Spark Excitation in Inert Atmospheres
Within the last few years, spark excitation in atmospheres other than air has proven its value in spectrochemical analysis. The beneficial influence of an inert atmosphere is undoubtedly due in part to the elimination of surface oxidation reactions which often strongly influence spectral intensities. Beyond this obvious chemical factor, there are other, more subtle influences due to the presence of the gas which can strongly influence the nature of the spectrum in different atmospheres. Thus, the spectrum of a given sample sparked in helium and in argon will behave quite differently in regard to total and relative line intensities, line form and width, relative strength of ion lines, line to background ratio, tendency of certain lines for self-reversal, reproducibility of intensity ratios in replicate exposures, etc. These observable effects depend upon the ionization and excitation potential of the gas, as well as upon the atomic weight and the collision cross section of the gas atoms with sample vapor. Since the sample vapor enters the spark gap in the form of supersonic vapor jets from the cathode, it is quite evident that the slowing down of these jets by collisions with gas atoms plays a major role in the excitation of spark spectra in different inert atmospheres. An elementary theory of this process will be developed and some of its predictions will be compared with experiments.
KeywordsInert Atmosphere Velocity Curve Collision Cross Section Half Cycle Sample Vapor
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