A solid-spiking matrix matched calibration strategy for simultaneous determination of cadmium and chromium in sediments by isotope dilution laser ablation inductively coupled plasma mass spectrometry†
A rapid solid-spiking matrix-matched strategy is applied to simultaneously determine Cd and Cr in sediments by laser ablation inductively coupled plasma isotope dilution mass spectrometry (LA-ICP-IDMS). For this, conventional instrumentation (i.e. nanosecond laser device and ablation chamber) frequently available in laboratories is used. Two drying procedures for the synthesis of the isotopically enriched solid spike were studied considering precision, as RSD values, and applying nested ANOVA statistics; where from it was found that rotary evaporation mixed the spike solution with the solid matrix best. Thus, the solid spike is prepared using a drying step which reduced significantly the time of the synthesis, from hours (reported in literature) to minutes. The elemental distribution of the target isotopes in the pellets was assessed for natural isotope composition materials, solid spikes and enriched blend samples, and it was statistically demonstrated that the ablation of only five lines per pellet provided satisfactory precision. The time of the measurement step of the mass spectrometer was reduced compared to conventional instrumental conditions, programming a unique sequence to measure Cd (in low mass resolution) and Cr (in medium mass resolution) in the same ablation line. The study of the variables affecting the ablation process by means of a Plackett–Burman experimental design revealed that the method was robust. Accuracy was studied, both trueness (experimental/certified values ratios around unity) and precision (relative standard deviations around 2% and 4% for Cd, and 7% and 10% for Cr, when analyzing SRM 1944 and PACS-3, respectively), with good results. Satisfactory relative uncertainties in the range 10–17% were obtained. The important task of the applicability of the proposed approach (not often found in the literature) was demonstrated by the analysis of marine sediments collected at the Northwest coast of Spain.