RU2213957C2 - System for injection of corrosive gases, for instance, uranium hexafluoride, into mass spectrometer - Google Patents

Abstract

FIELD: spectrometry. SUBSTANCE: invention refers to systems for injection of corrosives gases into ion source of mass spectrometer. Technical result of invention lies in development of simple but effective system for injection of chemically active gases, for example, uranium hexafluoride, into ion source ensuring minimal " memory effect " of mass spectrometer. Uranium hexafluoride is injected into ion source in the form of formed molecular beam. Employment of multivalve leak in mix of system secures formation of molecular beam with the help of emitter and diaphragm with rectangular section conduits under pressure of uranium hexafluoride in zone of beam formation in the order of 10^-3 mm Hg. Values of memory coefficients corresponding to isotope ²³⁵U with use of injection system amounts to less than 1.003 with isotope content of 0.5- 5.0 atomic per cents. EFFECT: development of effective injection system. 3 cl, 4 dwg, 1 tbl

Description

 The invention relates to systems for introducing aggressive gases into an ion source of a mass spectrometer and can be used, for example, in the analysis of uranium hexafluoride for isotopic composition in the nuclear and chemical industries.

 When analyzing aggressive gases using a gas mass spectrometer, the “memory effect” has the most significant effect on the measurement error. This effect is due to a change in the composition of the analyzed sample due to desorption of the molecules of previous samples from the internal surfaces of the input system and the elements of the ion source. The most intensive processes of sorption and isotope exchange with solid deposits occur on the surface of the ionization chamber of the ion source, the temperature of which is 500 ÷ 600K. To reduce the “memory effect” of the mass spectrometer, an aggressive gas, such as uranium hexafluoride, is introduced into the ion source in the form of a formed molecular beam, which excludes its interaction with the walls of the ionization chamber.

There is a known system for introducing HFCs [1] into the ion source of a MI1201 mass spectrometer using a capillary, the outlet of which is located directly in the ionization chamber of the source, while molecules escaping from the capillary at an angle of more than 30 ^° fall on the chamber walls.

The closest analogue of the claimed system is the input system used in gas mass spectrometers from Finnigan MAT (Germany) [2], in particular in the MAT-281 mass spectrometer [3], and consisting of several gas supply lines (usually one line for analyzed samples, others for standard samples of the isotopic composition of uranium (CO)). Each input line includes a needle-type valve 1, an expansion cylinder 4 (V = 0.5 l) and three solenoid valves 2, 3, 5 (Fig. 1). Before introducing HFCs into the ion source, the gas is preliminarily injected through valves 1, 2 into the expansion tank 4 to a pressure of 0.15 ÷ 0.2 mm Hg. Art. After filling the cylinder and closing the valve 2, open the valve 5, while the pipeline 7, the gas enters the emitter of the molecular beam (figure 2). The emitter is two bars of stainless steel, in one of which a rectangular channel 50 mm long with a cross section of 1x2.5 mm is cut. Corrugated and flat gold foil plates with a thickness of 0.025 mm are laid in the channel along the entire length, as shown in FIG. 3. This design divides the entire rectangular channel into 50 ÷ 60 strictly parallel channels with a size (diameter) of ~ 0.12 mm. For given channel sizes, the mean free path of UF ₆ molecules, which is determined by the pressure in front of the emitter, is commensurate with their size and the gas flow through the emitter is molecular, which leads to focusing of the molecular beam emerging from the emitter. To get rid of the molecules leaving the molecular beam as a result of scattering, between the emitter and the ionization chamber at a distance of 10 mm from the emitter is a diaphragm with a rectangular channel (Fig. 2), whose length is 8 mm, and the geometric dimensions of the cross section are equal to the dimensions of the channel in the emitter. After passing through the ionization chamber, the molecular beam enters the cryogenic element. Molecules cut off by the diaphragm condense on the cryogenic element located in front of it. The consumption of uranium hexafluoride is 0.7 ÷ 1.0 mg / h.

When determining the isotopic composition of the HFC sample into the ion source, the sample and the standard sample of the HFC isotopic composition are sequentially injected. After each inlet of HFCs, the volume of the pipeline 7, the length of which is more than one meter, is pumped by a turbomolecular pump to a pressure of less than 10 ^-2 Pa. The main material used to make this HFC vacuum system is stainless steel.

The disadvantages of the closest analogue of the claimed input system are:
1. A large area of the inner surface of the input system with which uranium hexafluoride of various isotopic composition interacts (pipeline 7, Fig. 1), which necessitates increasing the pressure of HFCs in this part of the system (to 0.2 mm Hg) in order to reduce the influence of sorption processes on the measurement results.

 2. The need to use turbomolecular pumps to quickly pump out HFC residues from the input system.

 3. The complexity of the design of the input system, due to the need to accurately equalize the pressure of the sample and CO in the inlet system (in expansion cylinders).

 4. The need to use expensive materials, in particular gold foil, for the manufacture of a molecular beam emitter.

 The problem to which the invention is directed, is to create a simpler and more reliable system of molecular input of uranium hexafluoride into an ion source, providing a minimum "memory effect" of the mass spectrometer.

The problem in the inventive input system is solved in that the molecular beam is formed in it at lower HFC pressures by changing the design of the diaphragm and emitter of the molecular beam having rectangular channels, free from corrugated plates, and the installation between the beam emitter and the samplers with HFCs multivalve leakage valve, consisting of several solenoid valves 1 and one needle valve 2 located on the leakage housing and interconnected as shown figure 4, using cylindrical channels in the housing with diameters of 1 ÷ 2 mm The multi-valve leak housing is made of a single circle of stainless steel. The internal leakage volume between the valves does not exceed 0.05 cm ³ , which ensures the minimum internal leakage surface area and the loss of HFCs when pumping from this volume.

 Thanks to the inclusion of a multi-valve leakage in the input system, an advantage is achieved over known injection systems, which can significantly reduce the internal surface area of the vacuum system with which uranium hexafluoride of various isotopic composition interacts. This part of the system includes the internal volume of the multivalve leakage, capillary 8 and the emitter of the molecular beam 4 (figure 4). Sorption processes occurring in the system area before the needle valve 1 do not affect the measurements due to the high pressure of HFCs in this part of the system: 50 ÷ 80 mm Hg. Art. The capillary 8 connecting the needle valve to the emitter is made in the form of a nickel or copper tube with a smooth inner surface and an inner diameter of 2–4 mm. To reduce the sorption activity of the surface of the capillary, it is preferable to use a tube made of fluoroplastic brand F4MB tightly inserted into a copper or nickel capillary. The length of the capillary is 15 ÷ 20 cm.

The decrease in the internal surface area of the input system and its adsorption activity can reduce the pressure of HFCs behind the needle valve to values (1 ÷ 5) • 10 ^-3 mm Hg This, in turn, allows you to obtain a molecular flow of HFC molecules in the emitter 4 using a rectangular channel 9 (figure 4) with a size (width) of 0.5 ÷ 1 mm The optimal channel length of the emitter is 30 ÷ 50 mm.

 To remove scattered molecules between the emitter 4 and the ionization chamber 6, a diaphragm 5 is installed with a rectangular channel at a distance of 5 ÷ 8 mm from the emitter. After passing through the ionization chamber, the molecular beam and molecules cut off by the diaphragm condense on cryogenic elements 7 (Fig. 4).

 When determining the isotopic composition of the HFC sample into the ion source, the sample and the standard sample of the HFC isotopic composition are sequentially injected through the needle valve by opening the corresponding electromagnetic valve of the multi-valve leakage. After each inlet of HFCs, the internal volume of the multi-valve leakage is pumped out by the foreline pump by opening valve 3, while the opening of the needle valve does not change.

 The height of the rectangular channel of the molecular beam emitter and the cross section of the diaphragm channel are selected depending on the size of the ionization chamber of the ion source. For example, for a mass spectrometer of class MI1201 (Ukraine) with a cross section of the ionization chamber 4x12 mm, the following optimal dimensions of the HFC molecular input system were determined: height and width of the emitter rectangular channel 0.3–0.5 mm and 1–2 mm, respectively; channel length in the emitter 25 ÷ 50 mm; the dimensions of the rectangular channel of the diaphragm are 20–40% larger than the dimensions of the emitter; diaphragm channel length 4 ÷ 7 mm; the distance between the emitter and the diaphragm 3 ÷ 7 mm; the distance between the diaphragm and the ionization chamber is not more than 12 mm. The consumption of uranium hexafluoride to the ion source of the MI1201 mass spectrometer with the proposed HFC input system was 1–1.5 mg / h, which indicates sufficient focusing of the molecular beam.

где декады пробы и СО по i-му изотопу соответственно равны

,
где U_i - интенсивность аналитического сигнала UF₅ ⁺, соответствующего i-му изотопу, В;
U₈ - интенсивность аналитического сигнала ²³⁸UF₅ ⁺, В.In the relative method for determining the isotopic composition of uranium in a sample of uranium hexafluoride, measurements are made using a standard sample (CO) with a known isotopic composition. Performing alternate inlets into the mass spectrometer of CO and samples (the minimum number of pairs of inlets of the sample and CO n≥2) and measuring the intensities of the analytical signals, determine the factor E _i for the ith isotope equal to the average ratio of the decades of the sample and CO:

where the decades of the sample and CO in the i-th isotope are respectively equal

,
where U _i is the intensity of the analytical signal UF ₅ ⁺ corresponding to the i-th isotope, V;
U ₈ - the intensity of the analytical signal ²³⁸ UF ₅ ⁺ , Century

,
где D_i ^B, D_i ^H - паспортные значения декад большего и меньшего СО;
D_{i измер} - измеренные значения декад, вычисленные согласно выражениям (1-2), в первом случае пробой считают CO₁, во втором - СО₂.The measured value of the decade of the sample is determined by the formula
D $\genfrac{}{}{0ex}{}{\text{ism}}{\text{i}}$ = E _i • D $\genfrac{}{}{0ex}{}{\text{co}}{\text{i}}$ (2)
Before starting the measurement of samples using two standard samples ("upper" - CO ₁ and "lower" - CO ₂ ) measure the memory coefficients M _i , which characterize the "memory effect" of the mass spectrometer. The calculation of the memory coefficients for each WITH produced by the formulas

,
where D _i ^B , D _i ^H - passport values of decades of more and less WITH;
D _{i measured} - measured values of decades, calculated according to the expressions (1-2), in the first case, the breakdown is considered CO ₁ , in the second - CO ₂ .

Пример 1. Проводилось определение коэффициентов памяти на масс-спектрометре MAT-281 (Германия) и МИ1201-АГМ (Украина), оборудованном предлагаемой системой ввода гексафторида урана в ионный источник. В таблице приведены результаты определения коэффициентов памяти по изотопу ²³⁵U, рассчитанные по приведенным выше формулам (1-3).True value of sample decade D $\genfrac{}{}{0ex}{}{\text{etc}}{\text{iist}}$ taking into account the memory coefficient is determined by the formula (in the case of measuring samples with CO ₂ )
D $\genfrac{}{}{0ex}{}{\text{etc}}{\text{i truth}}$ = D $\genfrac{}{}{0ex}{}{\text{H}}{\text{i}}$ + M $\genfrac{}{}{0ex}{}{\text{H}}{\text{i}}$ • (D $\genfrac{}{}{0ex}{}{\text{ism}}{\text{ipr}}$ -D $\genfrac{}{}{0ex}{}{\text{H}}{\text{i}}$ ) (4)
The atomic fraction of the ith isotope is calculated by the formula

Example 1. The memory coefficients were determined on a mass spectrometer MAT-281 (Germany) and MI1201-AGM (Ukraine), equipped with the proposed system for introducing uranium hexafluoride into an ion source. The table shows the results of determining the memory coefficients from the ²³⁵ U isotope, calculated according to the above formulas (1-3).

 The data in the table indicate that the "memory effect" in the MI-1201 AGM mass spectrometer with the proposed HFC input system is insignificant and comparable with the value of the "memory effect" on the MAT-281 mass spectrometer.

Literature
1. Sysoev AA, Artaev VB, Kashcheev VV Isotope mass spectrometry. - M .: Energoatomizdat, 1993 .-- 288 p.

2. C. Brunne, Advances in mass spectrometry, volume 2, "A new mass spectrometer for precision measurement of the ²³⁵ U / ²³⁸ U isotopic ratio of UF ₆ ", 1963, page 230-243.

 3. Finnigan MAT, 281 Operating Manual, Bremen, 1986.

Claims (5)

 1. A system for introducing aggressive gases, for example, uranium hexafluoride, into the ionization chamber of a mass spectrometer ion source in the form of a formed molecular beam, comprising a beam emitter with a rectangular channel, a diaphragm located between the emitter and the ionization chamber, a molecular beam after passing through the ionization chamber and the molecule , cut off by a diaphragm, condense on cryogenic elements, characterized in that between the emitter of the beam and the samplers with uranium hexafluoride a multivalve drip a luminaire containing one needle and several solenoid valves mounted on the leakage housing and interconnected by channels made in the housing with diameters of 1-2 mm, said leakage is connected to the beam emitter, and the height of the rectangular channel of the beam emitter is 0.2-0 5 mm.  2. The input system according to claim 1, characterized in that the multi-valve leakage housing is made of a single circle of stainless steel.  3. The input system according to claim 1, characterized in that the multi-valve leakage evacuation valve is connected to the foreline pump.  4. The input system according to claim 1, characterized in that the multi-valve leakage is connected to the emitter of the beam using a copper or nickel capillary, the diameter and length of which are 2 ÷ 4 and 100 ÷ 150 mm, respectively.  5. The input system according to claim 4, characterized in that a fluoroplastic tube is tightly inserted into the capillary.

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