392 BELL SYSTEM TECHNICAL JOURNAL 



nickel filaments, and strontium-barium ratios in vacuum tube filament 

 coatings. 



6. Quantitative Analysis of Minute Traces 

 It has often been pointed out that spectrochemical methods can 

 be used to assist classical and micromethods by establishing the purity 

 of precipitates, checking the completeness of separation, demonstrating 

 the presence or absence of interfering elements, etc. It has only 

 infrequently been realized, however, that extremely minute concen- 

 trations of impurities can be determined by a combination of the two 

 methods. In many cases a chemical separation can be made in which 

 all of the impurities are removed from the bulk of the base material 

 (it is not essential that all of the base material be separated as it is in 

 chemical analyses) and can then be concentrated to fall within the 

 spectrochemical range. In this way impurities can be determined by, 

 say, method (3) above which could not be detected by direct excitation 

 of the original sample. Scrupulous cleanliness and the carrying 

 through of a reagents blank are, of course, essential when dealing with 

 such small quantities. 



7. As A Research Tool 



Many times the spectrochemical method can be applied as a probe in 

 obtaining important clues in a research project. This is particularly 

 true when two presumably identical materials show unexpected dif- 

 ferences in behavior. The research man may use nominally pure 

 materials which are never really pure and thus introduce unsuspected 

 impurities into his system which vary with different lots of materials. 

 The spectrograph readily shows up differences in composition of the 

 final product and thus often indicates the beneficial or detrimental 

 effect of impurities. 



In certain types of research on thin layers, the spectrochemical 

 method is the only one of sufficient sensitivity to be used for quantita- 

 tive analysis of the layers. Thus in thermionics it has been possible to 

 measure the amounts of material on tungsten corresponding to changes 

 in work function of the surface. Many other applications to thin films 

 are possible and some are being worked on in this laboratory. 



Thus it will be seen that chemical analysis by the emission spectrum 

 is a powerful and versatile tool which enjoys and deserves an increas- 

 ingly important role in communication research. 



Acknowledgment 

 We are indebted to Mr. E. K. Jaycox for some of the method 

 development as well as several of the spectrograms reproduced here. 



