Spectral interferences in light element analysis

Interferences (overlaps) occurring when lines of other. elements affect eit her peak or background measurements can cause errors in quantitative WD ana lysis, bur may be minimised by suitable choices of analysis conditions such as spectromerer crystal, background offsets, and pulse-height analyser set tings. Computer spectrum-simulation is much more effective than reference t o wavelength tables for investigating interferences. The 'Virtual WDS' simu lation program developed by the present authors, hitherto applied only to ' ordinary' elements (Z greater than or equal to 11), has been extended to li ght elements for which evaporated multilayers are used in place of true cry stals. 'Virtual WDS' utilises experimentally recorded light-element K spect ra and L and M spectra of heavier elements in the same wavelength range. It is impractical to record all high-order peaks, so computed line profiles a re used, with widths and intensities interpolated from a limited set of obs ervations. The relative positions of first and higher order peaks are affec ted significantly by the refractive index of the multilayer, requiring modi fication of the Bragg equation. Suppression of high orders by pulse-height analysis is less effective than for 'normal' wavelengths, owing to the brea dth of the pulse-height distribution for low X-ray energies. Simulation usi ng a Gaussian expression aids optimisation of threshold and window-width se ttings.