Spectral Interferences in Light Element Analysis

 Interferences (overlaps) occurring when lines of other elements affect either peak or background measurements can cause errors in quantitative WD analysis, but may be minimised by suitable choices of analysis conditions such as spectrometer crystal, background offsets, and pulse-height analyser settings. Computer spectrum-simulation is much more effective than reference to wavelength tables for investigating interferences. The ‘Virtual WDS’ simulation program developed by the present authors, hitherto applied only to ‘ordinary’ elements (Z ≥ 11), has been extended to light elements for which evaporated multilayers are used in place of true crystals. ‘Virtual WDS’ utilises experimentally recorded light-element K spectra 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 are used, with widths and intensities interpolated from a limited set of observations. The relative positions of first and higher order peaks are affected significantly by the refractive index of the multilayer, requiring modification of the Bragg equation. Suppression of high orders by pulse-height analysis is less effective than for ‘normal’ wavelengths, owing to the breadth of the pulse-height distribution for low X-ray energies. Simulation using a Gaussian expression aids optimisation of threshold and window-width settings.