Ford R. Bryan Ford Motor Company Dearborn, Michigan
Polaroid Films Simplify Instruction in Spectrographic Analysis
Laboratory instruction in spectrographic analytical methods has been seriously handicapped by the need for rather elaborate darkroom facilities and considerable student training in photographic processing techniques. A3 a result, courses in spectrographic analysis can be afforded only by comparatively large schools, and class enrollments must he limited to relatively small numbers. Since conventional photographic processing is time-consuming and somewhat, incidental to the underlying principles of spectroscopic analysis, a means of simplifying the photographic process is important from the standpoint of efficient tearhing. Polaroid emulsions can be used to demonstrate both qualitative and quantitative analysis by modifying n conventional spectrograph to accomodate Polaroid film. The experiments to he described involve the use of Polaroid positive prints and are designed to illustrate a simple means of utilizing one-step photographic processing' in a system appropriate for laboratory instruction.
elaborate than the high precision facilities used for industrial analysis or university research, it offers order of magnitude benefits in initial costs and in experiment,al time required t,o demmdrat,e basic prinriples.
Equipmenf
Spectrograph, source, and recording medium are selected to he compatible with one another in degree of usefulness. The spectrograph which has been used is a medium dispersion, glass prism instrument2 with mechanically limited range of 3700 to 6500 A. The transmission of glass and the spectral sensitivity of Polariod emulsions extend to nearly 3000 A, however. The spectrograph's upper limit of wavelength range coincides with the emnlsions's limit of 6500 A. The region in the neighborhood of 4000 Ais especially useful for alloy analysis. In this region dispersion is approximately 4 A per mm, and 400 A can he photogra,phed in a single exposure. Several exposures can he recorded on one photographic frame. The spectrograph camera is modified to accomodate a Polaroid film holder for Professional Pan, Type 53, in 4 X 5 Film Packet size. Separate frames are thus loaded, exposed, and processed individually. An A.C. arc generator provides adequate sensitivity and reproducibility for demonstration of both qualit,ative and quantitative spectrochemical applications. The arc circuit is equipped with an ignitor and ballast resistor and operates from standard 110 or 220 v outlets. The complete arrangement of equipment can be acrommodated on a 44%.bench space as illustrated in Figure 1. Although apparatus of this type is less
LAND,E. H.,3 . Opt. SOC.Am., 37, 61-77
(1947). Metal-Spectroscope manufactured by R. Fuess. Distributed in the United States hy Applied Research Laboratories, Glendale, 1
California.
Figure I. Complete spectrographic equipment: A, orc ignitor; 0 , rpecimen stand; C, spectrometer; D,cornero; E, Poloroid (ilrn holder.
Use in Analysis
A simple arc source together with a low dispersion spectrograph is satisfact,ory for the qualitative ident,ification of metals. Excitat,ion need not he especially stable for producing qualitative spectra, and spectral range is often more important than high dispersion. A supply of Johnson-Matthey pure metals and a copy of the MIT Wavelength Tahles will serve to establish the identity of lines. Figure 2 illustrates spectra of three commercial alloys recorded in the near ultraviolet on the Polaroid, Type 53, emulsion. Both major and minor constituents can be identified by selection of arc current and exposure time. A one-minute exposure reveals the trace of iron impurity in the aluminum alloy. The alloying elements, copper and magnesium, are also apparent in the aluminum. The copper alloy contains several per cent aluminum which classifies it as an aluminum bronze. The magnesium metal, containing alloyed aluminum, is found to he relatively free of impurity elements in this region. The dispersion, range, and sensitivity of this equipment combined with the Polaroid emulsion are exceptionally well suited to rapid qualitative metallurgical analysis. Volume 37, Number 9, September 1960
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8.49-3838
A1~3961
Figure 2. Qualitative orc spectra of commercial alloys recorded on polaroid Rim: A, aluminum alloy; 8, olvminum bronze; C, magnesium alloy.
ciple is observable in Table 1. Chromium concentrations which are indistinguishable by means of chromium lime heights alone are reliably separated by consideration of the corresponding iron reference line heights. A graphical relationship between differences in line heights and chromium concentration may be presented as in Figure 4. This form of calibration curve emphasizes the inherent non-linearity of spectral response to concentration and graphically illustrates the high sensitivity of the spectrographic method a t relatively low concentrations. From Figure 4 it is also quite apparent that a constant photometric error produces a widely varing absolute error in concentration when the entire concentration range is considered. A more conventional analytical calibration curve is a plot of the logarithm of the ratio of line intensities versus the loearithm of element concentration. This version of the same data is shown in Fignre 5. The u
Principles of quantitative spectrographic analysis can be demonstrated by the photometric comparison of line intensities from unknowns with the intensities of corresponding lines from known standards. The concepts of internal standard lines and homologous line pairsS are essential eonsidrrat,ions in any quantitative procedure. Of the various methods of line photometry, the measurement of line height as produced by a logarithmic wedge4 seems attractive, since the spectra are produced on a paper-based emulsion and cannot he measured by st,andard transmission densitometry. Either the logarithmic sector disc or a neutral filter of the continuous logarithmic wedge type is appropriate when placed immediately adjacent to the spect.rograph entrance slit. For this work, a Wratten Neutral Density Wedge of A m size was photographically reduced on fiue-grained film to 15-mm dimensions and placed over the entrance slit to provide a density gradient of 0 . 0 to 2.0 along the length of the slit. Resulting spectral lines are thus tapered in appearance, and the difference in lengths" of t,wo limes within a spectrum is proportional to the logarithm of the ratio of the line intensities. A simple magnifying glass wit,h millimeter scale provides line height measurements reproducible to the nearest 0 . 6 mm value. An example of quantit,ative calibration utilizing this simplified spectrographic procedure is illustrated by means of a series of steel standards containing chromium ranging from 0.006% to 2.40% by weight. Each standard was excited for a period of 15 sec by means of a & A.C. arc. The spectra were recorded in the 4200-4.100 A region where several sensitive chromium lines exist (Fig. 3). The heights of the Cr14289.7 A line and the FeI 4292.3 A line were measured, and t,he difference in line heights determined for each standard. A listing of the standards together with the corresponding line heights and differences obtained is shown in Tahle 1. Importance of the inkrnal standard prina GERLACH, W., SCHWEITZER, E., "Foundations and Methods of Chemical Analysis by the ihission Spectrum," Adam Hilger, LM., London, 1931. 4 S ~ G., ~NEUEX.URSER, ~ ~ ~A,, ~Angew. , Chent., 41, 1218
Figure 3. Qumntitotive spectro of steels recorded thmugh o logarithmic wedge on Polaroid Rim.
difference in line heights, as determined by the optical wedge function, represents the logarithm of the intensity ratio; the concentration is then plotted on a logarithmic scale. The expected linear relationship is t,hus obtained over approximately three orders of magnitude concentration. From this type of calibration it is apparent that a given phot,ometric error results
DIFFERENCE IN LINE HEIGHTS imm 1 Figure 4. Response of the Cr 1 4289.7 A and Fe 14292.3 Aline pairwith changesof chramivmconcenamionin~tee~.
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Table 1.
Chromium concentration
(%)
Quantitative Photometric Data
Chromium line height (mm)
Iron line height (mm)
Difference line height,^ (mm)
the sample. Refinements in photometry could perhapr; reduce error to within f5%.= As with spectrographic methods in general, the degree of accuracy obtainable is most advantageous at. low concentrations. Conclusions
DIFFERENCE I N LINE HEIGHTS i m m
1
Figure 5. Conventional mdyticol calibration curve for chromium In steel udng arc excitation, wedged line photometry, end Polaroid emulsion.
in a constant error throughout the concentration range in terms of percentage of the amount of element present in the sample. I n this quantitative procedure the error in matching line heights, which approaches a 0.5 mm maximum is believed to be the limiting factor in accuracy of the method. I n this case of chromium in steel, the error averages about of the amount of chromium in
Spectrographic analysis, both qualitative and quantitative, can be accomplished with comparatively simple equipment and without a darkroom, by the utilizat,ion of Polaroid-Land films. Range and accuracy of the methods are appropriate for demonstration of the basic principles of emission spectrochemical analysis of metals and alloys. Savings in equipment costs and experimental time over conventional spectrographic procedures are sufficient to warrant inclusion of spectrochemical instruction in many more chemical and metetlurigical courses. "GREEN,
M.,AND POLK,M. L., Appl.
8peelloscopy, 8, 126-10
(1954).
Volume 37, Number 9, September 1960
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