Matrix effects in simultaneous microwave induced plasma optical emission spectrometry: new perspectives on an old problem

Abstract

Easily ionizable elements (EIEs) such as alkaline or alkaline earth elements are well known to cause matrix effects on analytically used plasma sources in general and on microwave-induced plasmas (MIP) in particular. We investigated the matrix effects of 18 major matrix elements on 105 emission lines of 42 elements using an axially viewed microwave-sustained, inductively coupled, atmospheric-pressure plasma (MICAP) attached to a simultaneous spectrometer for optical emission spectrometry (MICAP-OES). In contrast to many previous studies, we did not adjust the nebulizer gas flow or axial observation spot for every emission line but used compromise conditions for simultaneously recording all investigated lines. The obtained data clearly show that inter-element matrix effects were encountered not only for g L⁻¹ concentrations of matrix elements as reported in the literature but also as low as 20 mg L⁻¹ with signal suppression of up to 25% on particularly affected emission lines. The magnitude of the matrix effects was not similar for all investigated matrix elements but particularly pronounced for the alkaline elements. The matrix effect of the alkaline elements was found to decrease in the order Li > Na > K > Cs for the same concentration (mg L⁻¹). However, if equimolar amounts (mmol L⁻¹) of alkaline elements were introduced into the MIP, the matrix effect on concomitant elements was comparable in magnitude. This indicates that the absolute number of atoms in the plasma is responsible for this effect rather than the ionization energy of the respective element. Neither optimizing microwave power (range 1.1 to 1.5 kW) nor nebulizer flow (0.5–1.1 L min⁻¹) effectively diminished the encountered matrix effects in MICAP-OES.