Whole Rock Trace Element Geochemistry
Sample Preparation and Analysis
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 by 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, the owner of the powder (i.e., of the researcher requesting the analyses) is responsible for ensuring 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 certified Class 100 clean laboratories at PCIGR.
Acid digestion occurs 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;” see Practical Considerations below), and are measured against internal standards using either external calibration or the standard addition method as required. A detailed description of the analytical procedure is published in Pretorius et al. (2006). Matrix matched standard reference materials are also commonly used to perform calibrations to further improve analytical accuracy.
Instrumentation, Set Up and Costs
PCIGR’s Element2 laboratory is located in the basement of the EOS Main building (Room 036). Both the Agilent Quad and the Nu AttoM are located in the nUBC laboratories on the ground floor of EOS Main.
These instruments are operated on a cost recovery basis. Use of the instruments is made available to academic (University/NSERC), government and nndustry researchers at hourly rates of $60, $90 and $120 per hour, respectively (see Fees for more information). Assistance, training and troubleshooting by the lab managers is also available at the same rate structure as for the instruments.
The instruments are maintained and operated by the lab managers. During instrument start-up, the operator will configure the sample introduction apparatus prior to start-up, ignite the plasma, warm up the instrument, and tune and perform performance checks and mass calibrations at the beginning of the day. Daily start-up and tuning usually takes 1.5–2 hours of instrument and manager time that are charged to the user. Any extra time spent by managers on helping users to set up analyses is charged accordingly.
Thereafter, 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 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 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 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 that 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 in your samples, weigh out 10 g of a typical sample, dry it down and weigh what is left; the dry weight should be below 0.01 g.
• The most favourable sample matrix is 1% concentrated HNO3 (Fisher Scientific Optima or equivalent). The same sample matrix should be used for all standards and samples. When using self-aspirating nebulizers, this concentration can be critical; therefore, any deviations from 1% HNO3 should be carefully considered 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 permitted on PCIGR instruments. HF may be added to the matrix for some elements, but usually no more than 0.05% of concentrated HF is allowed.
• Internal standards must be used.
• The concentration limit for standards and samples of most elements is 100 ppb for the 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 that precludes subsequent measurements in the ppb and ppt ranges. If a user needs to measure elements in the ppm range or higher, either be prepared to do a large dilution or find a more appropriate instrument for these measurements.
• Users must prepare their own matrix matched standards using the same distilled deionized water and ultra-clean 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”.