SU1760577A1 - Time-of-flight mass spectrometer - Google Patents

Abstract

The invention relates to instrumentation, in particular to mass-spectrometric instrumentation. SUMMARY OF THE INVENTION 1 A meter 11 is inserted into a mass spectrometer between the actual and the time of flight of ions of the reference component set during tuning, the sync pulse output of which is connected to 8 square pulses , the input is to the output of the broadband amplifier 9, the analog output is to the power supply unit 7 of the ion reflector. 1 silt (L S

Description

The invention relates to instrumentation, and more specifically to mass spectrometric instrumentation.

The known time-of-flight mass spectrometers consist of an ion source, a drift chamber, an ion reflector, an ion receiver, an analyzer power supply unit that includes an ion reflector power supply unit, a square pulse generator (GUI), and a broadband amplifier. recording device 1, 2. When analyzing the percentage of elements or isotopes in a substance using these time-of-flight mass spectrometers, the recording system uses an oscilloscope to measure the amplitude of the peaks of the mass spectrum (according to which the content is judged). signals are output from the silo, the launch sweep which

carried out by the sync pulses produced in the GUI.

Of great importance when measuring the amplitude of peaks using an oscilloscope is the constancy of the time of flight of ions with the same mass to charge ratio, since in this case it is easier to identify the peaks of the mass spectrum, which prevents gross errors associated with it. no less than the relative error in measuring the amplitude of the peaks of the mass spectrum in this case is still significant and is about 10%,

The time-of-flight mass spectrometer 3, taken as a prototype, consists of an ion source, a drift chamber, an ion reflector, an ion receiver, an analyzer power supply unit, including an ion reflector power supply unit, a GUI, a silo unit, a recording device

m

)about

| About SL

h

2

The ion reflector consists of a series of electrodes parallel with each other, supported under respective potentials formed by a resistor divider voltage at the input of the ion reflector power supply unit (as in 1).

In addition to the oscilloscope, pulse-analog transducers (PAPs), the amplitude of the output signals of which is proportional to the amplitude of the peak of the corresponding component, and a measuring-computing complex, designed to measure the amplitude of the PAP output signals and process information, are used as the recording system in the prototype.

Signals from the silo output are supplied to the input of the PAI. The operation of the PAI is synchronized using GPI clock pulses. The tuning of the transducers to the peaks consists in setting the moment when the key of each IAP is triggered, which coincides in time with the appearance at the FDR output of the signal of the corresponding component.

To ensure high accuracy in measuring the amplitude of the peaks of the mass spectrum for a long time, it is necessary that the average transit time T in the ion analyzer with the same mass to charge ratio does not change over time, since in this case the setting of the PAI mass spectrum remains optimal. T (m / q) in each cycle of the mass spectrometer with fixed parameters of the analyzer elements significantly depends on the place of formation of ions in the ion source and on the action of electric fields in the elements of the analyzer. The location of the formation of ions is determined by the geometrical parameters of the electron beam in the ion source, which are fundamentally unstable due to the effect on the electron beam of uncontrolled, time-varying parasitic electric fields formed by charges on the surface of the source design elements. This leads to a change with time T (m / q) and, consequently, to a decrease in the measurement accuracy of the amplitudes of the peaks of the mass spectrum using the prototype. The electric fields of the elements of the prototype analyzer with fixed geometrical parameters are determined by the following factors: the output voltages of the power supply unit of the elements of the analyzer; the parameters of a GUI pulse accelerating ions by the action of potentials induced by parasitic charges on the surface of the analyzer's structural elements (ion source, drift chamber, ion reflector)

The instability of these factors also leads to a change with time T (m / q) and, consequently, to a decrease in the measurement accuracy of the amplitudes of the peaks of the mass spectrum with

using the prototype.

Within a few hours after adjustment (from one to four hours - depending on the complexity of the gas analysis task), the prototype provides enough

0 accurate determination of the amplitude of the peaks of the mass spectrum, resulting in an absolute error in determining the concentration of components does not exceed 0.4 vol.% 3 However, due to an increase in the error in determining the amplitude of the peaks of the mass spectrum after more than 4 hours, for example, one day, the absolute error in determining component concentrations can be significantly greater than 0.4

0 vol.% And in some cases reach 100 vol.%. This occurs as a result of a change in T (: n q) under the action of the reasons listed above, which violates the optimality of adjusting the IAP to the peaks of the mass spectrum. The absolute error of determining the concentration of a component of 100 vol.% Can lead to a change in T (m / q) of only 50 ns.

Thus, the prototype does not provide the necessary accuracy of measuring the amplitude of the peaks of the mass spectrum after i h of 4 hours or more.

The aim of the invention is to improve the accuracy of determining the amplitude of the peaks

5 mass spectrum during long-term mass spectrometer operation.

The goal is achieved by the fact that during a flight mass spectrometer 3, which consists of an ion source, a drift chamber,

0 an ion reflector, an ion receiver, an analyzer power supply unit, including an ion reflector power supply unit, a GUI, a silo generator, a recording device, a meter for the time interval between the actual and the reference component set during tuning of the ion component of the clock input of the GUI input - to the output of the silo, analog output - to the unit

About the power reflector ions.

In the known technical solution 3, the average time of flight of ions of each component of the analyzed mixtures during long-term operation (more than 4 hours) of the mass spectrometer changes significantly under the influence of these ions (starting from the influence on the place of their formation in the ion source) uncontrolledly changing parasitic parameters. electric fields in the various elements of the analyzer Additional changes in flight time

Ions are also associated with the instability of the output voltages of the power supply units of the analyzer elements and the parameters of impulses of the GUI accelerating ions.

The change in the transit time of the ions relative to the reference, at which the channels of the recording system were tuned, reaches values in absolute value greater than 50 ns, which leads to a significant decrease in the measurement accuracy of the amplitudes of the peaks of the mass spectrum

In the proposed solution, due to the introduction of the time interval meter between the actual and the ion time of the reference component set during tuning, the sync pulse output of which is connected to the GUI clock input, the input to the silo output, the analog output to the ion reflector power supply unit, the actual time The span of the reference component ions is maintained equal to the initial one, at which the PAI channels were tuned, with an error not exceeding ± 1 not during the entire device operation time.

From this, it follows that the time interval meter proposed for use in the inventive device generates synchronization pulses used to start the GUI, a time delay corresponding to that set when setting the time of flight of the reference component ions, and a voltage proportional to the specified time interval. Devices that perform the function of measuring the time interval (forming voltages proportional to the interval) are widely known in the art, for example, in radiolocation (see 4), and their specific implementation for the device being claimed does not cause difficulties for a specialist in the field of electronics.

During the transit mass spectrometer of this type (to which both the analogs and the prototype belong), the ions move in quasistatic electric fields. Therefore, in a fairly good approximation, the time of flight of ions is directly proportional to Vfyj / q. Consequently, simultaneously with the stabilization of the actual time of flight of the ions of the reference component, the time of flight of the ions of other components stabilizes.

The importance of connecting the analog output introduced into the time interval meter device, specifically to the ion reflector power supply unit, is that the mass mass reflector resolution changes only slightly with a significant change in the ion transit time by adjusting the ion reflector voltage (more ). When using the invention, the transit times of these ions are maintained equal to the initial ones, for which the PAI channels were tuned with an error not exceeding ± 10 not for a long time.

The full stabilization of the transit time of ions of other (non-reference) components does not occur mainly for the following reason.

In the source, during ionization, ions are displaced under the action of parasitic fields even before they begin to accelerate as a result of the delivery of an ejector impulse generated by the GUI. it

the displacement depends on the mass of the ion; therefore, the energies with which the ions fly into the drift chamber from a source are different for ions of different masses.

Parasitic field in the process

the device changes arbitrarily, which leads to a change in the drift energy of ions with different mass-to-charge ratios and, therefore, to the deviation of their transit times from the original, for which the PAI channels were tuned. This deviation, in principle, changes during the operation of the instrument and most significantly varies for lightweight ions (M1,2). However, for them it in practice does not exceed ± 10 no.

The indicated stability of the time of flight of ions, achieved due to the combination of the listed distinguishing features, is sufficient so that during the long-term operation of the mass spectrometer (almost constantly), the IAP setting remains optimal. This provides sufficient accuracy in measuring the amplitude of the peaks of the mass spectrum for a long time.

The claimed set of features is not known. Introduction during the transit mass spectrometer and connecting in this way the meter of the time interval between the actual and the time of flight of ions of the reference component set during adjustment is new and used for the first time. As a result, it was possible to create transit time.

mass spectrometer, in which, fundamentally, without deteriorating its mass resolution and sensitivity, one of the main analytical parameters is stabilized — the time of movement of ions (regardless of mass to charge ratio), which leads to an increase in the accuracy of determining the amplitude of peaks of the mass spectrum during long-term operation of the device.

The scheme of the claimed device is shown in the drawing.

The device includes a mass spectrometer 1 consisting of an ion source 2, a drift chamber 3, a reflector 4 ions, an ion receiver 5, a power supply unit 6 for the elements of the mass spectrometer, an ion reflector power supply 7, GUI 8, the output of which connected to a source of 2 ions, a silo 9, the input of which is connected to the output of the receiver 5 ions, a recording device 10, an analog input of which is connected to the output of the silo, an external trigger input to the output of clock signals of the GUI 8, the meter 11 between the actual and set when e time span fiducial component ions, whose output is connected to the clock input of GUI 8, input - to the output of the silo 9, the analog output to the power unit 7 of the reflector ions.

The device works as follows.

A portion of the analyzed gas mixture is injected into the mass spectrometer 1. Its elements are supplied with voltage from the power supply unit 6 of the mass spectrometer elements, including the ion reflector power supply unit 7, as well as voltage pulses from the GUI 8, which is started from using clock pulses generated in the meter 11 time interval.

All this ensures the implementation of the following physical processes: ionization by electron impact of atoms and molecules of the components of the test gas and acceleration of the formed ions in the ion source 2. The passage of these ions in the drift space from the ion source to the reflector A of ions, their reflection in the reflector 4 ions and the span in the drift space from the reflector ions to the receiver 5 ions. At the output of the ion receiver, current pulses are formed, the amplitude of which is proportional to the concentrations of the corresponding components in the analyzed gas. The delay of these pulses relative to the onset of acceleration of ions in the ion source associated with the impulse delivery of the GUI is equal to the time of flight of the ions of the corresponding components in the analyzer and proportional to V rn / n. At this delay it is determined which component corresponds to the pulses at the output of the ion receiver. The amplitude of these pulses determines the concentration of the corresponding component.

Such an analysis is carried out in a recording device 10, to which pulses are supplied from the output of the analyzer ion receiver through silo 9, intended for their additional amplification.

In order to synchronize the operation of the registering device, clock pulses are used that are generated in the GUI simultaneously with voltage pulses intended

for initial ion acceleration.

The analyzer's mode of operation is adjusted to the optimum in resolution and sensitivity (which is conveniently monitored with an oscilloscope) in the same way as in analogs and prototypes by adjusting the corresponding voltage levels produced by the power supply unit of the mass spectrometer and GUI elements. Indicative values of the levels of these

stresses can be estimated by calculation.

Etc:- ; setting the analog output of the time interval meter 1 must be disconnected from the reflector power supply unit 7

ions.

After analyzer setup is completed, it is necessary to determine (using an oscilloscope) the time of flight of the reference component ions (usually Hz, since nitrogen is present in most gas phases of technical objects) and install it on the time interval meter. A voltage proportional to the time interval between the actual moment of arrival of the pulses corresponding to the ions of the reference component supplied from the silo output 9 ns meter input 11 and the time corresponding to the established one, which may not coincide due to errors of measurement and setting the time of flight of the reference component ions.

The polarity of the output analog voltage of the meter 11, when applied to control the output voltage of the power supply unit of the V0rp ion reflector, is set so that a change occurs

0 Watr, resulting in a decrease in the difference in the time of flight of the reference component ions from the established time. This keeps the time of flight of the reference component ions equal to that established with high accuracy over a long time. In this case, the transit times of all other ions corresponding to other components of the mass spectrum are stabilized, since the time of movement of the ions during the transit mass spectrometer in a fairly good approximation is directly proportional to

At PTI, the claimed device was manufactured using a prototype mass spectrometer.

The ion reflector is two-gap. Potentials are supplied to the reflector electrodes from the resistive divider ion reflector installed at the output of the power supply unit.

Adjusting the operating mode of the mass spectrometer to the optimum in resolution and sensitivity (controlled with an oscilloscope) was made by adjusting the appropriate voltage levels generated by the power supply unit of the mass spectrometer and the GUI elements with the output of the ion reflector power supply off. Approximate values of these levels for the transit time of the ions of the component used as a reference (nitrogen; ions He +: M - 28), equal to 10 µs, were estimated by calculation. Ion transit time controlled with an oscilloscope, was set with an adjustment equal to 10 μs with an error of ± 200 ns. In this case, the ion drift energy in the zero-free space took values in the range from 700 to 800 eE (with a width of the ionization region of 1 mm). The width of the first gap of the ion reflector was 5 mm; electric field intensity is 100 V / mm. The width of the second gap is 40 mm; electric field intensity is 10 V / mm.

The duration of the peak corresponding to single charge ions with M 28 at the level of 10% of the peak height from the base, due to a number of factors, one of which is the ion drift energy spread, was not 50. In this case, as can be calculated using the above data, the time of movement of these ions in the reflector, depending on their drift energy in the no-field space, took values in the range from 2.351 (drift energy 700 eV) to 2.816 μs (drift energy 800 eV).

It can also be calculated that the movement time of the said ions from M 28 in the reflector while reducing the voltage of its power supply unit by 5% takes values in the range from 2.474 (drift energy 700 eV) to 2.964 μs (drift energy 800 eV).

From the data it follows that with a decrease in the voltage of the power supply unit of the reflector by 5% of its optimal value, the time of flight of one-charge ions from M 28 to 135 n does not increase and the duration of the corresponding mass spectrum peak increases by a maximum of 25 n.

Thus, in a mass reflectron, an insignificant change in the voltage on the ion reflector with other parameters unchanged leads to a significant change in the time of flight of ions for recording (since the time width of the windows of the recording channels is 100). What is happening

0 while a slight increase in the duration of the peaks (defocusing of the ion packets) for recording the intensity of the peaks of the mass spectrum is not significant.

Change by 100 is not flight time

5 in some other way, for example, by changing the ion drift energy, leads to a substantial defocusing of ion packets.

Hard time was established by

0 discrete switching in the time interval meter of 10 µs (Tdr), with an accuracy of the minimum switching step which was 100 ns.

When the output analog voltage was supplied from the time interval meter to the ion reflector power supply unit, the output voltage of the latter was set such that the passage time of the Na ions became equal to the reference voltage with a error less than t1 not (this was also controlled with an oscilloscope) and this equality was maintained during the entire operation time of the device. Tests carried out in continuous

5, the device operation (the claimed device) during the week showed that it provides a fundamentally higher accuracy of measuring the amplitudes of the peaks of the mass spectrum during long-term operation over

0 compared to the prototype. Thus, the prototype provided an analysis of the composition of the gas mixtures for each component with an error not exceeding 0.4 vol.% For only 4 hours, whereas the inventive device provides an analysis with the same error for 24 hours or more.

An additional positive effect when using the claimed device is an increase of 5 times

0 temperature stability analysis. This dramatically reduces the requirements for stabilizing the air temperature in the room where the mass spectrometer is located (from ± 1 to ± 5 ° C), which is of great importance when it is used in industrial conditions.

Claims (1)

Claims of the invention. A time-of-flight mass spectrometer comprising an ion source, a drift chamber, an ion reflector, an ion receiver, a power supply unit for the elements of the mass spectrometer, including an ion reflector power supply unit, a square pulse generator, the output of which is connected to an ion source, a broadband amplifier whose input is connected to an ion receiver and a recording device, characterized in that, in order to improve the accuracy of determining the amplitude of peaks of the mass spectrum at its long-term operation was additionally introduced a meter of the time interval between the actual and the time of flight of the reference component set during adjustment, the sync pulse output of which is connected to the synchronization input of the square pulse generator, the input to the output of the wideband amplifier, the analog output to the ion reflector power supply.