Digestions vs. suspensions: The influence of sample preparation on precision and accuracy in total-reflection X-ray fluorescence analysis by the example of waste incineration fly ash

The elements K, Ca, Ti, Mn, Fe, Cu, Zn, Rb, Sr, Pb, and Br in a fly ash sample from municipal waste incineration were determined by total-reflection X-ray fluorescence (TXRF) spectrometry. The fly ash was analyzed after microwave-assisted digestion and in the form of suspensions, each with several modifications. The results of the different sample preparation methods were compared with regard to precision and accuracy. Multiple sample spots were prepared for each sample preparation method, and the spread of the results was used as a measure of the precision. Accuracy was evaluated by comparing the median of the results with ICP-OES/ICP-MS analysis results. Additionally, the results were validated using a waste incineration fly ash reference material. Different internal standards and the influence of adding a surfactant were tested for the digestion solution. For the suspensions, the fly ash sample was subjected to different grinding levels and suspended both in water and in 1% w/w nitric acid. Light microscope images were taken of the dried sample spots on the sample carrier, and their size and shape were visually compared. Some sample preparation methods led to an inhomogeneous depth distribution of the analytes and/or the internal standard within the sample spots. This inhomogeneity was detected by angle scans and was found to be a main reason for systematic errors and/or a low precision. With optimized preparation of the suspension and of the digestion solution, comparable precision could be achieved, even in the range of the instrument precision. A suited sample preparation method that requires the least effort and least use of chemicals is presented for each element.