively: the voltage range of the extraction lens electrode is-6V-0V, and the voltage range of the transmission lens electrode is-80V-110V.
The anode voltage of the ion source glow discharge region is 400-550V, the discharge potential between the anode and the cathode is maintained between 350V and 400V, the pressure of the ion source glow discharge region is 10-100Pa, and the discharge current is 4-6 mA; the pressure of the ion transport region was 10-4In the order of mbar.
In order to obtain better ionization efficiency of reaction gas, the anode voltage of the glow discharge area of the ion source needs to be ensured to be 400-550V, and the cathode voltage is caused to float downwards to about 750V by the supply of glow discharge before the plasma starts after the glow discharge is switched on; below the anode set point. Therefore, if the anode voltage is 450V, the cathode voltage will drop to about-300V. When the plasma starts, the cathode voltage starts to change. At this time, a voltage difference of the sustain discharge between the anode and the cathode is maintained between 350V and 400V. The pressure of the glow discharge area of the ion source is about 10-100Pa, and the discharge current is 4-6 mA. Generation of large quantities of H by cathode glow discharge]+、[H3O]+And further reacting in the ion source drift region to generate high-purity [ H ]3O]+. Reactive gas ion [ H ]3O]+The mixture enters a reaction tube to collide with VOCs molecules R, wherein R represents the molecules, and the reaction is carried out to generate molecules to be detected [ RH [ -RH [ ]]+. In the reaction of the reaction gas ionsThe tube can simultaneously carry water vapor to different degrees, in order to reduce water molecules entering the reaction tube as much as possible and improve the purity of water and protons, a first differential vacuum chamber is designed and installed between the ion source drift region and the reaction tube, and neutral reaction gas (H) is pumped by a first mechanical pump at the tail end of the first differential vacuum chamber2O)。
The reaction chamber is provided with a reaction gas inlet which is connected with water vapor generation equipment, and the gas flow of the reaction gas inlet is 3-14 mL/min.
The gas flow at the reaction gas inlet is 3-14mL/min, so that the ion abundance of hydronium ions reaches 1.6 multiplied by 105The above.
The inner diameter of the first small hole is 0.05-0.5 cm; the diameters of the second small hole and the third small hole are both 0.1-1.0cm, and the inner diameter of the differential flow guide hole is 1-1.5 mm.
The pressure in the reaction tube is 5-10mbar, the applied voltage at two ends of the reaction tube is 1.5-3KV, and the sample gas inlet amount is 200-; the length of the reaction tube is 90-120 cm.
The direct current voltage applied to two ends of the reaction zone of the reaction tube is 1.5-3KV, and an equivalent voltage-dividing resistor is arranged outside the reaction zone to divide the direct current voltage outside the reaction tube, so that a uniform electric field is formed in the reaction tube. Keeping the E/n within the range of 170-. The material and length of the reaction tube have important influence on the sensitivity and resolution of the measurement of the instrument, and the reaction tube made of stainless steel can generate adsorption effect and memory effect, so that the inner wall of the reaction tube is modified by a polydimethylsiloxane coating, the adsorption and memory effect is reduced, and the measurement sensitivity is improved. The length of the reaction tube is designed to be 90-120cm, and the length can increase the probability of collision of VOCs and reaction reagent ions, improve the reaction efficiency and improve the measurement sensitivity of an instrument.
And a VOCs gas outlet is arranged on the second differential vacuum cavity.
The reaction chamber of the time-of-flight mass spectrometer is characterized in that the rear side of an ion transmission area is connected with a detection area, a differential diversion hole is positioned between the ion transmission area and the detection area, the detection area is just a conventional detection area, and an XY deflection plate and a detector are generally arranged in the detection area.
Wherein:
equivalent voltage dividing resistors are arranged outside the ion source drift region, the first differential vacuum cavity and the reaction region of the reaction tube, direct current voltage is applied, and the direction of an electric field is from left to right along the axial direction of the drift tube; the reaction tube focus area is provided with high-frequency alternating voltage, and the direction of an electric field is in the radial direction of the drift tube.
Through the isolation effect of the first differential vacuum cavity, the influence of the pressure increase of the reaction tube on the ion source is reduced, and the air input of the reaction tube is increased to improve the sensitivity of the instrument.
The outside of the reaction chamber is coated with a heat-insulating layer, and the inside of the reaction chamber is provided with a heating wire, a fan and a temperature controller.
The invention reduces the influence of the pressure increase of the reaction tube on the ion source and increases the air input of the reaction tube to improve the sensitivity of the instrument by the isolation effect of the first differential vacuum cavity. Because the concentration content of VOCs in the environmental air sample is low, in order to improve the accuracy of sample determination, the pressure in the reaction tube is set to be 5-10mbar, the first mechanical pump is used for pumping vacuum, the direct current voltage applied to two ends of the reaction zone of the reaction tube is 1.5-3KV, and the sample gas inlet amount is 200-; and simultaneously applying high-frequency alternating voltage to a focusing area of the reaction tube, wherein the frequency of the alternating voltage is 0.4-0.6MHz, and the amplitude of the voltage is 40-60V. The length of the reaction tube is 90-120 cm.
The ion source comprises an ion source glow discharge region and an ion source drift region, and the interior of the reaction tube is divided into a reaction region with no space blockage and a focusing region; the ion source is provided with a reaction gas inlet which is connected with a water vapor inlet.
The reaction gas may be steam or O2NO, etc., the gas flow at the inlet of the reaction chamber is 3-14mL/min, so that the reaction gas is positively ionized ([ H ]3O]+、[O2]+Or [ NO ]]+) The abundance of the ions reaches 1.6 multiplied by 105Thus, the volatile organic compound molecule R is successfully converted into VOCs positive ions [ RH ]]+Or R+。
The first differential vacuum chamber serves a dual purpose, first, to further purify the reactant gas ions. And voltage is applied to the outside of the first differential vacuum cavity and is connected with equivalent resistance to divide voltage, and a uniform electric field is formed inside the first differential vacuum cavity. Under the action of electric field force, positive ions move towards the reaction tube, and neutral molecules do not have obvious orientation due to the fact that the neutral molecules do not move under the action of the electric field force and are pumped away through the molecular pump, so that the purpose that the ions of the reaction gas are continuously extracted from the reaction gas in the process of moving towards the reaction tube is achieved. Secondly, the pressure in the reaction tube can influence the ion source to a certain extent, so the pressure in the reaction tube which can be born by the ion source is limited, the pressure of the traditional drift tube can only reach 1-2mbar, the ion source and the reaction tube can be isolated to a greater extent by virtue of the first differential vacuum chamber, and the pressure in the reaction tube can reach 5-10mbar, so that the ion number density in the reaction tube can be improved, the sample introduction amount of the sample gas is increased to remarkably enhance the response value of measuring each VOCs substance, the sensitivity of the instrument is improved, and the accuracy of the measuring result is ensured. Through an ion motion simulation experiment, the ion current is monitored by using a Faraday cup arranged at the tail end of the reaction tube, the ion current intensity is 500nA when the first differential vacuum chamber is not arranged, and the ion current intensity is increased to 700nA when a low-concentration sample is measured by means of the action of the first differential vacuum chamber. Here the increased ion current intensity versus the increased sensitivity of the instrument is embodied in a significant reduction of the detection limit: when the first differential vacuum cavity is not arranged, the detection limit of the instrument can only reach a few ppt, and by arranging the differential vacuum cavity, the detection limit of each substance of the VOCs can reach the level below the ppt level. The inner diameter of the first small hole is 0.05-0.5cm, and the reaction gas is further pumped from the rear part of the first small hole through a molecular pump, so that the unprotonated reaction gas entering the reaction tube is reduced to the maximum extent.
The inner wall of the reaction tube is modified, and the modified material can use polydimethylsiloxane to reduce the adsorption of VOCs molecules.
An equivalent voltage dividing resistor is arranged outside the reaction zone of the reaction tube to divide the direct current voltage outside the reaction tube, so that a uniform axial electric field is formed in the reaction tube, and axial power is provided for ion movement; the VOCs molecules collide with the positive ions of the reaction gas in the tube to react.
When the sample is measured, the pressure in the reaction tube is 5-10mbar, the direct current voltage applied to two ends of the reaction tube is 1.5-3KV, and the sample gas introduction amount is 200-; and simultaneously applying high-frequency alternating voltage to the focusing area, wherein the frequency of the alternating voltage is 0.4-0.6MHz, the amplitude of the voltage is 40-60V, and the voltage provides radial constraint force for the movement of ions so that the ions are focused to the center of the reaction tube, thereby increasing the ion number passing through the second differential vacuum pore. Through an ion motion simulation experiment, ion current is monitored by a Faraday cup, and the ion passing rate can be improved by 10 times through the focusing effect of the high-frequency alternating voltage arranged at the ion current. The length of the reaction tube is 90-120cm, and the increase of the length can increase the probability of collision of VOCs and reaction reagent ions, improve the reaction efficiency and improve the measurement sensitivity of an instrument.
The single lens is formed by placing three groups of electrodes left and right, each group of electrodes is two pieces which are opposite up and down, wherein, the two ends of the electrodes are extraction lens electrodes, the middle electrode is a transmission lens electrode, and the ion flow focusing is completed by adjusting the voltage value of the electrodes. The extraction lens electrode voltage is high, the transmission lens electrode voltage is low, and the voltages applied by the three groups of electrodes are respectively as follows: the voltage range of the extraction lens electrode is-6V-0V, and the voltage range of the transmission lens electrode is-80V-110V. The ion transport zone pressure is maintained at 10 by a first turbomolecular pump-4In the mbar class.
The ion transmission area is communicated with the detection area through a differential flow guide hole, the inner diameter of the differential flow guide hole is 1-1.5mm, and the pressure in the detection area is 10-5-10-7In the mbar class.
According to the invention, the positive ions of the reaction gas can be generated and purified under the action of the electric field applied outside the first differential vacuum cavity, so that the purity of the reaction ions is improved; the reaction with VOCs molecules is completed in the modified reaction tube, and the influence of pressure increase in the reaction tube on the ion source is reduced by virtue of the isolation effect of the first differential vacuum cavity, so that the sample air input is improved, the molecule number density in the tube is increased, the sensitivity of the instrument can be obviously improved, and the anti-adsorption effect of a modifier also plays an important role in improving the sensitivity of the instrument; the direct current applied to the reaction region of the reaction tube and the high-frequency alternating current applied to the focusing region can screen and focus ions generated by the reaction, increase the density of the ions entering the ion transmission region, and improve the resolution of the instrument.
The focusing action and high transmittance of the single lens also contribute significantly to the sensitivity and resolution of the instrument.
The use process of the reaction chamber of the time-of-flight mass spectrometer comprises the following steps:
water vapor enters an ion source glow discharge area of the reaction chamber from a reaction gas inlet and is ionized by a hollow cathode lamp of the ion source to generate hydronium ions (H)3O]+Ion source generated [ H ]3O]+The reaction tube is arranged in the first differential vacuum cavity and collides with VOCs molecules R in the sample to be detected, and the reaction is carried out to generate molecules to be detected [ RH ]]+(ii) a A first differential vacuum chamber is arranged between the ion source drift region and the reaction tube, an equivalent voltage-dividing resistor is arranged outside the first differential vacuum chamber, direct current voltage is applied and is connected with the equivalent resistor for dividing voltage, a uniform electric field is formed inside the first differential vacuum chamber, and the generated hydrated hydrogen ions [ H ] are generated under the action of the electric field force3O]+Move to the reaction tube, and the reaction gas is neutral molecule H2O has no obvious orientation due to no electric field force, the end of the first differential vacuum chamber is connected with a first mechanical pump, and the first mechanical pump pumps neutral molecules H in the reaction gas2O, prevention of neutral molecules H2O enters a reaction tube; reaction-generated molecule [ RH ] to be detected]+Under the action of a uniform electric field in the reaction tube, the ion flow enters an ion transmission area through a second small hole, a single lens is arranged in the ion transmission area, the single lens is formed by placing three groups of electrodes left and right, each group of electrodes is formed by two vertically opposite electrodes, the two electrodes are extraction lens electrodes, the middle electrode is a transmission lens electrode, and the ion flow is focused by adjusting the voltage value of the electrodes.
The reaction chamber of the time-of-flight mass spectrometer is characterized in that the rear side of an ion transmission area is connected with a detection area, a differential diversion hole is positioned between the ion transmission area and the detection area, the detection area is just a conventional detection area, and an XY deflection plate and a detector are generally arranged in the detection area. The ions from the reaction chamber horizontally enter the detection area after passing through the differential diversion holes after being focused, the ions with different mass-to-charge ratios in the detection area are distinguished in the process of continuous movement, and the ions are guided to impact the detector, so that the detector can detect each group of ions.
In summary, the invention has the following advantages:
(1) the first differential vacuum cavity applies direct current voltage and is connected with equivalent resistance to divide voltage, and a uniform electric field is formed inside the first differential vacuum cavity. Under the action of electric field force, positive ions move to the reaction tube, while neutral molecules do not have obvious orientation due to the fact that the neutral molecules do not move under the action of electric field force, and therefore ions ([ H ] in the reaction gas are obtained3O]+) Continuously reacting gas (H) in the process of moving to the reaction tube2O) is extracted. Simultaneously, a first mechanical pump is used for pumping neutral molecules (H) in the reaction gas in the first differential vacuum chamber2O), neutral water molecules are prevented from entering the reaction tube, the purity of hydronium ions entering the reaction tube is improved, and the sensitivity of the instrument is improved.
(2) The invention relates to a reaction chamber of a time-of-flight mass spectrometer, which can obviously improve the accuracy of the determination of a VOCs gas sample. The first differential vacuum chamber is additionally arranged between the ion source drift region and the reaction tube, and the influence of pressure increase in the reaction tube on the ion source is reduced through the isolation effect of the first differential vacuum chamber, so that the pressure in the reaction tube can be greatly increased, the sample air input is improved, the molecule number density in the tube is increased, and the detection accuracy is obviously improved.
(3) The invention can obviously improve the sensitivity of VOCs determination. The inner wall of the used reaction tube is modified by a coating, so that the adsorption effect of VOCs molecules can be reduced; applying high-frequency alternating voltage to the reaction tube focusing area to focus ion flow; and the transmittance of ions entering a detection area is further improved through secondary focusing of the ion transmission area, and the ion fragments are few after passing through the reaction chamber, the mass spectrum background noise is low, and the sensitivity is high.
(4) The invention can select positive ions generated by ionizing various reaction gases such as water vapor and the like to react with VOCs molecules in a modified reaction tube to generate charged ions, then the charged ions enter a novel lens of a transmission area for focusing without using the traditional electrostatic quadrupole DCQ focusing separation, thereby avoiding the loss of the charged ions, increasing the range of the measured molecular weight, finally measuring by a detection area, and realizing the high-resolution detection of the molecules of the object to be measured by the precise cooperation of all parts of the whole system. Interference factors in the detection process are few; the ionization source is soft chemical ionization, the generation of fragments is less, the spectrogram is simple, the analysis is easy, the range of the measured molecular weight is wide, and the resolution and the sensitivity are high.
Drawings
FIG. 1 is a schematic structural view of embodiment 1 of the present invention;
in the figure: 1-a reaction gas inlet, 2-an ion source glow discharge region, 3-an ion source drift region, 4-a first small hole, 5-a first mechanical pump, 6-an equivalent divider resistor, 7-a second small hole, 8-a third small hole, 9-a second mechanical pump, 10-electrodes, 11-a differential diversion hole, 12-an ion transmission region, 13-a VOCs gas outlet, 14-a second differential vacuum cavity, 15-a reaction tube, 16-a first differential vacuum cavity and 17-a VOCs gas inlet.
Detailed Description
The present invention will be further described with reference to the following examples.
Example 1
A reaction chamber of a time-of-flight mass spectrometer is shown in figure 1 and comprises an ion source glow discharge region 2, an ion source drift region 3, a first differential vacuum chamber 16 and a reaction tube 15 which are adjacent in sequence, wherein the reaction tube 15 is connected with a second differential vacuum chamber 14 and an ion transmission region 12 in sequence, the first differential vacuum chamber 16 is communicated with the reaction tube 15 through a first small hole 4, the reaction tube 15 is communicated with the second differential vacuum chamber 14 through a second small hole 7, the second differential vacuum chamber 14 is communicated with the ion transmission region 12 through a third small hole 8, a differential flow guide hole 11 is formed in the wall of one side, away from the second differential vacuum chamber 14, of the ion transmission region 12, a single lens is arranged in the ion transmission region 12 and is composed of three groups of left and right electrodes 10, and each group of electrodes 10 is composed of two vertically opposite electrodes.
Equivalent voltage dividing resistors 6 are arranged outside the ion source drift region 3, the first differential vacuum chamber 16 and the reaction region of the reaction tube 15.
The first differential vacuum chamber 16 is connected at its end to the first mechanical pump 5 and the second differential vacuum chamber 14 is connected at its end to the second mechanical pump 9.
A VOCs gas inlet 17 is provided in the wall of the reaction tube 15 adjacent the first differential vacuum chamber 16.
The single lens is composed of three groups of left and right electrodes 10, each group of electrodes 10 is composed of two electrodes which are opposite up and down, wherein, the two end electrodes 10 are extraction lens electrodes, the middle electrode 10 is a transmission lens electrode, the applied voltage is respectively: the voltage range of the extraction lens electrode is-6V-0V, and the voltage range of the transmission lens electrode is-80V-110V.
The anode voltage of the ion source glow discharge area 2 is 400-550V, the discharge potential between the anode and the cathode is maintained between 350V and 400V, the pressure of the ion source glow discharge area 2 is 10-100Pa, and the discharge current is 4-6 mA; the pressure of the ion transport region 12 is 10-4In the order of mbar.
The reaction chamber is provided with a reaction gas inlet 1, the reaction gas inlet 1 is connected with a water vapor generating device, and the gas flow of the reaction gas inlet 1 is 3-14 mL/min.
The inner diameter of the first small hole 4 is 0.05-0.5 cm; the diameters of the second small hole 7 and the third small hole 8 are both 0.1-1.0cm, and the inner diameter of the differential diversion hole 11 is 1-1.5 mm.
The pressure in the reaction tube 15 is 5-10mbar, the applied voltage at two ends of the reaction tube 15 is 1.5-3KV, and the sample gas inlet amount is 200-; the length of the reaction tube 15 is 90-120 cm.
And the second differential vacuum cavity 14 is provided with a VOCs gas outlet 13.
For better ionization efficiency, the anode voltage of the glow discharge region 2 of the ion source is 450V, and the supply of the glow discharge causes the cathode voltage to drop to about-300V before the glow discharge is turned on but the plasma begins. When the plasma starts, the cathode voltage starts to change. At this time, the voltage difference of the sustain discharge between the anode and the cathode was maintained at 400V. The pressure of the ion source glow discharge area 2 is 100Pa, and the discharge current is 5 mA; generation of large quantities of H by cathode glow discharge]+、[H3O]+And further reacting in the ion source drift region to generate high-purity [ H ]3O]+. Before a sample is measured, the reaction chamber is heated by the heating wires arranged in the reaction chamber, the fan promotes the internal temperature to be balanced, the heat preservation layer coated outside keeps constant temperature, and the temperature is strictly controlled by a temperature controller to be 100 ℃.
Ion source generated [ H ]3O]+The water enters the reaction tube 15 with the length of 96cm after passing through the first differential vacuum chamber 16 and collides with VOCs molecules R to react, at the moment, the first differential vacuum chamber 16 effectively reduces the water molecules entering the reaction tube 15 and improves hydronium ions [ H ] H3O]+The purity of the (D) has an important effect on improving the sensitivity of the instrument; the inner diameter of the first small hole 4 is 0.1cm, so that reaction gas ions can effectively enter the reaction tube 15, and the ion source is separated from the reaction tube 15 to a greater extent. The reaction tube 15 is modified by polydimethylsiloxane to reduce the adsorption of organic molecules.
When a gas sample containing propylene, benzene, toluene, styrene, acetone and the like is measured, the ion source and the reaction tube 15 can be separated to a greater extent through the first differential vacuum chamber 16, the influence of pressure increase in the reaction tube 15 on the ion source is reduced, the pressure in the reaction tube 15 is controlled to be increased to 10mbar through a computer, the voltage at two ends is set to be 3KV, the E/n is maintained at 175, the sample introduction amount of the sample gas is 260ml/min, and therefore the ion number density in the reaction tube 15 is increased, and the response value of each substance to be measured is obviously enhanced. And simultaneously, high-frequency alternating voltage is applied to the focusing area, the frequency of the alternating voltage is 0.5MHz, the amplitude of the voltage is 55V, and the voltage provides radial constraint force for the movement of ions so that the ions are focused towards the center of the reaction tube, thereby increasing the ion number passing through the second differential vacuum small hole and improving the sensitivity of the instrument. In particular in terms of a significant reduction in detection limit: the lowest detection limit of the instrument can only reach several ppt without the first differential vacuum chamber 16, and the detection limit of each substance can reach the ppt level or less by providing the first differential vacuum chamber 16.
The products of protonation of propylene, benzene, toluene, styrene and acetone [ RH ] generated by the reaction]+Under the action of uniform electric field in the reaction tube 15, the reaction tube passes through the second small hole 7 and the third small hole 8 with the diameter of 0.5cm and then enters the ion transmission area 12, the ion transmission area 12 and the reaction tube 15 are isolated by the second small hole 7 and the third small hole 8, and the first turbo molecular pump is used for vacuumizing to ensure that the pressure of the ion transmission area 12 is 10-4mbar. The single lens is composed of three groups of left and right electrodes 10, each group of electrodes 10The device is composed of two electrodes which are opposite up and down, wherein two end electrodes 10 are extraction lens electrodes, a middle electrode 10 is a transmission lens electrode, and the voltages applied by the three groups of electrodes 10 are respectively as follows: the voltage of the extraction electrode is-3V and the voltage of the transmission electrode is-100V, so that the formed curved electric field is opposite to the [ RH ] of the entering electric field]+The focusing is carried out to form a concentrated ion flow, so that the problem of ion divergence is prevented, and the resolution of the instrument is improved. The focused ion flow horizontally enters a time of flight mass spectrum TOF detection area after passing through a differential diversion hole 11, the inner diameter of the differential diversion hole 11 is 1.0mm, the ion passing rate is ensured, an ion transmission area and the time of flight mass spectrum TOF detection area are effectively isolated, the second turbo molecular pump is used for pumping, and the high vacuum degree of the time of flight mass spectrum TOF area is 10-7mbar。
The sample containing propylene, benzene, toluene, styrene and acetone is measured by using water vapor as a reaction gas, and the influence of the first differential vacuum chamber on the detection limit is examined. The results of calculating the detection limits of the VOCs corresponding to the first differential vacuum chamber are not shown in table 1:
TABLE 1
Through the above comparison experiment of the detection limits of the VOCs components in the presence or absence of the differential vacuum chambers, it can be found that the existence of the first differential vacuum chamber reduces the detection limit of VOCs substances such as propylene, benzene, toluene, styrene, acetone and the like by 4.30 times to 6.34 times, which indicates that the first differential vacuum chamber plays a crucial role in improving the sensitivity of the instrument.
The use process comprises the following steps:
taking water as reaction gas as an example, water vapor enters an ion source glow discharge region 2 of the reaction chamber from a reaction gas inlet 1 and is ionized by a hollow cathode lamp of the ion source to generate hydronium ions [ H ]3O]+Ion source generated [ H ]3O]+Enters the reaction tube 15 after passing through the first differential vacuum cavity 16 to collide with the VOCs molecules R in the sample to be detected, and reacts to generate the molecules to be detectedRH]+(ii) a A first differential vacuum chamber 16 is arranged between the ion source drift region 3 and the reaction tube 15, an equivalent voltage-dividing resistor 6 is arranged outside the first differential vacuum chamber 16, direct current voltage is applied and is connected with an equivalent resistor voltage-dividing resistor 23, a uniform electric field is formed inside the first differential vacuum chamber 16, and the generated hydronium ions [ H ] are generated under the action of the electric field force3O]+Moves to the reaction tube 15, and the reaction gas is neutral molecule H2O has no obvious orientation due to the action of no electric field force, the end of the first differential vacuum chamber 16 is connected with a first mechanical pump 5, and the first mechanical pump 5 pumps neutral molecules H in the reaction gas2O, prevention of neutral molecules H2O enters the reaction tube 15; reaction-generated molecule [ RH ] to be detected]+Under the action of a uniform electric field in the reaction tube 15, the ion enters the ion transmission area 12 through the second small hole 7, a single lens is arranged in the ion transmission area 12, the single lens is formed by placing three groups of electrodes 10 left and right, each group of electrodes 10 is formed by two vertically opposite electrodes 10, the electrodes 10 at two ends are extraction lens electrodes, the middle electrode is a transmission lens electrode, and the focusing of ion current is completed by adjusting the voltage value of the electrodes.
Claims (10)
1. A time-of-flight mass spectrometer reaction chamber, characterized by: comprises an ion source glow discharge region (2), an ion source drift region (3), a first differential vacuum chamber (16) and a reaction tube (15) which are adjacent in sequence, wherein the reaction tube (15) is connected with a second differential vacuum chamber (14) and an ion transmission region (12) in sequence, the first differential vacuum chamber (16) is communicated with the reaction tube (15) through a first small hole (4), the reaction tube (15) is communicated with the second differential vacuum chamber (14) through a second small hole (7), the second differential vacuum chamber (14) is communicated with the ion transmission region (12) through a third small hole (8), a differential diversion hole (11) is formed in the wall of one side, away from the second differential vacuum cavity (14), of the ion transmission area (12), a single lens is arranged in the ion transmission area (12), the single lens is composed of three groups of left and right electrodes (10), and each group of electrodes (10) is composed of two vertically opposite electrodes.
2. The time-of-flight mass spectrometer reaction chamber of claim 1, wherein: equivalent voltage dividing resistors (6) are arranged outside the ion source drift region (3), the first differential vacuum chamber (16) and the reaction region of the reaction tube (15).
3. The time-of-flight mass spectrometer reaction chamber of claim 1, wherein: the end of the first differential vacuum cavity (16) is connected with the first mechanical pump (5), and the end of the second differential vacuum cavity (14) is connected with the second mechanical pump (9).
4. The time-of-flight mass spectrometer reaction chamber of claim 1, wherein: a VOCs gas inlet (17) is provided in the wall of the reaction tube (15) adjacent to the first differential vacuum chamber (16).
5. The time-of-flight mass spectrometer reaction chamber of claim 1, wherein: the single lens is composed of three groups of left and right placed electrodes (10), each group of electrodes (10) is composed of two opposite electrodes up and down, wherein, the two end electrodes (10) are extraction lens electrodes, the middle electrode (10) is a transmission lens electrode, the applied voltage is respectively: the voltage range of the extraction lens electrode is-6V-0V, and the voltage range of the transmission lens electrode is-80V-110V.
6. The time-of-flight mass spectrometer reaction chamber of claim 1, wherein: the anode voltage of the ion source glow discharge area (2) is 400-550V, the discharge potential between the anode and the cathode is maintained between 350V and 400V, the pressure of the ion source glow discharge area (2) is 10-100Pa, and the discharge current is 4-6 mA; the pressure of the ion transport region (12) is 10-4In the order of mbar.
7. The time-of-flight mass spectrometer reaction chamber of claim 1, wherein: the reaction chamber is provided with a reaction gas inlet (1), the reaction gas inlet (1) is connected with a water vapor generating device, and the gas flow of the reaction gas inlet (1) is 3-14 mL/min.
8. The time-of-flight mass spectrometer reaction chamber of claim 1, wherein: the inner diameter of the first small hole (4) is 0.05-0.5 cm; the diameters of the second small hole (7) and the third small hole (8) are both 0.1-1.0cm, and the inner diameter of the differential diversion hole (11) is 1-1.5 mm.
9. The time-of-flight mass spectrometer reaction chamber of claim 1, wherein: the pressure in the reaction tube (15) is 5-10mbar, the voltage applied to the two ends of the reaction tube (15) is 1.5-3KV, and the sample gas introduction amount is 200-; the length of the reaction tube (15) is 90-120 cm.
10. The time-of-flight mass spectrometer reaction chamber of claim 1, wherein: the second differential vacuum cavity (14) is provided with a VOCs gas outlet (13).
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