SPECTROCHEMICAL ANALYSIS 389 



Another example of this sort of analysis is the identification of 

 aluminum alloys. It is a not uncommon occurrence for an alloy 

 supposed to be of a certain composition to exhibit physical properties 

 which would not be expected of such an alloy. Frequently it is 

 important, therefore, to check the composition of the alloy and still 

 preserve it for other tests. Now in the Bell System there are nine 

 commonly used aluminum alloys, each of which contains different 

 proportions of copper, manganese, magnesium, silicon, chromium and 

 nickel. By examination of the spectrum obtained from a few grains 

 of filings the analyst can easily tell by estimating the amounts of these 

 minor components which of the nine alloys comprises the sample in 

 question (Fig. 4). 



3. Quantitative Analysis With High Speed and 

 Moderate Precision 



The greatest advantages of the spectrochemical method are speed, 

 sensitivity, and flexibility of application. In many cases if a new 

 material can be analyzed quickly only a moderate degree of precision 

 is necessary. To fulfill such conditions an application of the method 

 has been developed ^ which can be used for almost any material that 

 can be dissolved in common solvents. The sample is first analyzed by 

 method (1) or (2) above to find its approximate composition if that 

 is not already known. Then a concentrated solution of the sample is 

 prepared. 



Another solution is prepared from pure specimens of the base mate- 

 rials present in the sample. To portions of this pure solution known 

 amounts of the impurities present in the sample are added so that 

 standard solutions are available for comparison with the sample. 

 Spectra of the sample solution and the standard solutions are photo- 

 graphed on the same plate, using arc excitation on graphite electrodes, 

 and the percentage of each impurity is estimated by comparison of line 

 intensities. The method is capable of a precision of better than ± 10 

 per cent of the amount determined and can frequently be applied with 

 little modification in cases where chemical methods would require 

 considerable development work before they could be used. Further- 

 more the total sample needed for analysis is always very much less 

 than is needed for a chemical analysis. 



Since this is the quantitative method most widely applied in this 

 laboratory numerous examples could be cited. To mention a few: 

 aluminum, arsenic, tin and zinc in lead-base alloys, zinc in tin-base 

 alloys, zinc in aluminum-base alloys, and cadmium, lead, tin, copper, 



3 Nitchie, C. C, Indus. & Engg. Chem. (Anal. Ed.), 1, 1 (1929). 



