SU754300A1 - Method of determining absolute sensitivity of vacuum ionization devices to gas flow - Google Patents

Description

The invention relates to methods for determining the absolute sensitivity of vacuum ionization devices (mass spectrometers, ionization manometric lamps, etc.); It can be used in the study of physical and chemical processes using mass spectrometers, when absolute measurements are necessary, as well as when comparing the characteristics of similar instruments. In addition, the method can be used to create a constant flow of gas of known size.

A known method for determining the absolute sensitivity (calibration) of a mass spectrometer to a flow of various gases. In this method, a constant flow of calibration gas enters the mass spectrometer from the inlet volume through a calibrated nagel. Absolute sensitivity is defined as the ratio of the magnitude of the corresponding ion current to the gas pressure in the region

2

ionization, which is determined by calculation based on the known gas pressure in the overlap volume and the gas conductivity known in advance of the calibrated leak. In this case, the flow of gas occurs in a strictly defined mode (molecular or viscous) m.

The disadvantage of the calibration method is the calculated determination of the pressure in the ionization region. The calculation is based on several assumptions that in this particular case may be incorrect. Due to clogging or other reasons, the value of hectares is

_{) The 5th} basic conductivity of a calibrated leak during the calibration can change in comparison with the previously measured one, and the neglect of this fact can lead to. errors. To avoid such mistakes

2 ₀ should be determined by the gas conductivity of the calibrated leak before each calibration, which lengthens and complicates the calibration process. Necessary to maintain constant conditions for pro754300

when calibrating, in particular, to maintain a constant gas temperature in the inlet volume, it is necessary to know the exact pressure of the gas and the volume of the inlet.

The aim of the invention is to improve the accuracy and reliability of determination.

This goal is achieved by heme that the gas in the overlap volume is cut off from the total gas volume, the known amount of gas is passed through the calibrated device, the integral of the corresponding ion current is measured over the gas flow time from the inlet volume through the calibrated device and the absolute sensitivity is determined by the formula

(to

where p is the gas pressure in the inlet volume, mm. Hg v.}

V is the volume of overlap cm ³ ;

AWP - the amount of gas passed

through the device;

Z _o is the area of the diagram below the recorded value of the ion go;

θθ “plot area of a diagram with ordinate equal to the value of the ion current recorded on the diagram at a certain time I., and the abscissa equal to the distance to which the diagram tape moves in 1 s;

C is the absolute value of the corresponding ion current in amperes, measured on the output device of the ion current registration channel;

the value corresponds to the value of the ordinate of the plot of $.

The meaning of the formula (1) is the following: 6 o is proportional to the amount of the passed gas d (pX); coefficient of proportionality is; derivative of 5o '

proportional to ° the gas flow O at time 1 itself

there is . Absolute chuvsgvitel about

. ness is

C ~ d (p \ /) 5 _o

In the proposed method does not require any assumptions about the constancy

 calibration conditions. In particular, constancy of the gas conductivity of the leak is not required. Moreover, spend—

four

The leakage gap may change during the calibration process, and this is all the same (due to the fact that a known amount of gas flows through the instrument) does not change the result and the accuracy of the calibration. The only requirement for the leak is at a given gas pressure in the inlet volume of the normal working gas flow through the device.

Further, the magnitude of the gas pressure in the amount of overlap when using the proposed method is not limited and can be chosen large enough (1 atm) so that it can be measured directly with sufficient accuracy.

The accuracy of the calibration method proposed should be no worse than 5%, if all 5 values that determine the magnitude of the absolute sensitivity, will be determined with the usual accuracy of 1%. In this case, since all these values are determined from experimental data, the obtained value of absolute sensitivity will have a very high reliability.

FIG. 1 shows a diagram of the device to explain the implementation of the proposed method; in fig. Figures 2 and 3 show the types of recording of the ion current on a chart tape in two different ways of carrying out the method, when all the gas in the inlet volume is passed through the device.

The device contains metal tubes 1 and 2 with a known cross-sectional area; The end 1 is attached to the device, the end 2, which plays the role of the volume of the inlet, is open in the 'container containing the gas, the clamping of the 3 metal tube, which plays the role of the leak; vessel 4 with liquid.

FIG. 2 and 3 - the variable value of the ion current; £ is the current time; 3 - the value of the corresponding ion current at a given time (in the figures, the moment of the start of the calibration process is taken as such a time); 1 ₀ - the initial time of calibration; square

diagram tape under the curve of the magnitude of the ion current corresponding to the amount of gas passed through the device;

* -> - the area of the diagram tape corresponding to the amount of gas in the inlet path above leakage 3 at the moment of time and does not participate in the calculations; 5 _С - area of the diagram tape, corresponding to the current gas flow rate

754300

time; Τ is the time of passing a known amount of gas through the device at a constant pressure in the volume of inlet 2 during calibration (option 6).

The proposed method is carried out 5 as follows.

Option A. In the initial position, the gas through natagel 3 enters from the inlet volume 2 communicated with the container (not shown in Fig.) Into the device. Fortunately, 10 giving the small gas conductivity of the leak 3, compared to the gas conductivity of the inlet volume 2, the gas pressure in the inlet volume 2 is the same as in the container. At the initial time of calibration 1. ₀ cut off the gas contained in the volume of the inlet 2 from the container by immersing the open end of the volume of the inlet 2 into the liquid 4 wetting the walls of the volume of the inlet 2. Under the action of pressure of the gas in the container liquid 4 fills the inlet volume 2 as the gas flows from the inlet volume 2 into the device through the leak 3. The gas in the inlet volume 2 is all the time 25

under pressure equal to the pressure of the gas in the container, this ensures the flow of gas into the device in the form of a stream of constant value. At the same time, the value of the corresponding ion current is recorded on the chart tape (see Fig. 2).

The magnitude of the absolute sensitivity determine

⁼ ρν

35

Where

p is the gas pressure in the container;

y is the volume of overlap 2.

Option B. The initial position as in option A. At the moment of time, the outer end of the overlap volume 2 is cleverly tightly closed, and the gas through the leak 3 departs from the overflow volume 2

into the device. A diagram of the change in the magnitude of the ion current is shown in FIG. 3

Claims (2)

Claim The method for determining the absolute sensitivity of vacuum ionization devices to the gas flow by supplying gas to the instrument being calibrated from the volume of overlap through the gel, is different. In order to improve the accuracy and reliability of the determination, the gas in the overlap volume of the total gas volume is cut off, a known amount of gas is passed through the calibrated device, the integral of the corresponding ion current over the passage time is simultaneously measured gas from the volume of inlet through the calibrated device and determine the absolute sensitivity by the formula in bulk p is the gas pressure start; V is the inlet volume; the amount of gas passed through the instrument being calibrated; $> ₀ is the quantity ;, ^ proportional to the integral of the ion current over time; • 5 ₀ - derivative of 5 at a given time; 3 - the absolute value of the ion current at the time of determining the value.