What are reference materials?
Reference Materials are control samples of known grade composition that are inserted into a batch of field samples with unknown composition to test and validate analytical measurement during the laboratory assay process. The use of CRMs in analytical measurement is not just QA/QC best practice, it is law. Their use is mandatory for all mining companies reporting publicly on stock exchanges and improves confidence in ore reserves, drill target selection and company aptitude.
More specifically for the Geologist, reference materials have application in grass roots exploration, resource definition, mine site exploration and grade control.
To the Chemist they facilitate the calibration of analytical equipment, evaluation and validation of analytical methods and routine in-house QA/QC programs.
Many terms are used to describe Reference Materials (RMs) or Certified Reference Materials (CRMs). The term ‘standard’ strictly refers to an International Standard Organisation (ISO) approved and documented procedure but its use as a synonym for RMs is widespread and commonly used throughout the mining and chemical industries. Other terms include Standard Reference Materials (SRMs), In-house or Internal Reference Materials (IRMs) and Matrix-Matched or Mine-Matched Certified Reference Materials (MMCRMs). See our Glossary for further details.
A quality reference material must have a proven high level of homogeneity such that any significant analytical error can be attributed to the laboratory. An evaluation of CRM sampling error must therefore be demonstrated to permit an appraisal of the CRM’s homogeneity. See the ‘Tolerance Interval’ section below for detail on how OREAS® provide this information.
Statistically Robust Characterisation
CRMs destined for use in the mining and analytical industries should be well characterised by round robin evaluation at a minimum of 10 recognised mineral testing laboratories and certified in accordance with ISO recommendations (Guides 31 and 35). Fusion XRF certified CRMs are an exception to this where a minimum of six laboratories is usually sufficient given the increased precision and inter-laboratory consensus often seen by this method. An evaluation program should also include an analysis of variance (ANOVA) treatment to establish uniformity of the measured property throughout the entire prepared batch.
In order to confidently make decisions with your analytical data and believe in the integrity of datasets it is imperative to have trust in the CRMs you are using. The task of assigning the source of error seen in suspect analytical data is otherwise fraught with uncertainty. The CRM producer’s credentials and reputation must be unassailable.
Certified reference materials are commonly used in the mining industry to monitor bias in chemical analyses of geological samples. Critical concentrations in mining operations are cut-off and head grades and CRMs are generally selected to approximate these grades. CRMs are usually inserted at a frequency of 1 in 20 to 1 in 30 into the sample stream and the results produced by the laboratory are then compared against the certified values. CRM blanks are devoid of the metal(s) of interest and are used to monitor contamination within the laboratory.
No analytical method is 100% accurate and therefore a certain amount of error is tolerated. This margin of error can be referred to as the window of acceptability, control limit or performance gate. Generally, results lying within two (or sometimes three) standard deviations either side of the certified value are deemed acceptable, although precise application of control limits should be at the discretion of the QC manager concerned. More information on Control Limits can be found in our Glossary.
Certificates of Analysis for CRMs must include a measure of uncertainty of each certified value. This is generally expressed as a 95% Confidence Interval or Expanded Uncertainty, this should not be confused with Control Limits. Put simply, Control Limits provide an expectation of acceptable analytical laboratory performance while Confidence Intervals provide an estimate of the reliability of certified values (i.e. the narrower the confidence interval, the greater the certainty in the certified value).
This parameter is a measure of homogeneity. For gold bearing CRMs we have pioneered a method of reduced analytical subsampling for evaluating homogeneity. This involves the analysis of gold by high precision neutron activation analysis (NAA) on analytical subsample weights of 0.09g to 1g (compared to typical 25g to 50g for the fire assay method). The corresponding standard deviation at a 25g to 50g subsample weight can then be determined from the observed standard deviation of the 0.09g to 1g data using the known relationship between the two parameters (Ingamells, C. O. and Switzer, P. (1973), Talanta 20, 547-568). The absolute homogeneity of gold is then determined from tables of factors for two-sided tolerance limits for normal distributions.
All OREAS® and custom gold CRMs undergo this stringent testing and without exception exhibit a very high level of repeatability consistent with excellent homogeneity. See Our quality assurance for further details.
For analytes other than gold, tolerance limits (ISO 16269:2014) are determined using an analysis of precision errors method and are considered a conservative estimate of true homogeneity.