'Sharing' Background Measurements in Wavelength Dispersive Electron Probe Microanalysis

Wavelength dispersive x-ray spectrometry in electron probe microanalysis requires subtraction of the Bremsstrahlung background, which is usually done by measuring the intensity in 1-2 positions close to the peak. Accurate background determination is crucial in trace element analysis where the peak to background ratio is low. To improve accuracy the multi-point background (MPB) technique has recently been developed, where the background is measured in multiple positions for each line, followed by fitting a curve to the background data. Compromised backgrounds can be excluded automatically or manually. MPB is well suited for high accuracy analysis of complex phases such as monazites, but acquiring multiple backgrounds for many elements can substantially increase the total acquisition time, which reduces throughput and can also aggravate electron beam irradiation effects. Backgrounds of neighbouring x-ray lines are also often measured in similar absolute positions.

We present a method to reassign conventional off-peak backgrounds (2 per element) to all elements measured on the same monochromator and process the data in the same way as for MPB, e.g., if 4 elements were acquired on a spectrometer, each element has an MPB array of 8 points after background "sharing". The final point selection for background fitting is optimized during postprocessing. As an example, wavescans and 30 point measurements were acquired on a natural scheelite crystal using a Cameca SX100. Settings and results are given in Table 1. The 3 datasets are based on the same measurements with the only difference being the background correction. The material shows considerable inhomogeneity, but for most elements the variations are within the precision and the levels below the detection limit of the method, hence the challenge in these cases is to accurately measure zero.