An improved multiple internal standard normalisation for drift in LA-ICP-MS measurements

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Abstract
One of the primary factors limiting precision in LA-ICP-MS measurements is the instrument fluctuation. To overcome this limitation one usually employs a normalisation based on a single internal standard. This article reports a new approach for the normalisation of LA-ICP-MS measurements, based on multiple internal standards. The novelty of the approach is that it (i) takes into account the measurement uncertainties, (ii) generates uncertainty bounds on the normalized measurements and (iii) allows one to check and dismiss the mass dependency of the normalisation procedure. As a result, it is shown that the improvement in normalisation factor increases proportionally to the square root of the number of internal standards (e.g., the use of two internal standards leads to a decrease of the standard deviation by 30%, three to 42% and four up to 50% compared with the classical normalisation). This is illustrated on the NIST 610 standard with a 266 nm UV LA-ICP-MS. An extra advantage of the use of multiple internal standards is that it allows the detection of artifacts in the measurements (internal quality check).

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