Reduction of acid effects in inductively coupled plasma optical emission spectrometry using internal standards selected by principal component analysis

A systematic procedure for the selection of optimal internal standards in ICP-OES, based on the principal component analysis, has been applied to compensate for the effects due to inorganic acids and their mixtures. An axially viewed ICP with multichannel detection based on coupled charge device was used and the operating conditions chosen in order to maximize the efficiency of internal standardization. The analysis of the score plot made it possible to obtain a simple and effective description of the characteristics of several reference and analytical emission lines with respect to single and combined acid effects and to perform the choice of the optimal internal standards, accordingly. Most of the considered emission lines displayed a quite similar behaviour, showing strong signal depression, mainly in the presence of sulfuric acid and acid mixtures, which does not depend on the energy and type of the emission line. By the analyses of synthetic samples, it was verified that the analytical errors due to hydrochloric, nitric, sulfuric acids and their mixtures can be reduced from 10-40% to less than 5%, using Sc 424 nm and Co 238 nm as reference lines. On the contrary, few emission lines (Se 196 nm, As 188 nm, As 193 nm, Sb 206 nm, P 213 nm) showed a specific behaviour and optimal correction by internal standardization was not achieved. The efficiency of the procedure was further verified by analyzing the certified reference material BCR 288B (lead with added impurities), after digestion with nitric acid. It was verified that when using Sc 424 nm or Co 228 nm reference lines, the uncorrected results are shifted towards the certified values and a significant improvement in accuracy obtained.