Simultaneous determination of Cd, Pb, Cu and Zn as total and labile fractions in soil using a small-sized electrothermal vaporization capacitively coupled plasma microtorch optical emission spectrometer after diffusive gradients in thin-film passive accumulation

Abstract

The study presents for the first time the figures of merit of a completely miniaturized instrumentation based on a capacitively coupled plasma microtorch as the core element interfaced with a small-sized electrothermal vaporization device and a low-resolution microspectrometer for the simultaneous determination of Cd, Pb, Cu and Zn by optical emission spectrometry in soil as total and labile fractions after diffusive gradients in thin-film (DGT) accumulation. The coupling of the low-power and low-argon consumption plasma (15 W; 150 mL min⁻¹) with the DGT passive accumulation technique, although requiring a too-long time for sample preparation, allowed a considerable improvement of the detection limits and avoidance of the non-spectral matrix effects, otherwise a recognized process in low power microplasmas, when a complex matrix is analysed, like environmental samples. The detection limits for the total content in soil were (mg kg⁻¹), 0.10(Cd), 0.40(Pb), 0.15(Cu), and 0.03(Zn), one order of magnitude better than in the procedure without DGT accumulation and 10–3300-times lower than the guide values in soil. In the DGT-based labile fraction exhibiting the highest bioavailability the detection limits were (μg kg⁻¹) 0.01(Cd, Cu, and Zn) and 0.03(Pb), which allowed the determination of Cd, Pb, Cu and Zn in the concentration range (μg kg⁻¹) of 0.3–2.0, 0.8–18.4, 2.4–56.3 and 9.4–60.6, respectively. Validation through the analysis of certified reference materials (CRMs) showed a recovery of 85–123% with a relative expanded uncertainty of 19–35% (k = 2) for the total content of analytes. The analysis of the certified reference materials highlighted that the DGT accumulation was not affected by the multielemental matrix, since the experimental diffusion coefficients of the analytes were similar in the four analyzed CRMs and to those provided by the manufacturer, respectively. Precision for the measurements of the total content and DGT-labile fraction in real samples evaluated from the combined uncertainty was 10–19% and 10–15%, respectively. The Bland–Altman plot applied to the results of real samples indicated the lack of statistical differences versus line-source graphite furnace atomic absorption spectrometry for both the total content and DGT-based labile fraction.