RU2616927C1 - Stand for calibrating device of mass-spectrometric gas flow measurements - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: stand for calibrating the device of mass spectrometric gas flow measurements comprises a gas inlet chamber connected to the pressure sensor, not sensitive to the gas genus, a gas flow recording camera coupled with the mass spectrometer and the combined full-range gas pressure sensor, a vacuum pumping system for chambers, the gas inlet chambers and the gas flow recording chambers are connected to the line with a valve, wherein a gas-permeable membrane is installed on the line end introduced into the gas inlet chamber, in addition, the gas inlet chambers and the gas flow recording chambers are connected by the line with the two valves, between which a calibrated flow of the gas flow molecular mode is installed.

EFFECT: invention provides calibration of the mass spectrometric device in a wide range of the measured gas flows.

2 cl, 4 dwg

Description

The claimed invention relates to vacuum technology, mass spectrometric technology and can be used in the study of gas permeability of materials and tasks associated with accurate measurement of gas flows (in the range of operating pressures of a mass spectrometer (10 ^-13 -10 ^-2 Pa). the calibration of the device for mass spectrometric measurement of gas flows, the cross-section (conductivity) of a calibrated leak (or a set of them) should provide a molecular regime of gas flow, including ultra-small (about 10 ⁸ molecules / s or 10 ^-14 Pa⋅m ³ / s).

A device for calibrating a mass spectrometer is known - calibrated leak (a reference source of molecular gas flow), [http://www.lacotech.com/leakstandardsandcalibrations/heliumreservoirleakstandards.aspx]. The device contains: gas cylinder, calibrated leak and shutoff valves. When the device is connected to a vacuum system, in which it is necessary to calibrate the mass spectrometric equipment according to the gas flow, the corresponding valves are opened, as a result, gas from the cylinder is supplied to the user system with a given flow value. The device is able to create a flow of the corresponding gas at the outlet regulated by the manufacturer for a long time. The gas pressure in the calibration cylinder is several atmospheres, and the gas conductivity of the leak is extremely small. Thus, the gas flow will be constant for a sufficiently long time, due to the small absolute value of the flow and, therefore, a small change in pressure in the cylinder. Such systems are quite common and easy to use. The disadvantages of this device include the need for periodic (every 1-2 years, depending on the frequency of use and gas consumption) inspection, refueling, and calibration of the device from the manufacturer. The total cost of operating such a system, taking into account transportation and services, is quite high. Also, if it is necessary to calibrate the mass spectrometer in order to measure the gas flow, which can vary over a wide range, it is necessary to use several similar devices. The task is further complicated if the measured gas stream is a mixture of gases with different molar masses.

In the patent of the Russian Federation No. 81442, publ. 10.20.2008, a device for calibrating a mass spectrometer is described, which can be used in the field of studying the isotopic composition of substances. The device comprises an ionization chamber, an evaporator, a laser radiation source, and an ion guide with an extractive lens; above the ionization chamber is an electron source consisting of a wire cathode and a ring anode mounted coaxially with the holes of the ionization chamber and an ion guide traction lens. The operation of the device is based on the principle of photoionization and cannot be applicable to our tasks due to the fact that it is not suitable for studying a gas mixture consisting of light chemical elements.

In the patent of the Russian Federation No. 2367939, publ. 09/20/2009, describes a method for quantitative mass spectrometric analysis of the composition of the gas mixture. This method will allow to determine the concentration of the individual components of the gas mixture in the presence of unknown gas components in it. The instrument is calibrated for individual gases by simultaneously recording the mass spectrum and absolute gas pressure in the mass spectrometer inlet system, then absolute sensitivity coefficients are determined.

In the patent of the Russian Federation No. 110541, publ. 05/17/2011, a device for mass spectrometric analysis of hydrogen purity and quantitative composition of gas impurities is described, comprising a mass spectrometer associated with a pumping system and a continuous input system for the analyzed gas mixture containing a device for enriching the initial gas mixture with analyzed impurities, a gas mixture pressure meter installed at the inlet of the mass spectrometer and a gas mixture flow rate meter installed at the inlet of the enrichment device of the initial gas mixture with the analyzed impurities, system continuously introducing a gaseous mixture of the analyzed device enrichment feed gas mixture being analyzed by impurities formed as a permeable only to hydrogen and impermeable to other gases, any selective membrane, placed in a sealed enclosure connected to a mass spectrometer via an adjustable valve. This device contains some structural elements that coincide with the device for mass spectrometric measurement, which is calibrated at our stand.

An analysis of the publications did not reveal technical solutions that allow the calibration of mass spectrometric devices for measuring gas flows both for each registered gas individually and for a gas mixture without significant structural changes to existing devices.

The technical result is the creation of a calibration bench with wide functional capabilities, allowing for minimal design changes of the calibrated mass spectrometric gas flow measuring device to calibrate it when operating in a wide range of measured gas flows (10 ^-14 -10 ^-3 Pa⋅m ³ / s or 10 ⁸ molecules / s - 10 ¹⁷ molecules / s), while the stand does not require regular maintenance / verification, which makes this technical solution cost-effective.

To achieve the specified technical result, a stand is proposed for calibrating a mass spectrometric measurement device for gas flows, comprising a gas inlet chamber connected to a pressure sensor not sensitive to the gas genus, a gas flow registration chamber connected to a mass spectrometer and a combined full-range gas pressure sensor, systems for vacuum pumping chambers, gas inlet chambers, and gas flow recordings are connected by a line to a valve, and at the end of the line introduced into the chamber for gas start-up, a gas-permeable membrane is installed, in addition, the gas inlet and gas flow detection chambers are connected by at least one line with two valves, between which a calibrated flow is established, the cross-section d of which ensures the molecular flow regime of the gas stream.

In addition, the number of highways with calibrated leaks may be equal to n, while d _n / d _n-1 is in the range of 10-30.

The invention is illustrated by drawings.

In FIG. 1 and 2 schematically depict a stand for calibrating a mass spectrometric measurement device for gas flows.

In FIG. Figure 3 shows the time dependences of ion currents (mass spectrometer readings) when studying gas permeability of an EK-181 steel membrane (Rusfer) and performing calibration when the stationary value of measurable gas flows is reached.

In FIG. Figure 4 shows the dependences of ion currents (mass spectrometer readings) on pressure P ₁ in the gas inlet chamber when determining the sensitivity coefficients of the mass spectrometer k _mi for gases: D ₂ , Н ₂ , Ar - “Calibration 1” and D ₂ , Ar - “ Calibration 2 ".

The positions indicated:

1 - Gas inlet chamber

2 - Gas flow recording chamber

3 - High vacuum valves, VT1 to the left of the leak and VT2 to the right of the leak

4, 4.1, 4.2, ..., 4.n - Calibrated leak

5 - VT3 high vacuum valve

6 - System of high vacuum pumping gas from the gas inlet chamber and the gas flow recording chamber

7 - Pressure sensor, not sensitive to the type of gas

8 - Combined full-range gas pressure sensor

9 - Mass spectrometer

10 - Gas permeable membrane

11 - Calibrated gas flow J

12 - Measured gas flow J ^ISM

13, 14 - Highways

The calibration stand (see Fig. 1) contains a gas inlet chamber 1, which is a stainless steel vacuum chamber in which a working gas pressure P ₁ is created (gas storage tanks are not shown in the figures) connected by a line 13 to a valve 5 s a gas flow recording chamber 2, also representing a stainless steel vacuum chamber; at the end of the line 13, introduced into the gas inlet chamber 1, a gas-permeable membrane is installed 10. The gas inlet chamber 1 and the gas flow recording chamber 2 are connected to the high-vacuum pumping systems 6. The gas flow recording chamber 2 is connected to a mass spectrometer 9 and a full-range pressure sensor 8 The gas inlet chamber 1 is connected to a pressure sensor, for example, of a capacitive type, which is not sensitive to the type of gas 7.

For high-precision calibration, it is advisable to use pressure sensors of capacitive type 7, which are not sensitive to the type of gas, but other pressure sensors (deformation, ionization, etc.) can also be used if their accuracy (absolute and relative measurement errors) is sufficient .

The gas inlet chamber 1 is connected to the gas flow recording chamber 2 by a line 14 with high-vacuum valves 3 and a calibrated leak 4.

The number of lines 14 and calibrated leaks 4 with decreasing bore d may be n, as shown in FIG. 2, while d _n / d _n-1 lies in the range of 10-30. With this ratio, it is possible to provide an ultra-wide range of measured gas flows:

J ^meas _i + J _nat + J _i = S⋅ (10 ^-2 -10 ^-13 ), where:

- измеряемый поток газа выбранной массы (Па⋅м³/с);

- the measured gas flow of the selected mass (Pa⋅m ³ / s);

J _NAT - gas leakage into the registration chamber (Pa⋅m ³ / s);

J _i - calibrated gas flow (Pa⋅m ³ / s);

S is the gas pumping rate (m ³ / s);

(10 ^-2 -10 ^-13 ) - pressure range of the mass spectrometer (Pa).

The cross section d is related to the conductivity of the leak S (moreover, S ~ D ² / h, where D is the diameter of the hole in the diaphragm / leak, h is the thickness of the diaphragm / leak), and the conductivity S is used for further calculations.

At the stand, a calibration device for mass spectrometric measurement of gas flows can be performed when the values measured by the mass spectrometer 9 are stationary (the time dependences of the magnitude of the ion currents do not change).

Calibration is based on determining the coefficients connecting the ion current intensity of each gas with individual gas streams flowing through calibrated leak 4 or a set of leaks (by simultaneously recording the mass spectrum and absolute gas pressure in the mass spectrometer inlet system), and further recalculating the relative the readings of the mass spectrometer (ion currents) obtained by measuring the gas flow (including the gas mixture flow) and the gas flow passing through a calibrated flow with a flow cross section d is small conductivity S _mi (l / s), in the absolute values of the measured gas flows. You can also determine the magnitude of the flows of the individual components of the gas mixture flowing through the registration chamber of the gas stream 2.

Calibration of the device is as follows.

Gas is introduced into the inlet chamber 1, through which the mass spectrometer 9 is calibrated. The valve 5 opens, the time dependence of the ion currents from the mass spectrometer 9 is recorded, which corresponds to the change in partial pressures due to the gas flow penetrating through the membrane 10 of J ^IS 12. Upon reaching the stationary Valves 3 open the ion current of the mass spectrometer. The time dependences of the ion currents are taken from the mass spectrometer 9, the increase in the values of which is due to the additional calibrated gas flow J 11 through the cal Bathroom leak 4.

Calibration of the device for the gas mixture under study is made by converting the relative readings of the mass spectrometer (ion currents) into absolute values (Pa⋅m ³ / s) according to the formula (in the case of registration by the main masses):

- in the case of known pressures P _mi (for example, in the case of a calibration mixture of non-interacting gases with pressures P _m1 ... P _mi ). Or, in general,

Where:

J ^ISM _mi - measured gas flow of the selected mass (Pa⋅m ³ / s);

I ^ISM _mi - ion current of the i-th mass (more precisely m / Z), with closed valves 3 (A);

I ^νt _mi - ion current of the i-th mass (more precisely m / Z), with open valves 3 (A);

S _mi is the conductivity of calibrated leak 4 in the gas used (l / s);

S ¹ is the gas conductivity with a molar mass of M = 1 (l / s);

P _mi is the difference between the gas pressure in the inlet chamber and the registration chamber (P ₂ -P ₁ );

P ¹ - pressure in the inlet chamber (Pa);

k _mi is the sensitivity coefficient of the mass spectrometer by the i-th gas mass;

M _i is the molar mass of the i-th gas.

The determination of calibration coefficients connecting the ion current intensity of each gas with its pressure is carried out for individual gases (by simultaneously recording the mass spectrum and absolute gas pressure in the mass spectrometer inlet system).

In the case of a mass spectrometer (including a quadrupole) operating on the principle of electron impact ionization, ion currents of the corresponding masses are recorded, more precisely M / Z, where: M is the molar mass of the gas; Z is the charge of the ion that is formed in the mass spectrometer. For example, during argon ionization by electron impact, Ar ⁺ ions with M = 40 and Z = 1 are often formed, as well as Ar ⁺⁺ with M = 40 and Z = 2. When registering polyatomic gases (hydrocarbons, oxides, alcohols, etc.), gas molecules can partially or completely decompose and form a whole mass spectrum (M / Z). The proposed approach can be applied in the case of polyatomic complex gases.

In FIG. Figure 3 shows the permeability curve of the studied gas-permeable membrane 10 made of steel EK-181 (Rusfer), when gaseous deuterium is admitted into the chamber 1 (mixture D ₂ : HD: H ₂ ≈86%: 12%: 2%), at a membrane temperature of 10-600 ° C and pressure P ₁ = 3⋅10 ^-2 hPa. At the initial moment of time, the membrane is warmed up and is in vacuum, when gas is injected into chamber 1 (the moment in time is indicated by a dashed line), the readings of the mass spectrometer I _m4 , I _m3 and I _m2 (M / Z = 4, 3 and 2, respectively ), upon reaching the stationarity of the penetrating flow in the time range of ~ 1750–3250 sec in FIG. 3, when VT2 3 valves were opened, the corresponding masses increased sharply as a result of adding an additional calibration stream J 11 to the measured flow J ^ISM 12.

In an exemplary embodiment, chambers 1 and 2 were made of stainless steel. All connections are made using Conflat standard vacuum fittings. Due to the fact that the line 13 from the permeable membrane 10, connecting the volumes of the gas inlet chambers 1 and gas registration 2, has the DN40CF connection standard, the transitional connection DN40CF-D to two DN16CF is specially made for the stand, and a calibrated leak 4 is made from the flange DN16CF. Connections are made using the bellows CF16FX250R, high vacuum valves 3 type DN16CF. To measure gas pressure, PBR260 was used - a combined full-range gas pressure sensor 8 (1000 ÷ 5 ^{-10 -10} hPa) and CMR - a gas pressure sensor of capacitive type 7 (0.1 ÷ 10 ^-4 hPa for CMR365 and 1-10 ^-3 hPa for CMR364). A 9-QMS quadrupole mass spectrometer was used.

Description of the method for converting the relative readings of the mass spectrometer to absolute values

The gas stream J 11 passing through the calibrated stream 4 in the case of the molecular regime of gas flow is a stream of a mixture of gases (in this case, a mixture with molar masses M ₄ , M ₃ , M ₂ ), which can be described by the expression:

,

,

Where:

S _mi is the gas leakage conductivity with a molar mass M _i (l / s);

P _mi ¹ is the partial pressure of a gas with a molar mass M _i in the inlet chamber 1 (Pa);

P _mi ² is the partial pressure of a gas with a molar mass M _i in the registration chamber 2 (Pa);

P ₁ - pressure of the gas or gas mixture, measured using a pressure sensor 7 in the inlet chamber 1 (Pa) (in this case, the mixture of gases D ₂ , HD, H ₂ in which deuterium predominates).

As a calibrated leak, it is recommended to use a low conductivity leak sufficient to satisfy the inequality: P _mi ¹ >> P _mi ² in a wide range of gas flows 11. The following can be used as a leak: thin capillary; aperture with a small hole (10-50 microns); double-sided stainless flange with a threaded hole into which a stainless steel screw is screwed (used in the example of a specific embodiment of the claimed invention); factory solutions.

The conductivity of a leak 4 or capillary, in the case of a molecular regime of gas flow, depends on the temperature and molar mass of the gas as:

Consequently, the conductivity of calibrated leak 4 measured on one gas can be recalculated for any other gas.

As can be seen from FIG. 3, during the calibration process, when adding to the measured gas flow, the flow of a mixture of gases passing through the calibrated leak (with open valves 3), an abrupt increase in the readings of the mass spectrometer is observed. In this case, the gas mixture stream 11 (D ₂ , HD, H ₂ ) through the calibrated stream 4 can be represented by the expression:

Where:

K _calib - calibration factor;

k _mi is the sensitivity coefficient of the mass spectrometer by the i-th gas mass;

I _mi ^νt2 is the value of the ion current (partial pressure) recorded by the mass spectrometer 9 with open valves 3 for the mass m _i (M / Z) (A);

I _mi is the stationary value of the ion current (partial pressure) recorded by the mass spectrometer 9 (A). If there is no stationary measured gas flow 12, the background (zero) value can be taken as I _mi .

Determination of sensitivity factors for individual gases

для каждого газа из смеси (с молярной массой M_i), варьируя давление газа Р_mi ¹ в камере напуска 1 и регистрируя изменение соответствующих показаний масс-спектрометра при открытых высоковакуумных вентилях 3 в камере регистрации 2.The sensitivity coefficients of the mass spectrometer k _mi must be determined separately for each gas with a molar mass M _i entering the gas mixture. It is advisable to determine the coefficients k _mi when the measured gas stream 12 does not enter the recording chamber 2. The ratio of the coefficients k _mi can be determined from the relationship of dependencies

for each gas from the mixture (with a molar mass M _i ), varying the gas pressure P _mi ¹ in the inlet chamber 1 and recording the change in the corresponding readings of the mass spectrometer with open high-vacuum valves 3 in the registration chamber 2.

может быть представлена как:From the expressions presented above, for each gas with mass M _{i the} dependence

can be represented as:

- прирост значения тока (парциального давления), регистрируемого масс-спектрометром при калибровке, относительно фонового значения.Where

- an increase in the current value (partial pressure) recorded by the mass spectrometer during calibration relative to the background value.

и

для водорода (М₂) и для дейтерия (М₄). Для этого в камеру напуска 1 чистые газы (водород и дейтерий) подавались отдельно и регистрировался прирост соответствующих показаний масс-спектрометра

. Также отдельно была снята зависимость для аргона.In the case considered as an example, the dependencies were removed

and

for hydrogen (M ₂ ) and for deuterium (M ₄ ). For this, pure gases (hydrogen and deuterium) were supplied separately to the inlet chamber 1, and an increase in the corresponding readings of the mass spectrometer was recorded

. Also, the dependence for argon was separately removed.

, представленные на Фиг. 4, были аппроксимированы линейной зависимостью в диапазоне, в котором происходило определение коэффициентов чувствительности масс-спектрометра. На Фиг. 4 представлена калибровка для определения коэффициентов чувствительности масс-спектрометра k_mi для газов: D₂, Н₂, Ar - «Калибровка 1» и D₂, Ar - «Калибровка 2».In this example, dependencies

shown in FIG. 4, were approximated by a linear dependence in the range in which the sensitivity coefficients of the mass spectrometer were determined. In FIG. Figure 4 shows the calibration for determining the sensitivity coefficients of the mass spectrometer k _mi for gases: D ₂ , H ₂ , Ar - "Calibration 1" and D ₂ , Ar - "Calibration 2".

, как видно из Фиг.4, могут отличаться до трех раз.Despite the fact that calibrations (1 and 2) were carried out at close settings of the mass spectrometer (they were the same: electron energy, pulling potential, and measurement scale), dependences

as can be seen from Figure 4, can vary up to three times.

и

, что соответствует токам ионов Ar⁺⁺ и Ar⁺. Таким образом, поток аргона 11 через калиброванную течь 4 может быть представлен и учтен в дальнейшем любым удобным для конкретного случая способом, например:As can be seen from FIG. 4, the flow of argon 11 passing through a calibrated leak 4, under the influence of ionization by electron impact, is recorded by the mass spectrometer 9 in the form of two ion currents -

and

, which corresponds to the currents of ions Ar ⁺⁺ and Ar ⁺ . Thus, the flow of argon 11 through a calibrated leak 4 can be represented and taken into account in any convenient way for a particular case, for example:

,

,

Where:

- прирост значения тока (парциального давления), регистрируемого масс-спектрометром при калибровке, относительно фонового значения (А);

- an increase in the current value (partial pressure) recorded by the mass spectrometer during calibration, relative to the background value (A);

- коэффициенты чувствительности масс-спектрометра 9 для каждого конкретного способа представления калиброванного потока.

- sensitivity coefficients of the mass spectrometer 9 for each specific way of representing the calibrated stream.

In the exemplary case of FIG. 4, the following ratios of the sensitivity coefficients of the mass spectrometer were obtained (for the case of “Calibration 1”):

;

;

It should be noted that the sensitivity coefficients of the mass spectrometer 9 depend on many factors and can vary significantly for each specific mass spectrometer.

и определить отношение коэффициентов чувствительности для используемых коэффициентов усиления (шкал измерений) масс-спектрометра.Thus, in order to calibrate the mass spectrometer 9, it is necessary to verify the linearity of the dependencies

and determine the ratio of the sensitivity factors for the used gain factors (measurement scales) of the mass spectrometer.

Thus (in our particular case), it is possible to translate the relative readings of the mass spectrometer of 9 ion currents: I _m4 and I _m3 into the measured gas flow J ^ISM 12 according to the formula:

Calibrated Leak Conductivity Assessment

To implement the calibration of the device for mass spectrometric measurement, used to convert the relative readings of the mass spectrometer 9 into the absolute values of the measured gas flows, it is necessary to use one (Fig. 1) or several leaks (Fig. 2) of low conductivity (providing molecular gas flow ) In general, the gas conductivity of a leak can be determined by the method of leakage of a gas stream through a leak into a non-pumped vacuum volume and by the method of pumping this volume through a leak.

In an example of a specific embodiment of the claimed invention, the conductivity of a calibrated gas leak 4 was measured as follows.

The balance of gas flows in the registration chamber 2 can be described by the expression:

, где:

where:

S is the gas pumping rate (l / s);

V ₂ and P ₂ - the volume and pressure of the gas in the gas registration chamber (m ³ and Pa);

J ^rev _. - measured gas flow through the membrane 10 (Pa⋅m ³ / s);

J _nat - gas leakage into the gas registration chamber (Pa⋅m ³ / s);

J - calibrated gas flow (Pa⋅m ³ / s).

There is gas in the inlet chamber 1 and P ₁ >> P ₂ during the entire measurement time, there is no gas flow through the membrane 10 (J _ISM = 0), and the high-vacuum valves 3 (VT1 and VT2) are in the open position:

and closed position:

Т.е. калиброванный поток газа J 11 равен разности угловых коэффициентов кривых натекания газа

и

.The gas flow into the registration chamber 2 is caused only by the degassing of the chamber and the calibrated gas flow through the calibrated leak 4. Closing the gas pumping in the registration chamber 2 (S = 0) and measuring the pressure increase (leak) of the gas in the registration chamber 2 with open and closed high-vacuum valves 3 (VT1 and VT2), we get the difference

Those. calibrated gas flow J 11 is equal to the difference of the angular coefficients of the curves of gas leakage

and

.

и Р₂<<P₁), получим выражение для определения проводимости калиброванной течи 4:Thus, from the expression J = S _mi ⋅ (P ₁ -P ₂ ), where S _mi (l / s) is the conductivity of the calibrated leak 4 through this gas, and P ₁ and P ₂ are the gas pressures in the inlet chamber 1 and the chamber registration of gas flow 2, respectively, given that (

and P ₂ << P ₁ ), we obtain an expression for determining the conductivity of a calibrated leak 4:

The main source of error in determining the conductivity of a calibrated leak 4 by gas is the accuracy in measuring pressure (P ₁ and P ₂ ), as well as the error in determining the volume. In the case under consideration, the conductivity value of the calibrated gas leak 4 (D ₂ ), determined by the expression above, was S _D2 = (2.123 ± 0.012) ⋅ 10 ^-4 (l / s).

Thus, the claimed device allows you to calibrate the mass spectrometric device for any working gases during the experiment and create calibrated gas flows in a wide range (3 orders or more) to obtain quantitative values of the gas flow (for each recorded mass is the isotopic composition of the gas mixture) for almost any vacuum circuit with a minimum set of additional vacuum components and measuring devices.

Claims (2)

1. Stand for calibrating a mass spectrometric measurement device for gas flows, comprising a gas inlet chamber connected to a pressure sensor insensitive to the gas genus, a gas flow recording chamber connected to a mass spectrometer and a combined full-range gas pressure sensor, vacuum chamber evacuation systems , the gas inlet and gas flow recording chambers are connected by a line with a valve, and at the end of the line introduced into the gas inlet chamber, a gas-permeable membrane is installed, except , The gas inlet chamber and registration of the gas flow are connected to at least one manifold with the two valves, between which a calibrated leak orifice d which provides a molecular flow regime of the gas stream. 2. The stand according to claim 1, characterized in that the number of highways with calibrated leaks is n, while d _n / d _n-1 is in the range of 10-30.

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