Improved precision of quantitating calcium in biological electron probe analysis

We describe the use of direct multiple least-squares fitting of (unfiltered ) biological X-ray spectra. Test of this method on computer-generated spect ra and experimental specimens shows that, for the average biological specim en, direct fitting improves the precision of Ca measurements by an approxim ate factor of 1.6 for a given electron dose. The influence of non-statistic al, instrumental sources of measurement errors is considered, and we show a simple method for reducing the effect of these sources of error in the dir ectly fitted spectra.