Until comparatively recently, trace analysis techniques were in general directed toward the determination of impurities in bulk materials. Methods were developed for very high relative sensitivity, and the values determined were average values. Sampling procedures were devised which eliminated the so-called sampling error. However, in the last decade or so, a number of developments have shown that, for many purposes, the distribution of defects within a material can confer important new properties on the material. Perhaps the most striking example of this is given by semiconductors; a whole...