Customized sample preparation for TXRF analysis of soil organic matter and amendments using a multivariate experimental design

Total-reflection X-ray fluorescence (TXRF) spectrometry is a well-established technique for the elemental analysis of many kind of samples, from water solutions to ores. This technique offers many advantages, among which the micro-analytical and multi-elemental character, along with simple and fast sample preparation. Indeed, minimal amounts of sample (5-10 μL) as solutions or prepared as slurries are required for analysis. In case of slurries, various factors can influence the analytical performances, including the mass of sample, the volume of the dispersant, the internal standard concentration, etc. Furthermore, it has been observed that the combination of these factors can affect the analysis much more than the variations of single parameters. Therefore, a multivariate experimental design should be applied to assess the effects of single and combined factors’ variations, in order not to find one of the possible acceptable slurry preparation (e.g. evaluating the recovery after the analysis of a reference material), but the best one. In this work, a full factorial experimental design was applied to develop the most suitable protocol for the preparation of soil organic matters and amendments as slurries for TXRF analysis. Two factors were considered, e.g. sample mass (10 mg – 100 mg) and dispersant volume (1 mL – 5 mL of 1% Triton X-100 solution). Firstly, the study was carried out on the reference material NIST 1635a and using recovery as monitoring parameter. The best results were obtained with 10 mg in 3 mL and 10 mg in 5 mL. After that, the validation of these two sample preparation combinations was performed using another reference material (NIST 1573a). Based on recovery, relative standard deviation, limits of detection and quantification values, it was found that the best sample preparation method was the one using 10 mg of samples in 3 mL of dispersant. This procedure was therefore adopted for the TXRF analysis of a set of soil organic matter and amendments samples including three humic acids, three humates, three peats, two biochars, one digestate and one compost. The analyses were performed with two Bruker S2 Picofox TXRF spectrometers, equipped with a Mo and a W sources, respectively. The obtained results are statistically similar to those obtained analyzing the same matrices with ICP-OES, which is the recognized method for the analysis of this class of samples. This demonstrates the good analytical performances of the new developed method. Agritech National Research Center, European Union Next-Generation EUPNRR – MISSIONE4 COMPONENTE2, INVESTIMENTO1.4 – D.D. 1032 17/06/2022, CN00000022