UDC 541.183

DETERMINATION OF OPTIMAL ADMISSION CONDITIONS AND ANALYSIS ON AN MSKh-3A MASS SPECTROMETER OF GASEOUS DECOMPOSITION PRODUCTS OF COMPLEX ORGANIC COMPOUNDS IN PLASMA

G. S. Tikhonov, V. I. Kiselev, V. I. Efremov

(Presented by the scientific seminar of the Faculty of Physics and Engineering)

In this work, the admission regimes into the ion source were studied for gaseous products of the destruction of the solid polymer vinylidene fluoride in the plasma of an h.f. torch discharge, and their composition was analyzed on an MSKh-3A time-of-flight mass spectrometer.

For this purpose, an experimental setup was created that makes it possible to carry out investigations at temperatures on the order of 3500°K and gas pressures of 30–75 torr.

As a result of the investigations it was established that the composition of the gaseous products formed by decomposition of the polymer depends on the nature of the plasma-forming gas (helium or air) and is a complex mixture of gases.
Illustrations 1, tables 2, bibliography 7.

The field of scientific and technical application of mass spectrometry is currently expanding rapidly. Recently, there has increasingly arisen a need for studying plasma, photochemical reactions, combustion processes, and other rapidly occurring processes. One of the methods for investigating these processes is the mass-spectrometric method. The aim of the present work is to determine the optimal admission conditions into the ion source of an MSKh-3A time-of-flight mass spectrometer for gaseous destruction products of complex organic compounds in low-temperature plasma and to carry out their mass analysis.

In most mass-spectrometric analyses of gaseous mixtures, the small-orifice method is used [1–3]. The essence of this method is that a gas at low pressure is expanded through a very small orifice into the ion source of a mass spectrometer in order to create a molecular beam. In doing so it is usually assumed that the gas taken for analysis has the same composition as the gas under investigation. Nevertheless, because of the complex nature of the sampling process, the composition of the sampled gas undergoes changes, and it is not easy to correct deviations in the composition of the sample.

A number of authors [1–5] have shown that the inlet system being designed must possess such properties that the time for establishment of molecular flow would be less than 1 μsec. If these conditions are not observed, reactions with large activation energies will be frozen more rapidly, since a relatively small drop in temperature will lead to a strong change in their rate, which will result in a change in the composition of the products under investigation.

In this work we investigated the admission regimes into the ion source for gaseous products of destruction of vinylidene fluoride

in plasma, and their mass analysis was carried out on an MSKh-3A time-of-flight mass spectrometer.

Experimental procedure

To carry out experiments on the decomposition of a polymer of the type \((-\mathrm{CF}_2-\mathrm{CH}_2-)_{n}\) under h.f. plasma conditions and to analyze the products of its decomposition, an apparatus was constructed, shown schematically in the figure, comprising the following main units: plasmatron—2, gas-inlet system—3, MSKh-3A mass spectrometer—5: mass analyzer—4; h.f. generator LGE-3B—1; vacuum pumping station—6.

Fig. 1.

The experimental procedure consists primarily in selecting the optimal regimes for the admission of gaseous decomposition products of a solid organic polymer into the ion source of the time-of-flight mass spectrometer.

Optimization of the admission regimes was aimed at selecting diaphragms with an optimal aperture diameter, at which the flow through the diaphragm would be close to molecular, without changing the temperature in the discharge zone. Molecular effusion of gas into vacuum through an aperture is described by the Knudsen equation [6].

\[ Q=\left(\frac{RT_{0}}{2\pi M}\right)^{1/2} P_{0} A_{e}, \]

where \(Q\) is the gas flow, \(\mathrm{cm}^{3}\ \mathrm{torr}\cdot \mathrm{sec}^{-1}\); \(R\) is the gas constant; \(T_{0}\) is the temperature, \({}^{\circ}\mathrm{K}\); \(M\) is the molecular weight; \(P_{0}\) is the gas pressure, torr; \(A_{e}\) is the area of the aperture of the sampling diaphragm, \(\mathrm{cm}^{2}\).

Molecular effusion of gas occurs only when the mean free path of the molecule is greater than the size of the aperture. The mean free path is determined by the formula

\[ \lambda=\frac{kT_{0}}{\sqrt{2}\pi P_{0}\delta^{2}}, \]

where \(k\) is the Stefan–Boltzmann constant, \(\delta\) is the molecular diameter, and \(P_{0}\) is the gas pressure in the system.

As is evident from the formula given, the value of \(\lambda\) depends on temperature and pressure. For h.f. plasma, the dependence of temperature on pressure is given in the work of M. S. Khokhlov [7]. According to the experimental conditions, the temperature in the plasmatron was maintained by us within the range 3000–3500°K. From the data of [7] it is seen that the specified temperature interval lies in the pressure region 25–70 torr. Varying the temperature and pressure, we determined that the most acceptable diaphragms, with which the molecular character of the flow is preserved, are diaphragms with an aperture of 10–30 microns. Experimental studies showed that diaphragms with an aperture diameter of 30 microns are the most acceptable, since the sensitivity of the instrument remains sufficiently good and the flow through the aperture is close to molecular.

On the basis of the above computational and experimental studies of the optimal admission regimes, we selected a procedure

mass-spectrometric analysis of the decomposition products of the polymer \((—\mathrm{CF}_2—\mathrm{CH}_2—)_n\) under rf-plasma conditions. It consists of the following: the plasmatron chamber was evacuated to a pressure on the order of 0.1 torr, and then air or helium was introduced into it and an rf torch discharge was excited. The pressure in the plasmatron chamber was maintained at about 75 torr; at the same time, the temperature in the chamber, as already noted above, was about \(3500^\circ\mathrm{K}\). At this temperature, the gaseous decomposition products of the polymer were sampled for analysis on an MSKh-3A time-of-flight mass spectrometer.

Results of the Mass-Spectrometric Analysis

The polymer \((—\mathrm{CF}_2—\mathrm{CH}_2—)_n\), decomposing under the action of the rf plasma of the mass spectrometer’s sampling system, causes the appearance of a large number of mass peaks. Exact interpretation of the observed ion peaks is possible only after calibration of the instrument. Such calibration is most simply carried out by introducing calibration gases into the sampling system; as these we used \(\mathrm{H}_2\), He, \(\mathrm{CH}_4\), \(\mathrm{H}_2\), Ar, \(\mathrm{CO}_2\), \(\mathrm{C}_2\mathrm{H}_4\), and \(\mathrm{C}_2\mathrm{H}_6\), \(\mathrm{C}_3\mathrm{H}_8\).

The results of mass-spectrometric analysis of the polymer decomposition products under helium-plasma conditions are summarized in Table 1.

Table 1

Table 2 gives the mass spectra of the polymer decomposition products under air-plasma conditions.

Table 2

Discussion of Results

From the mass-spectrometric results presented (Tables 1 and 2), it follows that the main components of the gas mixture entering the detector of the time-of-flight mass spectrometer during decomposition of a polymer of the type \((—\mathrm{CF}_2—\mathrm{CH}_2—)_n\) in helium plasma are: hydrogen (H and \(\mathrm{H}_2\)), helium, carbon, acetylene, CF and \(\mathrm{CH}_2\), fluoroethylene and difluoroethylene. Traces of monomeric and polymeric carbon (\(\mathrm{C}_1\), \(\mathrm{C}_2\), and \(\mathrm{C}_3\)) were also detected in the mass spectra.

In the mass spectra of the decomposition products of the polymer \((—\mathrm{CF}_2—\mathrm{CH}_2—)_n\) in air plasma, in addition to those indicated above, the following components are added: nitrogen, oxygen, FCN, \(\mathrm{CO}_2\), CO. It should be noted that in all the measured spectra a general tendency is observed, namely, a certain decrease in the intensity of the ion peaks with increasing atomic weight.

Thus, we have investigated the admission of gaseous products of the decomposition of a polymer of complex chemical composition into the ion source of a time-of-flight mass spectrometer from the zone of an RF torch discharge.

A qualitative mass-spectrometric analysis of the products obtained was carried out, and it was established that the composition of the products is a complex mixture of gases consisting of elements included in the polymer or resulting from the reaction of these elements with air.

REFERENCES

1. J. Cuthbert, Advances in Mass—Spectrometry, London, 3, 821 (1966).
2. M. Witthaus und K. Schugerl. Berichte der Bunsengesellschaft für Physikalische Chemie, vol. 4, 73 (1969).
3. K. T. Greene, T. A. Milne. Advances in Mass—Spectrometry, London, 3, 841 (1966).
4. A. Kontrowitz and Grey. Rev. Scient. Instrum., 22, 382 (1951).
5. N. H. Reis and J. B. Feen. Chem. Phys., 39, 3240 (1963).
6. M. Knudsen. Annls. Phys., 28, 999 (1908).
7. M. S. Khokhlov, “Optics and Spectroscopy,” 4, 439 (1958).