On a Method of Spectral Analysis by Means of X-rays.

Manne Siegbahn, Axel E. Lindh and Nils Stensson. Über ein Verfahren der Spektralanalyse mittels Röntgenstrahlen. Z. f. Phys. 4, p. 61. 1921.

X-ray science at the present time can answer the question of whether a given element is present in a given sample of substance, and tenths of one percent of content are sufficient for it to be detectable.

To extend the question in the direction of the simultaneous determination of all the elements contained in a sample, the X-ray spectrographs used in practice have not yet permitted this.

Theoretically, the problem appears entirely feasible, since X-ray spectra possess a very simple and regular structure. The most intense lines of the two principal \(K\) and \(L\) groups are so characteristic that even an approximate determination of the wavelength makes it possible to conclude to which elements they belong.

Moreover, the X-ray spectrum of an element is an atomic property of it and is not at all dependent on molecular compounds or on the presence of foreign bodies.

The authors of the paper under review propose a practical method of qualitative chemical analysis by means of X-rays—a method which, under certain conditions, also makes it possible to judge the approximate quantitative ratio of the elements entering into the composition of the body whose spectra (\(Na—Ur\)) are known.

The X-ray tube employed is, in its essential features, constructed as follows: the body of the tube is a rectangular metal box, somewhat larger than a matchbox, with a drilled opening for the cathode, the anticathode, and the tube leading to the molecular pump.

The cathode is an incandescent spiral, giving on the anticathode a rather extended focus, which makes it possible to fulfill the basic design condition in this apparatus—the production of considerable (\(40^\circ\)) solid angles both for the beam coming from the anticathode and for the beam reflected from the crystal. Opposite the anticathode, placed at the smallest possible distance from the opposite wall of the tube, a slit is cut (\(4.0 \times 0.1\) mm), to which, before assembly with the spectrograph, a thin aluminum or gold plate is applied, pressed by a copper ring screwed to the tube; to this ring the crystal is attached with cement (in the experiments carried out, Iceland spar or gypsum). The inclination of the crystal is such that the reflected rays go at an angle of \(10^\circ—50^\circ\).

The proximity of the crystal and anticathode to the slit helps to satisfy the above-mentioned condition—the obtaining of large solid angles. The vacuum spectrograph, screwed to the tube, has in cross-section the form of a sector, along the arc of which photographic film is introduced through a side opening. The center of the arc is located at the place of the image of the slit on the reflecting plane of the crystal.

This makes it possible to regard the lengths measured on the spectrograms as proportional to the angles of reflection of the rays. During operation the spectrograph is connected by a tube with the fore-vacuum of the molecular pump.

The cathode, anticathode, and box are cooled with water.

The tube operates with a one-ampere transformer at approximately \(15000\ V\) and 50 cycles, and with a rectifier on the motor axis. The cathode is heated by a small transformer at \(10\ V\). The current in the tube during exposures is \(20—30\) milliamperes.

For first-order spectra, with the grating constant of Iceland spar \(2d = 6.060\ \text{Å}\) and reflection angles \(\varphi\) from \(10^\circ—50^\circ\), from the relation \(\lambda = 2d \sin \varphi\) we obtain \(\lambda\) \(1.0—4.6\ \text{Å}\); with gypsum (\(2d = 15.15\ \text{Å}\)), \(\lambda = 2.6—11.6\ \text{Å}\), i.e. two exposures are sufficient for recording wavelengths \(\lambda\) from \(1.0—11.6 \cdot 10^{-8}\ cm\) (duration of an exposure about 2 hours).

For rapid orientation, instead of directly measuring spectrograms, one may use a special “key”: the principal lines of the \(K\), \(L\), and \(M\) groups of elements are plotted on a scale divided uniformly into degree divisions, at the corresponding divisions. The spectrogram under study is placed against such a scale, and, from the coincidence of its lines with the lines of the scale, it is determined which elements are present in the substance being analyzed. By regulating the voltage of the tube, one can eliminate the lines of shorter wavelengths, i.e., spectra of higher order, and thereby simplify the analysis of the spectrogram.

The principle of the method described is a valuable addition to the existing methods of X-ray examination and provides a new and interesting means of chemical analysis, having the particular advantage that no combinations of elements can affect the result of determining the minimum quantities of a substance.

A. Trapeznikov.