ANALYTICAL PRECISION AND ACCURACY IN X-RAY FLUO- RESCENCE ANALYSIS

When analytical samples are irradiated with x-rays emitted from an x-ray tube or radioactive source, fluorescent x-rays are generated in the sample and can be measured for quantitative analysis of its elements. X-ray fluorescence analysis is rapid, precise and non-destructive. X-ray intensity, which is measured from the number of accumulated counts of x-ray photons per unit time, is always accompanied by a small counting statistical fluctuation which conform to the Gaussian distribution with a standard deviation equal to the square root of the total counts. The precision of an x-ray measurement can, therefore, be predicted by the measured intensity. For example, an accumulated intensity of one million counted x-ray photons has a standard deviation of 0.1%, and for one hundred million counts the standard deviation is 0.01%. When an x-ray beam propagates through a sample, its intensity is modified by matrix element effects, including the generation of characteristic x-rays, absorption of the emitted xrays along their paths, and the enhancement effect due to secondary excitation. Studies of these modification processes and related x-ray physical phenomena lead to the derivation of mathematical correction formulae. The development of these x-ray correction methods dominates the analytical performance of the x-ray fluorescence method.