Combined LA-ICP-MS with empirical coefficient correction and Bi fire assay for the determination of ultra-trace Pt, Pd, and Au in geochemical samples using a 194Pt isotope spike as the internal standard.

In this work, a novel and environmentally friendly method based on bismuth fire assay (Bi-FA) combined with laser ablation ICP-MS (LA-ICP-MS) was established for the determination of ultra-trace Pt, Pd, and Au in geochemical samples, using a Pt isotope spike (194Pt) as the internal standard. Due to the low toxicity and low, stable blank values of the Bi2O3 collector, Bi-FA could be directly employed for the pre-concentration of Pt, Pd, and Au prior to analysis. The Ag slices prepared by annealing and tableting were analyzed at multiple points by LA-ICP-MS. This approach offered advantages including rapid analysis, good measurement stability, and the elimination of polyatomic ion interference and dilution effects associated with solution-based analysis. As the internal standard, the Pt isotope spike was thoroughly mixed with the target elements (Pt, Pd, and Au in the sample) in each Ag prill, and its distribution was consistent with that of the target elements. This method resolved the issue of uneven distributions of Pt, Pd, and Au in the Ag slices, thereby enabling internal standard correction for LA-ICP-MS by calculating the intensity of 194Pt from the Pt isotope spike. In addition, optimization experiments were conducted for key parameters, including laser frequency, energy density, ablation spot diameter, and ICP-MS dwell time. The Ag slices prepared from certified reference materials (CRMs) were used as the calibration standards. The standard samples had the same matrix as the real samples, which reduced laser ablation fractionation effects, minimized analytical errors, and improved accuracy. The standard curves were plotted by the empirical coefficient method and showed excellent linearity with correlation coefficients (R2) of 0.9999 (Pt), 0.9998 (Pd), and 0.9995 (Au). The limits of detection (LODs) of the method were 0.066 (Pt), 0.052 (Pd), and 0.090 (Au) ng g-1. The proposed method was validated by determining the Pt, Pd, and Au in CRMs, showing excellent agreement with certified values. When applied to geochemical samples, the results were consistent with those obtained by lead fire assay combined with graphite furnace atomic absorption spectrometry (Pb-FA-GF-AAS). The recoveries were 2.5-107.8% (Pt), 91.4-108.0% (Pd), and 91.0-109.2% (Au), demonstrating the good accuracy of the proposed method.