Selected-Area Analysis

Abstract

If the conventional aluminum-specimen-mask technique is applied to very small specimens or to small areas on large specimens, analyte peak-to-background ratio may decrease prohibitively. As analyte-line intensity decreases with specimen area, primary scatter from the increasing mask area increases, and primary scatter from air in the specimen compartment assumes greater significance. Scatter from the mask is reduced by use of a high-Z metal, but then the spectrum of this element is strongly excited, increasing the probability of spectral-line interference. Small specimens can be supported on thin Mylar or fine fibers, but air scatter remains, unless helium or vacuum is used. A better approach is to confine the primary beam to the area of interest, or to exclude all secondary radiation from the optical path except that originating in the area of interest.