Under favorable conditions, impurities in solids can be detected at a part-per-billion atomic fraction in a mass-spectrographic analysis using photographic recording. An important factor limiting the concentration that can be thus detected is the diffuse background against which a faint
mass line must be detected. The billionfold excess incidence of major-component ions produces secondary effects of sufficient magnitude to appreciably fog certain regions of the plate, thereby substantially increasing the concentration needed for detection of elements in these fogged regions.
This fogging in an MS–7 mass spectrograph is substantially reduced on the high-mass side of the incident ions by removing that segment of the photographic plate on which the major components are incident and trapping these ions and their secondary products by an appropriate collector.
The most likely mechanism for fogging is thought to be scattering of positive ions from the point of impact.
Bell Telephone Laboratories, Incorporated, Murray Hill, New Jersey 07971
Publication date: November 1, 1966
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