When the elements of interest in a specimen occur at extremely low concentrations, or when the available amount of material in a specimen is very small, it is desirable to apply the special techniques of trace analysis. In most fluorescence spectrometers the analyzed area on the surface of the specimen is designed to be approximately 5 cm2• If the specimen is larger in area than this sensitive region and is much thicker than the thickness yielding 99% of the emitted x-ray intensity, then it can be considered to be of infinite dimensions. Many of the quantitative models and analysis techniques have been developed for this type of specimen. Occasionally, it is necessary to analyze a specimen which is thin compared to the 99% yield thickness or a specimen which cannot cover the total sensitive area of the fluorescence spectrometer. This type of specimen is considered to be of limited quantity. More frequently, specimens of unlimited quantity are available, but several or all of the elements of interest are present at trace concentration levels.