Validation strategy for an ED-XRF method to determine trace elements in a wide range of organic and inorganic matrices based on fulfilment of performance criteria

Abstract

This work describes the approach used to validate a method based on the use of energy-dispersive X-ray fluorescence (ED-XRF) which should serve the purpose of determining routinely trace elements in organic and inorganic matrices in a wide range of mass fractions. A high energy ED-XRF (up to 100 kV), equipped with 3D Cartesian polarising optics was used. The novelty of the validation is that it is based in the fulfilment of performance criteria empirically established, and moving away for mathematical algorithms built in the ED-XRF software which frequently gave unrealistic results not fit for their intended purpose. The following performance characteristics were determined: limit of quantification (LoQ), repeatability, intermediate precision, standard uncertainty, trueness, working range and robustness; matrix effect and selectivity were also evaluated. The method was applied to the determination of 25 elements (Al, As, Ba, Br, Ca, Cd, Cl, Cr, Cu, Fe, Hg, K, Mg, Mn, Mo, Ni, P, Pb, Rb, S, Se, Sr, Ti, Zn, Zr), in organic matrices and 12 elements (As, Br, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Se, Zn,) in inorganic matrices and for mass fractions ranging from sub mg kg-1 level up to percentages, depending on the specific element. The LoQ was empirically calculated as the mass fraction for which a certain trueness and uncertainty was achieved, on the basis of information gathered from the ratios between the calculated and the certified or reference mass fractions (Ccalc/CCRM) in the calibration curves. The lowest LoQ, 0.16 mg kg-1 was achieved for Ni. Trueness was evaluated using 29 certified reference materials (CRMs) and reference materials (RMs). Repeatability and intermediate precision was found to be mass fraction-related, the higher the mass fraction the larger the repeatability/intermediate precision. The method was satisfactorily applied to the determination of trace elements in sediments, soils, cereals (wheat and rice), tobacco, feed, fish and milk.