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Use of Mineral Reference Standards in EPMA: Instrumental Calibration, Standards Comparison, and Quality Control

In electron-probe microanalysis (EPMA), concentration C is calculated from the k ratio of measured Xray intensity emitted from a sample relative to a primary standard where k = (P-B) /(P-B), and corrected for matrix effects via C = k * ZAF. The accuracy of this measurement depends on the instrumental calibration, k measurement from background corrected peak intensity on samples and standards, and conversion to concentration units by the correction algorithm. For standards this equation is used to calculate kcalc from known C using a given correction algorithm, and for samples the measured kmeas is used to calculate C using the same algorithm. The kmeas depends on all aspects of measurement including spectrometer alignment and the calibration intensity measurement, whereas kcalc depends on all aspects of correction including the standard composition and the correction algorithm. The ratio kmeas/kcalc should be unity if measurement and calculation are error-free. Measurements on a suite of microprobe standards using this unbiased comparison has been used to identify errors in alignment of wavelength-dispersive spectrometers (WDS), standard compositions, background determination, and to demonstrate the internal consistency of a suite of standards and accuracy of correction algorithms [1]. A diagnostic plot of kmeas/kcalc vs. concentration is the multielement equivalent of the error histogram historically used to evaluate binary element k data (Fig. 1).