The use of diffuse reflectance mid-infrared spectroscopy for the prediction of the concentration of chemical elements estimated by X-ray fluorescence in agricultural and grazing European soils .

The aim of this study was to develop partial least squares (PLS) models to predict the concentrations of 45 elements in soils extracted with aqua regia (AR) using mid-infrared (MIR) spectroscopy. A total of 4130 soils from the GEMAS European soil sampling program (geochemical mapping of agricultural soils and grazing land of Europe) were selected and MIR spectroscopy used for the development of models to predict Ag, Al, As, B, Ba, Be, Bi, Ca, Cd, Ce, Co, Cr, Cs, Cu, Fe, Ga, Hg, In, K, La, Li, Mg, Mn, Mo, Na, Nb, Ni, P, Pb, Rb, S, Sb, Sc, Se, Sn, Sr, Th, Ti, Tl, U, V, W, Y, Zn and Zr concentrations extracted by AR. From the full soil set, 1000 samples were randomly selected for the development of the calibration models, with the remaining 3130 samples used for model validation. Partial least-squares calibration models were used to relate the infrared (IR) spectra and the elemental concentrations in soils. The PLS calibrations were validated using cross validation and elements classified as a function of residual predictive deviation (RPD) values and R2 of the predictions. According to the RPD and R2 values of the validations, the 45 elements were allocated into two main groups; Group 1 (successful calibrations), 30 elements including those elements with RPD and R2 values equal or higher than 1.5 and 0.55, respectively: Ca (3.3, 0.91), Mg (2.5, 0.84), Al (2.4, 0.82), Fe (2.2, 0.79), Ga (2.2, 0.79), Co (2.1, 0.77), Sc (2.1, 0.77), Ni (2.0, 0.76), Ti (2.0, 0.75), Li (1.9, 0.73), Sr (1.9, 0.73), Cr (1.8, 0.69), Th (1.8, 0.69), K (1.8, 0.68), Be (1.7, 0.66), V (1.7, 0.63), S (1.6, 0.64), B (1.6, 0.62), Y (1.6, 0.61), Zn (1.6, 0.61), Rb (1.6, 0.61), Zr (1.6, 0.59), Na (1.5, 0.57), In (1.5, 0.57), Nb (1.5, 0.57), Cs (1.5, 0.57), Ce (1.5, 0.56), Cu (1.5, 0.56), Bi (1.5, 0.55) and Mn (1.5, 0.55); and group 2 for 15 elements with RPD and R2 values lower than 1.5 and 0.55, respectively: As (1.4, 0.52), La (1.4, 0.52), Ba (1.4, 0.52), Tl (1.4, 0.51), P (1.4, 0.46), U (1.4, 0.46), Sb (1.3, 0.46), Mo (1.3, 0.43), Pb (1.3, 0.42), Se (1.3, 0.40), Cd (1.3, 0.40), Sn (1.3, 0.39), Hg (1.2, 0.33), Ag (1.2, 0.32) and W (1.1, 0.19). The success of the PLS calibration models to predict AR extracted elemental concentrations in soils was found to be dependent on their relationships (directly or indirectly) with soil components that showed significant absorbances in the MIR region.