RU2337427C1 - Method of determination of effect of substance isotope composition "discrimination" in units of multiple-collector mass spectrometer - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: invention is related to the field of electric engineering, in particular, to analytical equipment, namely, to development of isotope multiple-collector mass spectrometers, which are used for determination of isotope composition of different gaseous substances. Method consists in change of substance passage conditions in investigated unit, while passage conditions are preserved constant in other units of mass spectrometer, at that availability and value of effect of "discrimination" of isotope composition of substance in investigated unit is determined by change of atomic fraction value of one of isotopes of analysed substance. The following units may be selected as investigated units of mass spectrometer: metering valve is system of sample preparation and introduction; ion source; dispersing mass analyser; ion receiver and others. Method implementation is possible in any mass spectrometer equipped with multiple-collector system of ions registration. Invention makes it possible to discover and quantitatively determine value of "discrimination" effects of substance isotope composition with the purpose of creation of mass spectrometer equipment with higher analytical and technical characteristics, which is technical result of invention.

EFFECT: provision of possibility of creation of mass spectrometer equipment with high analytical and technical characteristics.

4 cl, 3 dwg, 1 tbl

Description

The invention relates to analytical equipment, namely to the development of isotopic multi-collector mass spectrometers used to determine the isotopic composition of various gaseous substances. Such devices are widely used in various fields of the chemical industry and geology.

There is a set of standard analytical parameters of mass spectrometers that characterize the mass spectrometer and the improvement of which they pay special attention to. These parameters of the isotope mass spectrometer include [A.A.Sysoev, VB Artaev, VV Kashcheev. Isotopic mass spectrometry. - M .: Energoatomizdat, 1993. - 288 p.]: Resolution; isotopic sensitivity (detection limit); mass dispersion; the magnitude of the "memory" effect; standard deviation of the measurement results, etc. There are generally accepted methods for determining the magnitude of the above parameters, which are mostly determined using standard samples of the isotopic composition of the analyzed substances (CO) introduced into the mass spectrometer.

However, these parameters do not allow both developers and users of mass spectrometers to obtain information about the effects of "discrimination" (changes in the isotopic composition of the substance) that occur at different nodes of the mass spectrometer as the substance arrives from the balloon (sampler) to the ion receiver. Such effects to one degree or another exist in all modern mass spectrometers and are often a limitation of their analytical capabilities and the consequence of using only relative measurement methods, in which, together with the analyzed sample, it is necessary to simultaneously introduce CO into the mass spectrometer.

Figure 1 shows a well-known schematic diagram of the passage of a substance in a mass spectrometer, in which, when conducting mass spectrometric measurements, the sample (or CO) located in the cylinder of the sample preparation and injection system passes through the following nodes of the mass spectrometer in series: metering valve, source ions with a system of collimating and corrective lenses; dispersing magnet; ion receiver.

In each of these nodes, as a result of various physical processes, a change in the isotopic composition of the substance is possible, as a result of which the relative value of the recorded output signals will not correspond to the initial isotopic composition of the substance in the container.

It is known [A.A. Sysoev, V. B. Artaev, V. V. Kascheev. Isotopic mass spectrometry. - M .: Energoatomizdat, 1993. - 288 p.] That the effect of "discrimination" of the isotopic composition of a substance in a mass spectrometer as a whole can be determined by entering a standard sample (CO) of the isotopic composition of a substance and determining the isotopic composition by the formula:

where K _i is the coefficient of relative calibration of the registration channel of the i-th isotope;

U _i - the value of the output signal or ion current, when registering the i-th isotope, In (A).

The value of the relative calibration coefficients K _i is determined previously before measuring the isotopic composition of the substance by applying a constant ion beam or a reference current from a constant source to each collector.

The difference in the obtained value of the atomic fraction of the ith isotope of a substance from the nameplate value will characterize the effect of "discrimination" of the isotopic composition in the mass spectrometer as a whole. However, to eliminate this effect, which reduces the analytical capabilities of mass spectrometric equipment, it is necessary to know the magnitude of the "discrimination" of each node of the mass spectrometer.

The objective of the invention is to create a method for diagnosing a mass spectrometer, which would allow to determine the presence and magnitude of the effect of "discrimination" of the isotopic composition of a substance in any of the nodes of the mass spectrometer.

The problem is achieved by the fact that in the known method for determining the effect of "discrimination" of the isotopic composition of a substance in the entire mass spectrometer, which includes determining the calibration coefficients of the channels for detecting ion currents, introducing the substance into the mass spectrometer with simultaneous registration of the ion currents of each isotope of the substance in a separate collector and the calculation of the atomic fraction of the ith isotope according to the formula (1), in the node under study change the conditions for the passage of matter, keeping the conditions for passage in other nodes of the mass spec rometra, and at a fixed magnitude change the atomic fraction of i-th isotope (C _i) determine the magnitude of the effect of "discrimination" isotopes substances in this mass spectrometer, the test node.

The value of the atomic fraction of the ith isotope (C _i ), determined by formula (1), depends both on the accuracy of the relative calibration coefficients K _i and on a number of parameters characterizing the passage of the substance from the balloon to the ion receiver and affecting the correspondence of the recorded signal U _{i the} initial content of the isotope. In the general case, C _i can be represented as a function depending on the following parameters:

where K _i , K _i are the calibration coefficients of the registration channels of the i-th and j-th isotopes of the substance;

С _{0, i} - initial (in the balloon) content of the i-th isotope of the substance;

P is the gas pressure in the sample inlet system of the mass spectrometer;

t is the measurement time;

γ ₁ , γ ₂ , γ _n are the “discrimination” coefficients of the i-th isotope in the 1st, 2nd, and nth nodes of the mass spectrometer.

n is the number of nodes of the mass spectrometer in which a change in the isotopic composition of a substance can occur.

In turn, the values of γ _n depend on the parameters characterizing each node of the mass spectrometer. For example, the transmittance of the ith isotope of a substance through a metering valve (γ ₁ ) will depend on the flow regime of the substance in the metering valve, i.e. on the pressure of the substance in front of the valve and the characteristic size of the valve channel.

The table provides a list of possible variable parameters for each of the nodes of the isotopic multicollector mass spectrometer.

Mass spectrometer assembly Possible variable conditions (parameters) Sample preparation and injection system - Gas pressure in front of the metering valve; - Characteristic dimensions of the metering valve channel;
- Isotopic composition of the analyzed gas; Ion source (electron impact ionization) - Gas pressure in the ionization region;
- The intensity of the focusing magnetic field;
- The magnitude of the flux and energy of the electron beam;
- The magnitude of the ion current of the analyte;
- The size of the slit of the collimating lens;
- The value of the potentials of the electrodes of the ion source; Corrective and collimating lens system - The sizes of slits of corrective and collimating lenses;
- Values of potentials on lenses;
- The magnitude of the ion current of the analyte; Dispersing magnet - The size of the analyzer gap in a dispersing magnetic field;
- Forms of the input and output boundaries of the magnetic field;
- magnetic field strength;
- The magnitude of the ion current of the analyte; Ion receiver - Sizes of input diaphragms;
- The value of antidinatron potential;
- The characteristic dimensions of the collectors;
- The magnitude of the ion current of the analyte.

Thus, the problem of determining the magnitude of the effect of "discrimination" of the isotopic composition of a substance in the nth node of an isotopic multicollector mass spectrometer is solved by performing the following steps:

1. Determine the coefficients of the relative calibration of the channels for detecting ion currents (K _i );

2. Enter the analyte into the mass spectrometer, registering each isotope of the substance on a separate collector;

3. Select as an indicator the i-th isotope of the substance, the content of which is calculated by the formula (1);

4. Change the conditions for the passage of the substance through the n-th node of the mass spectrometer, keeping the conditions of passage in the remaining nodes constant, and observe the change in the value of C _i .

The implementation of the proposed method can be demonstrated by the example of diagnostics of the MTI-350G mass spectrometer (Novouralsk, Russia), designed to determine the isotopic composition of uranium hexafluoride (HFC). The size of the effect of "discrimination" of uranium isotopes was determined on a needle-type metering valve located in the sample preparation and injection system. This valve ensures that the analyzed gas enters the ion source at the required flow rate. The nominal HFC flow rate is about 1 mg / h; the HFC pressure before and after the valve is ~ 100 and ~ 10 ^-3 mm Hg. respectively.

As an indicator, the measured value of the uranium-235 content in a standard sample of the uranium isotopic composition was observed, the passport value of the atomic fraction of this uranium isotope in which was (5.3797 ± 0.0025)%. The conditions for the passage of HFCs through the metering valve was changed by reducing the pressure of the HFCs in front of the valve and opening the valve to restore the previous intensity of the analytical signal in order to keep the conditions for the passage of the substance in the following nodes of the mass spectrometer constant. The value of the output signal of the ²³⁸ UP ₅ ⁺ ion was maintained equal to ~ 5 · 10 ^-9 A.

Thus, the expression (2) in this case can be represented as:

where P is the pressure of the HFC in front of the metering valve;

d is the characteristic size of the valve channel.

Figure 2 presents the obtained dependence of the uranium-235 content, determined by the formula (1), on the pressure of HFCs in front of the metering valve, which shows a continuous increase in the measured value of the uranium-235 content with a decrease in pressure of HFCs, which indicates an increase in the effect of "discrimination" due to the approximation of the mean free path of HFC molecules to the characteristic channel size of the metering valve. With a change in pressure of 100 mm Hg the maximum change in the atomic fraction of uranium-235 was ~ 0.010%.

The maximum possible separation of isotopes in the case of the implementation of the molecular flow regime will be:

Thus, the obtained dependence, on the one hand, proves the presence of the “discrimination” effect in the metering valve, and on the other hand, indicates the implementation of the molecular-viscosity regime of gas flow in the valve of the mass spectrometer, because in the case of a molecular gas flow regime, the change in the uranium-235 content amounted to 0.023%.

After finalizing the design of this needle valve, aimed at creating a more viscous flow regime of HFCs, the dependence C ₂₃₅ = f (P) presented in FIG. 3 was re-taken. The results indicate a significant decrease in the effect of isotope separation. The maximum recorded value of ΔС ₂₃₅ was ~ 0.002%, which indicates the implementation of a more viscous regime of HFC flow. In this case, the change in pressure before the needle is from 40 to 100 mm Hg. does not lead to any noticeable change in the content of uranium-235.

Thus, the above example confirms the possibility of using the indicated method for diagnosing a mass spectrometer and allows one to identify the presence and magnitude of the effect of “discrimination” of the isotopic composition of the analyte in the metering valve of the mass spectrometer, on this basis, to optimize its design and thereby significantly reduce the effect of separation of uranium isotopes in this node of the mass spectrometer.

The invention improves the efficiency and quality of the developed mass spectrometric equipment and helps to increase its competitiveness.

Claims (3)

1. A method for determining the effect of "discrimination" of the isotopic composition of a substance in the nodes of a multi-collector mass spectrometer, including preliminary determination of the calibration coefficients of the ion current detection channels (K _i ), introducing a standard sample of the isotopic composition of the substance into the mass spectrometer, simultaneously recording the ion currents of each isotope of the substance (U _i ) on a separate collector and calculation of the atomic fraction of the i-th isotope of a substance according to the formula

where n are the recorded isotopes of the substance, characterized in that in the studied node of the mass spectrometer by enumerating the parameters of the node, the conditions for the passage of the substance through the node are changed, keeping the conditions for the passage of the substance in the remaining nodes of the mass spectrometer constant, while the magnitude of the effect of "discrimination" of the isotopic composition of the substance in this node is determined by the change in the atomic fraction of the ith isotope of the substance (C _i ). 2. The method according to claim 1, characterized in that when determining the magnitude of the effect of "discrimination" in the metering valve of the sample preparation and injection system of the mass spectrometer, the gas pressure in front of the metering valve and the degree of valve opening are selected as variable parameters for the passage of the substance. 3. The method according to claim 1 or 2, characterized in that uranium hexafluoride is used as an analyte, and the atomic fraction of uranium-235 is used as an indicator.

Images