ANALYSIS OF SOLIDS USING A MASS SPECTROMETER

In recent years the mass-spectrometric method for analyzing vapors and gases has been considerably improved and has found wide application. An equally interesting and important field of use of mass spectrometry is the analysis of substances in the solid phase. Until recently it had not received serious success. This is explained mainly by the difficulty of creating an ion source, since mass spectrometers of the usual type require a source producing an ion beam of very stable intensity.

A source suitable for solving certain problems arising in the analysis of solid samples was used by Hickam^1. The sample under investigation was evaporated in a furnace located in the chamber of the mass spectrometer. The resulting molecular beam was subjected to electron bombardment.

In 1947 a mass spectrograph with a spark ion source was built for the chemical analysis of solid substances^2.

In the spark source, first proposed by Dempster^3, a high-frequency discharge is created between metallic electrodes. If the sample under investigation is used as one of the electrodes, then ions of all the elements contained in the sample will be present in the discharge.

Fig. 1

The design of the instrument^2 was based on the principle of double focusing^4, ^5. The schematic arrangement of the electric and magnetic fields and of the slit system is shown in Fig. 1.

According to Mattauch’s conclusions^4, the focusing conditions on a single photographic plate for a large mass range are as follows: 1) the ion beam must be deflected by the radial electrostatic field through an angle \(\Phi = 31^\circ 50'\) (see Fig. 1); 2) the distance \(l\) from the entrance slit \(S_1\) to the entrance

the condenser must be equal to \(\dfrac{a}{\sqrt{2}}\); 3) the deflection of the ion beam in the magnetic field \(\Phi_{mn}\) must make an angle \(\dfrac{\pi}{2}\); 4) the angle \(\chi\) must be equal to \(\dfrac{\pi}{4}\).

It is clear from Fig. 1 that \(a\) is the radius of curvature of the ion trajectory in the electric field, \(a_{mn}\) is the corresponding radius of curvature in the magnetic field, \(\chi\) is the angle between the plane of the photographic plate and the direction of the ion beam upon entry into the magnetic field, and \(\rho_n\) is the distance between the point at which the ion beam enters the magnetic field and the place where it is focused on the photographic plate.

The quantities \(a\) and \(a_{mn}\) do not affect the focusing, but the resolving power of the instrument depends on \(a\), while the dispersion depends on \(a_{mn}\) or \(\rho_n\). The resolving power is determined by the formula

\[ \frac{\Delta M}{M}=\frac{2S_1}{a}, \tag{1} \]

where \(S_1\) is the width of the entrance slit.

The dispersion, which determines the separation on the photographic plate of lines corresponding to two effective masses differing from one another (the ratio of mass number to ion charge, expressed in units of \(m_0\)), is determined by the formula

\[ d\rho_n=\frac{\rho_n}{2n}\,dn=\frac{\sqrt{2}}{2}\frac{a_{mn}}{n}\,dn, \tag{2} \]

where \(n\) is the effective mass.

The electrodes in the spark ion source were two coaxial cylinders: the outer one was a hollow cylinder of tantalum, copper, or nickel, and the inner one was made of the metallic specimen under investigation. A nickel or silver tube into which the substance under investigation was placed could be used as the inner electrode.

A high-frequency discharge, supplied from a Tesla transformer, was struck between the electrodes. The source was equipped with a device allowing the mutual arrangement of the electrodes to be adjusted.

The ions formed in the discharge were accelerated and passed through the entrance slit \(S_1\), forming a beam.

The slits \(S_2\) and \(S_3\) (Fig. 1) are not provided for by the theory. However, their presence improves the operation of the instrument. The slit \(S_2\) restricts the ion beam so that the ions do not fall on the plates of the condenser and do not distort the electric field in it. The slit \(S_3\) restricts the angle of divergence of the beam entering the magnetic field. At the exit of the magnetic analyzer a photographic plate was placed, registering the ion lines (with an exposure time of about 3 min). On a single plate it proved possible to record a mass range from \(M\) to \(10M\).

With a slit width \(S_1=0.1\) mm, the value of the resolving power of the instrument, according to the theory, should have been equal to \(\dfrac{1}{750}\). In practice, a resolution of \(\dfrac{1}{250}\) was obtained.

The accuracy of the results obtained in carrying out isotopic analysis of elements on the mass spectrograph was \(1 \div 5\%\), when deter-

From Current Literature

...the determination of the relative content of elements in metallic alloys—\(5 \div 10\%\), and in determining the ratio of two elements in oxides—\(30 \div 50\%\).

A shortcoming of the described instrument is the instability of the high-frequency discharge serving as the ion source. This instability is especially noticeable when the photographic method of registration is used. The use of an electrical method of registration improves the operation of the instrument.

Gorman, Jones, and Hippie\(^6\) constructed a Demister-type double-focusing instrument with a spark source and electrical recording. With this instrument, in the analysis of metallic specimens, an accuracy was obtained comparable with the accuracy of chemical analysis and equal to the accuracy obtained with mass-spectrometric gas analyzers.

To suppress the influence of electrical fluctuations in the spark, the circuit of Nier, Ney, and Inghram\(^7\), developed for measuring the relative content of two isotopes, was used (Fig. 2). Two ion beams corresponding to the two masses under investigation are simultaneously focused on two collectors. The ion current \(i_2\) produces across the resistance \(R_2\) a voltage supplied through an electrometer stage to the galvanometer.

Labels in Fig. 2: \(i_2\), Collector 2; \(R_2\); \(B\); \(C\); \(G\); \(i_1\), Collector 1; \(R_1\); \(V_1\); “Amplifier with feedback”; “Amplification”; \(V_0\); \(A\); \(X\); \(R_0\).

Fig. 2.

If a voltage equal to \(-i_2 R_2\) is applied between points \(B\) and \(C\), the galvanometer will give no deflection. This compensating voltage is taken from a potentiometer connected at the output of the feedback amplifier that amplifies the current \(i_1\). It changes automatically when the current \(i_1\) changes, and the balance is not disturbed when the intensities of both ion currents change simultaneously in the same way.

To obtain the complete spectrum of the specimen under investigation, one of the collectors was placed at the entrance of the magnetic analyzer and recorded the total ion current of the source. The second collector was located at the exit, beyond the exit slit. By changing the magnetic field, a beam of ions of one or another mass was directed to the exit slit. The currents of both collectors were amplified and fed toward each other to a self-recording instrument connected at the output of the amplifiers. The latter directly recorded the relative content of ions of the selected mass.

The described measurement method was tested on six samples of stainless steel. The spectrum of one of the samples is shown in Fig. 3. Calibration of the instrument was carried out using a sample with a known component content.

Fig. 3.

For all the samples investigated, good agreement with the data of chemical analysis was obtained. This indicates the possibility of using a mass spectrometer for rapid and accurate quantitative analysis of solid substances.

L. L.

References

1. W. M. Hickam, Phys. Rev. 74, 1222 (1948).
2. A. E. Schaw and W. Rall, Rev. Sci. Instr. 18, 278 (1947).
3. A. J. Dempster, Proc. Am. Phil. Soc. 75, 755 (1935).
4. J. Mattauch, Phys. Rev. 50, 617 (1936).
5. J. Mattauch und R. Herzog, Zeits. f. Physik 89, 786 (1934).
6. J. G. Gorman, E. J. Jones and J. A. Hipple, Anal. Chem. 23, 438 (1951).
7. A. O. Nier, E. P. Ney and M. G. Inghram, Rev. Sci. Instr. 18, 294 (1947).