Whole Rock Trace Element Geochemistry
Whole rock samples are first reduced to mm-sized granules using a Rocklabs hydraulic crusher fitted with tungsten carbide plates to minimize contamination (i.e., crushing is by the percussion method, not grinding). The coarse-crushed samples are then mixed and aliquoted by the cone-and-quarter method.
An aliquot of 100 grams is then reduced to a fine, homogeneous powder (<200 mesh) with a either a Fritsch Pulverisette Series 5/4 or Series 6 planetary mill equipped with agate jars and milling balls. The jars are cleaned with pure quartz sand between samples to prevent cross-sample contamination. For contract work, it is the researcher’s responsibility to examine a sub-selection of powders at high magnification with a microscope to ensure sample homogeneity and purity.
All subsequent handling of powder, including transfer of aliquots for digestion, takes place in Certified Class 100 clean laboratories at PCIGR. Acid digestion can be performed by two methods: 1) hotplate “flux” digestion in Savillex® PFA sample vials, or 2) oven (“bomb”) digestion at high pressures and temperatures in high-pressure Teflon vessels, as required for samples containing refractory minerals (e.g. zircon, sulphides). Acid mixtures are optimized for complete digestion of the full range of trace elements (e.g., Pretorius et al., 2006).
Concentrations of geologically relevant trace elements in the samples are analyzed on the Element 2 HR-ICP-MS in low, medium or high-resolution mode. Sample solutions (<100 ppb, TDS <0.1%) are measured against internal standards using either external calibration or the standard addition method. A detailed description of the analytical procedures is available in Pretorius et al (2006). Matrix-matched standard reference materials are often included as the checked standard, or as calibration standards to further improve analytical accuracy (Carpentier et al, 2013; Schudel et al, 2015; Fourny et al., 2016).
Guidelines for ICP-MS Users (Agilent 7700x, Nu AttoM and Element 2)
All PCIGR facilities are located in the EOS Main building. The Element 2 laboratory is located in Room 036E, and both the Agilent 7700x and the Nu AttoM are located the nUBC facilities. These instruments, along with the rest of the facility, are operated on a cost-recovery basis. Use of the ICP-MS instruments is made available at hourly rates adjusted to Academic (University/NSERC), Government, and Industry researchers, respectively. Assistance, training and troubleshooting by lab personnel are also available at the same rate structure as for the instruments.
The instruments are carefully maintained by lab personnel who set up and tune the instruments before users analyze their samples. A fixed charge of 1.5–2 hours of instrument time and staff time will be applied to the user in addition to instrument time for the analytical session. Any extra time spent on helping users set up the analyses is charged accordingly. Thereafter, users take over running their own blanks, standards, samples and quality control checks as necessary.
Initial training for novice user requires about 3 hours of time, during which the instrument is not in use. This training covers theory of instrument operation, practice, familiarization with the instrument software, and a discussion of the user’s particular analytical needs.
Sample Preparation Requirements:
• Samples to be introduced to the ICP-MS instruments must be free of particles. Particles will lodge within the sample path preceding the mass spectrometer and can cause increased signal noise and memory effects. Therefore, samples must be either: a) completely digested (no solid matter remaining), b) centrifuged and an aliquot diluted, or c) filtered through a 0.45-mm or finer filter. Filtration is the preferred method since it provides the best assurance that particles have been completely removed and only dissolved components remain.
• Dissolved organic matter in samples often causes changes in sensitivity due to differences in viscosity, plasma loading and polyatomic interferences. Organic components within the sample may also be deposited within the sample introduction lines, causing poor washout and nebulizer performance. Self-aspirating nebulizers will often stop aspirating if too much organic matter is present in a sample. Samples containing organic matter should be acid digested prior to analysis.
• The sample matrix for all samples prepared for the Element 2, AttoM and Agilent 7700x must be under 0.1% dissolved solids. If you are unsure of the dissolved solids content of your samples, weight out 10 g of a typical sample, dry it down and weigh what is left; it should be below 0.01 g.
• The most favourable sample matrix is 2% concentrated HNO3 (e.g., Seastar Chemicals BASELINE grade nitric acid, Fisher Scientific Optima nitric acid, or equivalent). The same sample matrix should be used for all standards and samples. This concentration can be critical when using self-aspirating nebulizers, so any deviations from 2% HNO3 should be considered carefully and tested. Using HCl instead of HNO3 causes confounding spectral (polyatomic) interferences with chlorine-35 and 37, while H3PO4 causes rapid corrosion of the cones and is not authorized on PCIGR instruments. HF may be added to the matrix for some elements but usually no more than 0.05% of concentrated HF.
• Internal standards must be used.
• The concentration limit for standards and samples of most elements is 100 ppb for the Element 2, AttoM and Agilent 7700x. The Element 2 is a highly sensitive instrument and operates best in the ppb (parts per billion), ppt (parts per trillion) and sub-ppt concentration ranges. While it can measure ppm concentrations, this usually leads to shortened detector life and contamination of the instrument sample introduction lines that precludes subsequent measurements in the ppb and ppt ranges. If one needs to measure elements in the ppm range or higher, please be aware that a large dilution is necessary, otherwise, we recommend selecting a more appropriate instrument for these measurements.
• Users must prepare their own matrix-matched blank solutions with the same distilled deionized water and ultraclean acids used to dilute/prepare their samples.
• Be prepared to learn that matrix matching is the shortest path to good analytical results.
• There are two golden rules for the ICP-MS lab:
Fourny, A., Weis, D., Scoates, J. (2016) Comprehensive Pb-Sr-Nd-Hf isotopic, trace element, and mineralogical characterization of mafic to ultramafic rock reference materials. Geochemistry, Geophysics, Geosystems, 17(3): 739–773.
Pretorius, W., Weis, D., Williams, G., Hanano, D., Kieffer, B., Scoates, J. (2006) Complete trace elemental characterisation of granitoid (USGS G-2, GSP-2) reference materials by high resolution inductively coupled plasma-mass spectrometry. Geostandards and Geoanalytical Research, 30(1): 39–54.