Tandem calibration methodology: dual nebulizer sample introduction for inductively coupled plasma atomic emission spectrometry

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Abstract
A calibration method based on the simultaneous introduction of sample and standard into the plasma by two independent devices is described. In this specific implementation, two nebulizers are linked to the ICP by a Y-shaped tube. Figures of merit were determined by comparing the results obtained with those produced using the method of external calibration. ‘Difficult samples’ containing matrices of C, Na, Al or K in concentrations ranging from 1000 to 10 000 ppm were used to provide matrices that produced significant errors when used with external calibration. The test solutions used for this study were mixtures of some of the following elements: Al, Cu, Mg, Fe, Ni, Pb and Zn in one of the matrices described previously. The comparison between the proposed tandem calibration method (TCM) and the method of external calibration clearly indicates that the TCM appears to be a valid alternative to traditional calibration methods, such as standard additions, matrix-matching and internal standards, that are commonly used when external calibration fails to give valid results for fficult samples' introduced into the ICP by means of a nebulizer. Improvement factors in accuracy based on the ratio of the relative error were, on average, of the order of 4 and, in some cases, well in excess of a full order of magnitude. For liquid samples, the precision of the results obtained by the proposed method was limited mainly by the noise of the sample introduction device (about 5% RSD on fficult samples'versus roughly 1% on clean standards) while the accuracy was limited by both the short- and long-term stability of the arrangement, typically around 7% relative error. In addition, the method was found to be easy to implement on existing equipment, inexpensive and potentially well suited for automation.

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