科研之友
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摘要
Compared with solution ICP-MS, LA-ICP-MS studies have thus far reported comparatively few external reference data for accuracy estimates of experiments. This is largely the result of a paucity of available reference materials of natural composition. Here, we report an evaluation of natural glass (obsidian) as an inexpensive and widely available external reference material. The homogeneity of over forty elements in six different obsidian samples was assessed by LA-ICP-MS. Accuracy was tested with two obsidian samples that were fully characterised by electron probe microanalysis and solution ICP-MS. Laser ablation experiments were performed with a variety of ablation parameters (fluence, spot sizes, ablation repetition rates) and calibration approaches (natural vs. synthetic reference materials, and different internal standard elements) to determine the best practice for obsidian analysis. Furthermore, the samples were analysed using two different laser wavelengths (193nm and 213nm) to compare the effect of potential ablation-related phenomena (e.g., fractionation). Our data indicate that ablation with fluences larger than 6Jcm-2 and repetition rates of 5 or 10Hz resulted in the most accurate results. Furthermore, synthetic NIST SRM 611 and 612 glasses worked better as reference materials compared with lower SiO2 content reference materials (e.g., BHVO-2G or GOR128-G). The very similar SiO2 content of the NIST SRM glasses and obsidian (i.e., matrix and compositional match) seems to be the first-order control on the ablation behaviour and, hence, the accuracy of the data. The use of different internal standard elements for the quantification of the obsidian data showed that Si and Na yielded accurate results for most elements. Nevertheless, for the analysis of samples with high SiO2 concentrations, it is recommended to use Si as the internal standard because it can be more precisely determined by electron probe microanalysis. At the scale of typical LA analyses, the six obsidian samples proved to be surprisingly homogenous. Analyses with a spot size of 80m resulted in relative standard deviations (% RSD) better than 8% for all but the most depleted elements (e.g., Sc, V, Ni, Cr, Cu, Cd) in these evolved glasses. The combined characteristics render obsidian a suitable, inexpensive and widely available, external quality-control material in LA-ICP-MS analysis for many applications. Moreover, obsidian glass is suited for tuning purposes, and well-characterised obsidian could even be used as a matrix-matched reference material for a considerable number of elements in studies of samples with high SiO2 contents.
- 出版日期2013-6
