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A systematic procedure for the selection of optimal internal standards in ICP-OES has been developed. According to the procedure, a preliminary set of measurements is performed before analysis and processing by principal component analysis. The result is a plot (score plot) in which the emission lines of analytes and potential internal standards are grouped according to their empirical behaviour with respect to the considered matrix. The analysis of the score plot makes it possible to choose the optimal reference line, for each analytical line or group of analytical lines. In order to verify the efficiency of the method, trace element determination in marine sediments was considered and the procedure applied to compensate for the matrix effects due to large amounts of iron, aluminium, calcium, sodium and potassium. A robust axially viewed ICP with multichannel detection based on a coupled charge device was used in order to provide real-time internal standardization and to permit the simultaneous measurement of several emission lines. By the analyses of both synthetic and real samples, it was verified that the proposed procedure is suitable for choosing the optimal reference lines, allowing the full effectiveness of the internal standard method.

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