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In an analytical laboratory, reference materials are used for several purposes, including the calibration of analytical instruments, periodic standardization of these instruments, and verification that the standardization is correct. Reference materials are also used to confirm that an analysis is performed with optimum precision and is not biased, and to determine the presence and effects of interferences. All of these applications require that proposed reference materials be evaluated for homogeneity. To this end, specimens are cut at regular intervals from bars selected for evaluation and identified accordingly. Across the entire cross section of each specimen, a flat surface is prepared as specified by the analytical method. The specimens are analyzed in random order by optical emission spectrometry, once at the center of the specimen and twice as close to the edge of the specimen as possible. If X-ray emission (fluorescent) spectrometry is used, two analyses are performed with a fresh specimen preparation before each analysis. Statistical analyses are performed on the values obtained for each element to determine whether there are significant differences in composition between center and edge or among specimens. This procedure has been used successfully for ten years to evaluate proposed reference materials prior to certification.
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