Improvement approach for determination of cadmium at trace levels in soils by handheld X-ray fluorescence analyzers

Determining trace levels of cadmium in soils by the handheld X-ray fluorescence analyzer is severely hampered by the spectrum baseline. To improve the peak-to-background ratio of the characteristic Cd-Kα X-ray, the filter structure, material, and thickness were optimized; to find the most informative variables for the PLS analysis of trace Cd, the related information enhancement competitive adaptive reweighted sampling (rieCARS) was proposed as a variable selection approach, which contains correlated spectral selection (CSS), double logarithmic processing(DLP), competitive adaptive reweighted sampling (CARS). To eliminate the interference of the K-series line of the Ag anode target on trace Cd, the X-ray tube was used with the Tungsten anode target. One combined filter containing a 0.4 mm thickness Cu and a 0.5 mm thickness Fe was selected, which was placed between the X-ray tube and the sample. The instrumental limit of detection for the Cd descended from 32 mg/kg to 0.02 mg/kg . Sixty-four soil samples were used to establish the model for the filtered signal. The model was compared with PLS, CARS+PLS, DLP + CARS+PLS, CSS + PLS, and CSS + CARS+PLS. The application of rieCARS improved the performance of the calibration model for trace Cd. The coefficient of determination R2 of the rieCARS+PLS model on the calibration set was 0.990; the RMSEP on the test set was 0.28 mg/kg; the average of the relative error in the Poyang Lake soil samples was 0.18; the relative standard deviation for the thirteen replicates of the GSD17 was 0.07. The developed analytical procedure will lay the foundation for the analysis of heavy metal pollution sources in soils and ecological sustainability assessment.