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samples containing known amounts of that element. This mefhod was Bccurate to about *25% of the amount present. Since that period, quantitative spectrochemical analysis has 80 advanced that its accuracy is comparable to that of chemical methods and its speed is much greater. One of the important factprs which have made this advance possible is improved spectrographic equipment. Applied Research Laboratories, Glendale, Calif., and the Harry W. Dietert Company, Detroit 4, Mich., recently announced a large twometer grating spectrograph. This instrument, shown in Figure 1, was designed to fill a long-felt need for a compact spectrograph of exceptional dispersion, resolution, and versatility. By employing a grating ruled with an unusually large number of lines per inch, high dispersion is achieved without resorting to a bulky instrument of long focal length. The grating is mounted so that a large portion of the Rowland circle can be used, which makes available a spectrum length of approximately 5 feet. The two gratings available have a ruled area 2.5 X 1.25 inches. The first haa 36,600 lines per inch, giving an almost uniform dispersion of 3.4 A. per mm. in the first order or 1.7 d. per 111111. in the second order. The spectrum available in the first order is from 2100 to 7000 8. and in the second order, 1850 to 4600 A. The second grating has 24,400 lines per inch, giving almost uniform dispersion of 5.2 A. per mm. in the first order and 2.6 A. per mm. in the second order. The spectrograph can be supplied with either or both of these gratings. When both gratings are supplied, one or the other is brought into use by a small angular shift of the incident beam from one grating to the other. A 24-inch camera, providing a 20-inch spectrogram on 35mm. spectrum film,is standard. It is movable on a radius arm which allows positioning along the Rowland circle so that any 20-inch portion of the spectrum may be photographed. With the first grating 1660 can be photographed in the first order or 830 A. in the second order. For the second grating the corresponding figures are 2500 and 1250 A. Sixteen spectra suitable for quantitative analytical work can be photographed on a single strip of 3 5 - m . film. TWO 1OO-foot rolls of film are stored in the camera (Continuedon page 9%)
cornerstone of spectroscopy is the principle that emitted light is characteristic of the atom or molecule that produces it. The development of this principle was a slow process. Newton, in 1666, showed that sunlight is heterogeneous and that a prism separates it into its components. He knew that by the use of a lens he could obtain better separltion and a purer spectrum. His apparatus was therefore a nearly modern form of spectroscope. In the next two hundred years many workers nearly grasped this fundamental principle, but none completed the development. It remained for Kirchhoff to state this important generalization. He and Bunsen were the first to make practical use of spectrochemical analysis. Their work on the spectra of the alkali metals led them to the discovery of cesium and rubidium and permitted them to identify some of the elements in the atmosphere of the sun. Qualitative spectrochemistry was firmly established by these achievements. Quantitative spectrochemical analysis is based on the fact that the intensities of the lines due to a small amount of an element in a matrix decrease as the concentration of the element is decreased. Hartley, in 1882, made the first quantitative spectrochemical analyses. He and other early spectrochemists, such as de Gramont, compared visually the spectra of samples containing unknown amounts of an element with those of a series of similar HE
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case. Each has a counter to show the number of feet used. Film from either roll can be placed in the camera track without opening the camera. The unused portion of the film is protected from stray light by a mask which also linrits the length of the spectrum lines. The camera is racked electrically at the end of an exposure to make the instrument ready for the next spectrum. The instrument can be supplied either with fixed slits in widths of 15, 30,BO, and 100 microns or with an adjustable bilateral slit calibrated in 2-micron steps. An electromagnetically operated slit shutter within the instrument is controlled by a timer mounted on the control panel of the spectrograph. The optical bench is fitted with a universal arc-spark stand of rugged design and an intensity control stand and lens holder. All portions of the spectrograph, including the optical bench, are fastened to a massive cest-aluminum base. This is mounted by a three-point suspension in a heavy sheet-metal case. Metal covers which can easily be removed for adjustment or inspection close the top of the base casting. The design of the instrument is such that it can be built into a wall with the camera portion in the dark rqom and the source-slit portion of the instrument in the lighted part of the laboratory.
Although quantitative Spectrochemical analysis using photographic techniques haa reached the point where samples of various types can be analymd for as many as six minor constituents in a total elapsed time of 6 minutes, there are many applications where it would be valuable to decrease the time required. The recent development of electron multiplier-type phototubes has made possible direct-reading spectrochemical apparatus. Two of these instrur ients were described a t the October meeting of the Optical Society of America in New York, one by M. F. Hasler and H. W. Dietert of A.R.L.-Dietert, the other by J. L. Saunderson, V. V. Caldecourt, and E. W. Peterson of Dow Chemical Company. Both instruments consist of grating spectrographs of special design, arranged so that sensitive spectral lines of the constituents to be determined and an internal standard line pass through exit slits and fall on the cathodes of electron multiplier phototubes. Suitable amplifiers and electrical computing circuits are provided to measure the intensities of the unknown lines relative to that of the internal standard line. The Dow instrument is especially designed to control magnesium production by analyring, in the shortest possible time, samples from the pots before pouring. It supplies a record from which the percentages of seven elements may be read from calibrated scales 55 seconds after the sample electrodes are placed in the instrument. The A.R.L.-Dietert instrument had been used on aluminum alloys and steels in addition to magnesium. Its designers concluded that much quantitative spectrochemical analysis of a routine nature which does not involve spectrographic traces of elements or basic materials possessing complex spectra can be carried out more rapidly and accurately by a direct-reading instrument than by present photographic instruments. These instruments are a good example of the impact of electronics on instrumentation. Here electronic methods achieve a result unattainable by other means. I n addition, there is the possibility of greatly improving these direct-reading spectrochemical instruments by the application of still newer electronic devices. George R. Harrison has suggested that a special iconoscope or television pickup tube might be substituted for the phototubes now used. This would have the advantage that spectral lines too close together for present direct-reading instruments could be resolved, which would greatly increase the utility of direct-reading spectrochemical equipment.
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