Importance of internal standard selection in direct routine determination of L series elements concentration in water samples by application of the total reflection X-ray fluorescence technique

The aim of the research presented in this article is to select best internal standard for use in the direct routine analysis of L series elements concentration in water samples by applying the Total Reflection X-ray Fluorescence (TXRF) technique. In the study, aqueous reference materials were used to determine detection limits of the following potentially toxic elements: Pd, Ag, Cd, In, Sn, Sb, Ba, Nd, Gd, Er, Ta, W, Tl, Pb, and Bi. The elements were in the form of: monoelemental, two-elemental (Ba and Ti, W and Cu, Pb and As), and multielemental solutions with different concentrations of elements. The detection limits of chemical elements were analyzed using various internal standards (Co, Ga, Nd, and Sr), and the TXRF spectrometers with the Mo-anode and W-anode X-ray tubes. For the analyzed L series elements, the detection limits, normalized to the same measurement time (t = 1000 s), were calculated. Finally, the internal standards were proposed to analyze the studied elements for two different X-ray sources (the Mo-anode and W-anode X-ray tubes). The detection limits were also discussed with respect to the ISO 20289:2018 standard. Based on the measurements, the relative sensitivities were calculated for each of the applied TXRF spectrometers.