A fully automated direct injection nebulizer (d-DIHEN) for MC-ICP-MS isotope analysis: application to boron isotope ratio measurements

Abstract : This work presents a fully automated setup for using direct injection nebulization as an introduction system for solution measurements by MC-ICP-MS, here applied to boron isotopes in pure boric acid solutions and natural samples. In this setup, a direct injection nebulizer (d-DIHEN) is plugged into the plasma torch without any spray chamber, and an automated 6-port valve interfaces the nebulizer and the autosampler. The advantages of a d-DIHEN for boron isotope ratio measurements are high sensitivity and short washout times, allowing for sample-standard bracketing (SSB) measurements at a higher rate than spray chambers. The measurement of boron isotopes by MC-ICP-MS at an unprecedented sub 0.1 parts per thousand, repeatability level (2 standard deviation = 2SD) was achieved for pure boric acid solutions. The improved precision is allowed by a better stability of the introduction system with continuous operation of the peristaltic pump (which was manually switched off between samples before automation) and due to the possibility of multiple analyses of the same sample solution. However, such a good repeatability was not systematically obtained for boron isotopes SSB measurements of natural samples (in fine 2SD are between 0.02 and 0.5 parts per thousand). Boron from natural samples has to be extracted before isotope analysis, with one to four steps depending on the sample type. Repeated analyses of boron independently separated up to ten times from the same sample lead to an external reproducibility no better than 0.2 parts per thousand. Boron chemical separation from the samples prior to MC-ICP-MS analyses seems to remain the main limitation to precise measurements of boron isotope ratios.