Evaluation of calcium-, carbon- and sulfur-based non-spectral interferences in high-power MIP-OES: comparison with ICP-OES

Abstract

Fundamental studies about non-spectral interferences in high-power microwave induced plasma optical emission spectrometry (MIP-OES) are very limited so far. The goal of this work is to investigate non-spectral interferences originating from calcium (0.5% w w⁻¹ calcium nitrate), carbon (5.0% w w⁻¹ glycerol) and sulfur (5.0% w w⁻¹ sulfuric acid) matrices in commercial high-power MIP-OES based on a Hammer cavity. To this end, the influence of the nebulizer gas flow rate on the signal of several atomic and ionic lines of a total of 10 elements (As, Co, Cu, Mg, Mn, Mo, Sc, Se, Sr and Zn) has been studied for each matrix. Compared to the 1.0% w w⁻¹ nitric acid reference solution, the emission signal for atomic lines with E_sum values lower than 3.26 eV was enhanced in the presence of calcium but it was suppressed for the remaining atomic and ionic lines tested. Calcium matrix effects originate from changes in both plasma excitation conditions and ion–atom equilibrium. With regard to the 5.0% w w⁻¹ glycerol and 5.0% w w⁻¹ sulfuric acid solutions, no signal changes were noticed for the 1.0% w w⁻¹ nitric acid solution since no significant differences were observed either in the aerosol size and transport or in the plasma excitation conditions. A comparison of matrix effects in MIP-OES with those obtained in inductively coupled plasma optical emission spectrometry (ICP-OES) under similar experimental conditions reveals that the former source is more sensitive to the presence of the calcium matrix. In contrast, MIP-OES is a more robust technique for elemental analysis when operating with 5.0% w w⁻¹ glycerol and 5.0% w w⁻¹ sulfuric acid solutions. In fact, matrix-based signal enhancements observed in ICP-OES for As and Se are eliminated when operating in MIP-OES.