Determination of elemental bioavailability in soils and sediments by microwave induced plasma optical emission spectrometry (MIP-OES): Matrix effects and calibration strategies

Abstract

In the past few years, microwave induced plasma optical emission spectrometry (MIP-OES) has generated great interest as an alternative technique to inductively coupled plasma-based techniques due to its lower operational cost. Since MIP-OES suffers from severe matrix effects due to easily ionizable elements (EIEs) (Na, Ca, etc.), it is unclear whether this technique could be employed for elemental bioavailability studies in soils and sediments since the main extractant solutions employed in such works may contain high levels of these elements. Thus, the aim of this work was to evaluate the feasibility of MIP-OES as a detector for such applications. To this end, the influence of different extractant solutions (0.25 mol L−1 MgCl2, 0.25 mol L−1 CaCl2, 0.10 mol L−1 acetic acid, 0.05 mol L−1 Na2EDTA, 0.25 mol L−1 NaNO3, 0.25 mol L−1 NaOAc/HOAc and 0.10 mol L−1 NH2OH·HCl) on the analyte emission of several elements (As, Cd, Co, Cr, Cu, Fe, Mg, Mn, Mo, Ni, Pb, Rh, Se, Sr and Zn) was investigated. Results were compared to those obtained using a reference solution made of 5% w w−1 HNO3 solution. For saline extractant solutions, both the optimum nebulizer gas flow rate (Qg) and analyte emission were modified with regard to the reference solution. In general, the optimum Qg was reduced by between 0.1 and 0.2 L min−1 for both ionic and atomic lines. Under optimum Qg conditions, analyte emission was supressed by saline solutions except for atomic lines with an upper electronic state below 4 eV, which were enhanced. The magnitude of matrix effects was strongly dependent on EIE ionization energy. The lower the ionization energy, the greater the matrix effects were registered. No measurable matrix effects were registered on both Qg and analyte emission within experimental uncertainties for NH2OH·HCl and acetic acid extractant solutions. Experimental data suggest that matrix effects were related to changes in plasma characteristics and the analyte excitation/ionization mechanism. To mitigate matrix effects and improve long-term MIP-OES performance, internal standardization using either Rh (343.489 nm and 369.236 nm) or OH molecular emission band (308.958 nm) was required. This calibration methodology was successfully applied to the study of the elemental bioavailability in soil samples from a vineyard affected by copper-based fungicides and sediment samples from an area affected by mining waste.