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 with no 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) using a Fritsch Pulverisette (series 5 or 6) planetary mill equipped with agate jars and agate milling balls. The jars are cleaned with pure quartz sand between samples to prevent cross-sample contamination. For contract work, it is the responsibility of the owner of the powder (i.e. of the researcher requesting the analyses) to ensure the homogeneity and purity of the powders by examining a sub-selection of their powders at high magnification with a microscope.
All subsequent handling of powder, including transfer of aliquots for digestion, takes place in our Certified Class 100 clean laboratories at PCIGR. Acid digestion is undertaken in either Savillex® PFA sample vials (i.e. hotplate “flux” digestion method), or high-pressure Teflon vessels (i.e. oven “bomb” digestion method) as requested for samples containing refractory minerals (e.g. zircon, sulphides); acid mixtures are optimized for complete digestion of the full range of trace elements.
Elemental concentration in the samples are analyzed on the Thermo Finnigan Element2 HR-ICP-MS for the complete suite of geologically relevant trace elements in low, medium or high resolution mode, after appropriate dilution (under the maxim of “less is more”) and measured against internal standards using external calibration, or using standard addition method as may be required. A detailed description of the analytical procedure is published in Pretorius et al. (2006). It is also common to use matrix matched standard reference materials to perform calibrations to further improve analytical accuracy. Carpentier et al. (2013) and Schudel et al. (2015)
Rules for ICP-MS Users (Agilent quad, Finnigan Element 2 and Nu AttoM)
The PCIGR Element 2 laboratory is located in the basement of the EOS Main building, room 036; both the Quad and the Nu AttoM are located in the nUBC laboratories on the ground floor or EOSM. These instruments, as the rest of the facility, are operated on a cost recovery basis. Use of the ICP-MS instruments is made available to Academic (University/NSERC), Government and Industry Researchers at hourly rates of $60, $90 and $120 per hour respectively. Assistance, training and troubleshooting by the lab managers is also available at the same rate structure as for the instruments. The instruments are maintained by the lab managers who also configure the sample introduction apparatus prior to start-up, ignites the plasma, warms up the instrument, tunes and performs performance checks and mass calibrations at the beginning of the day. Daily startup and tuning usually takes 1.5-2 hours of instrument and manager time which are charged to the user. Any extra time one of the managers spends on helping the users to set up the analyses is charged accordingly. Thereafter the users take over running their own blanks, standards, samples and quality control checks as necessary. The quality and acceptability of the results are the responsibility of the user; the lab and manager offer no guarantees, besides original set-up. Initial training for novice user requires about 3 hours time during which the instrument is not in use. This training covers theory of the instrument operation, practice, familiarization with the instrument software and often a discussion of the user’s particular analytical problem.
• Samples to be introduced to the ICP-MS instruments must be free of particles. Particles will lodge within the sample path preceding the mass spec 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 poor nebulizer performance. Self aspirating nebulizers will often stop aspirating if too much organic matter is present in a sample.
• The sample matrix for all samples prepared for the Element2, 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 1% conc HNO3 (Fisher Scientific Optima or equivalent). Same sample matrix should be used for all standards and samples. When using self-aspirating nebulizers this concentration can be critical so any deviations from 1% 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 Element2, AttoM and Agilent 7700x. The Element2 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 which precludes subsequent measurements in the ppb and ppt ranges. If one needs to measure elements in the ppm range or higher either be prepared to do a large dilution or to find a more appropriate instrument for these measurements.
• Users must prepare their own matrix matched standards using 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: “LESS IS MORE” and “SLOW IS FAST”.