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Author: Admin | 2025-04-28
Most silicate and resistate minerals in the sample will not be dissolved by aqua regia, a potential problem if the elements of interest reside in these mineral phases. A four-acid digestion is considered a ‘near total’ method, in that most mineral phases will be digested. A few volatile elements such as Hg are not available from this stronger leach (they are lost during sample dissolution), and precision is generally slightly higher due to the more complex solution matrix caused by dissolution of silicate minerals. Some analytical methods such as instrumental neutron activation analysis (INAA) do not require any sample digestion and are considered ‘total’. Disadvantages of INAA include the smaller suite of elements determined and the long turn-around times typically required (Hoffman 1992). Once digested, element concentrations may be determined by the more sensitive instrument, ICP-MS, or by ICP-AES which may be more economical, but has higher detection limits and a smaller suite of elements determined. Improved instrumentation in the past decade has resulted in lower detection limits for most pathfinder elements, in many cases well below natural background concentrations; for example lower detection limits for Au, Bi, Te and the rare earth elements (REEs) of 0.2 parts per trillion (ppt), 0.5 parts per billion (ppb), 3, and 2 ppb, respectively, are attainable. The resulting stream sediment data have improved precision and accuracy at lower concentrations, a greater suite of pathfinder elements, and the potential to acquire individual element isotopes (e.g. Pb207, Pb206). A discussion with the laboratory geochemist or chemist to select the best method is recommended for new programmes. Detailed description of various methods is included in the International Union of Geological Sciences Manual of Standard Methods (Demetriades et al. 2022). Quality assurance/quality control (QAQC) for stream sediment surveys A quality management system for a standard geochemical survey includes both quality assurance (QA) and quality control (QC) elements. The QA focus is mainly in the analytical laboratory environment, where the components of standard operating procedures, instrument logs, training records, data acceptance/rejection criteria and lab audits are covered. The QC element provides measures of the accuracy and precision of geochemical data produced by an analytical method. The accuracy and precision are established through the analysis of standard reference materials (SRMs), blank samples and field and analytical duplicate samples. The accuracy of an analytical method is an attempt to quantify the ‘correctness’ of analysis, by including SRMs with the sample batch and comparing their analysis to the published value for those SRMs. Accuracy is calculated as the percent recovery by dividing the mean concentration by the target value of the SRM used and multiplying by 100. SRMs should be used that match the anticipated concentration of the stream sediment samples. If SRM concentrations are significantly higher, the lab may need to use a different method for analysis, and these higher-concentration reference samples may risk contaminating survey samples. Blank samples are included for the purpose of identifying contamination within the sample preparation and analytical processes. If lower concentrations are anticipated, blanks
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