Matrix-induced shift effects in axially viewed inductively coupled plasma atomic emission spectrometry

In order to study matrix effects and efficiency of internal standardization with an axially-viewed ICP-AES system, a large number of elements with atomic lines in the 2.9–7.8-eV excitation energy range and ionic lines in the 7.7–16.5-eV energy sum range were selected and matrix effects were evaluated using a multichannel detector having a wavelength coverage in the range 167–785 nm. Na and Ca were selected as interferent elements at a concentration of up to 10 g l−1, along with nitric acid, up to a concentration of 20% (v/v). Several operating conditions were used, ranging from robust conditions (1500 W, 0.65 l min−1 for the carrier gas) to non-robust conditions (800 W, 1.2 l min−1). Under robust conditions, a rather flat, depressive effect was observed for ionic lines and atomic lines. This depressive effect was mainly assigned to the aerosol transport and filtering phenomena and was element or acid concentration-dependent. Within this depressive effect, some small departures from the flat effect were observed. However, these small effects exhibited the same patterns over the interferent element concentration range of 1–10 g l−1, i.e. these effects were shifted as a function of the interferent concentration rather than multiplied. A consequence is that internal standardization to compensate for matrix effects on ionic lines was found to be more efficient when based on additive (i.e. shift) effects rather than on multiplicative (i.e. proportional) effects. Under non-robust conditions, signal compensation was found to be inefficient.