Calibration strategies for elemental determination using direct solid analysis by x-ray spectroscopy and laser-induced breakdown spectroscopy

Abstract

Direct analysis of solid samples using techniques such as wavelength dispersive X-ray fluorescence (WD XRF), X-ray absorption near-edge structure (XANES) spectroscopy and laser-induced breakdown spectroscopy (LIBS); have many advantages for the development of new analytical methods. However, as the sample is analyzed in its entirety (analyte and matrix present in the analyzed portion) by these techniques, strong matrix effects and inadequate preparation of the solid calibration standards can occur and compromise the determination with the necessary accuracy and precision. Several univariate and multivariate calibration strategies and different preparations of solid calibration standards were evaluated for the direct determination of i) Ca, P and S in mineral supplements for cattle by WD XRF, including the speciation analysis of S (elemental sulfur and sulfate species); ii) species of P (phosphate and phosphite) in fertilizers and mineral supplements for cattle XANES spectroscopy, and iii) of Ca and P in mineral supplements, and Al and Pb in electronics waste using LIBS. The evaluated strategies were matrix-matching calibration, internal standardization, calibration-free, single-sample calibration, one-point and multi-line calibration, two-point calibration transfer, partial least squares regression and linear combination fitting. Two new univariate calibration strategies have been proposed for the direct analysis of solids by LIBS: i) multi-energy calibration and one-point gravimetric standard addition. These strategies enable efficient matrix-matching and minimize matrix effects. Considering the advantages and limitations of the evaluated calibration strategies, the intrinsic characteristics of the analyzed samples and the determined analytes, associated with appropriate chemometric tools; new methods of determination and analysis of chemical speciation were proposed in this thesis, with excellent parameters of analytical performance.