Introduction

While geological reference materials (RMs) have existed for many years, their use has become widespread only in the last decade. This increase in popularity stems from the recognition of the critical role RMs play in monitoring the quality of assay data generated in analytical laboratories. To the geologist, reference materials have application in grass roots exploration, resource definition, minesite exploration and grade control. To the chemist they facilitate the calibration of analytical equipment, evaluation and validation of analytical methods and routine in-house Quality Assurance/Quality Control.

Many terms are used to describe RMs or Certified Reference Materials (CRMs) and this can be confusing to the uninitiated. The term ‘standard’ strictly refers to an (ISO) approved and documented procedure in industry but its use as a synonym for RMs is widespread and entrenched throughout the mining and chemical industry. Other terms in common usage 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 of Terms for further detail.

Essential attributes

  • Proven Homogeneity

    The CRM in question must have a proven level of homogeneity such that the observed variance in repeat assays can be attributed almost exclusively to measurement error. In other words, any sampling error resulting from inhomogeneity of the reference material should be small enough in comparison to measurement error that it’s negligible.

  • Statistically Robust Characterisation

    The CRM should be well characterised by round robin evaluation at a minimum of 10 recognised mineral testing laboratories and certified in accordance with International Standards Organisation (ISO) recommendations. This evaluation program should include analysis of variance (ANOVA) treatment to establish uniformity of the measured property throughout the entire batch.

  • Reputation

    A reference material is no better than the user’s perception of it. Therefore, it is critical, that the user has total confidence in its quality. If this is not the case and analytical problems are suspected, the task of assigning the source of error to the suspect laboratory is fraught with uncertainty. It is imperative, therefore, that the CRM producer’s credentials and reputation are unassailable and that the certification documentation is sufficiently comprehensive.

Using CRMs

CRMs are most commonly used in the mining industry to monitor bias in chemical analyses of geological samples. Critical concentrations in mining operations are cutoff 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.

Control Limits

No analytical method is 100% accurate and therefore a certain amount of error is tolerated. This margin of error is variously referred to as a 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.

There are various methods used to determine the standard deviation. These methods are empirically derived and based on an analysis of errors contributing to the spread of results obtained in the round robin certification program. These are laboratory measurement errors and sampling errors. Measurement errors include between-laboratory bias, between-batch bias (reproducibility errors) and within-batch precision (repeatability errors). Sampling errors relate to the level of homogeneity of the CRM and should be negligible in comparison with measurement errors.

Confidence Interval

ISO requires that Certificates of Analysis include a measurement of uncertainty of the certified value. This is generally expressed as a 95% Confidence Interval and should not be confused with Control Limits. Put simply, Control Limits provide an expectation of acceptable laboratory performance while Confidence Intervals provide an estimate of the reliability of the certified value.

Tolerance Interval

This parameter is a measure of homogeneity of the CRM. We have pioneered a method of reduced analytical subsampling for evaluating the homogeneity of gold in CRMs. This involves the analysis of gold by high precision neutron activation analysis (NAA) on analytical subsample weights of 0.5g to 1.5g (compared to 25g to 50g for the fire assay method). By employing a sufficiently reduced subsample weight in a series of determinations by the same method, analytical error becomes negligible when compared with subsampling error. The corresponding standard deviation at a 25g to 50g subsample weight can then be determined from the observed standard deviation of the 0.5g to 1.5g 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.

Glossary of Terms

The following terms are used here and in literature elsewhere to discuss reference materials.

ILAC – The International Organisation for Accreditation Bodies operating in accordance with ISO/IEC 17011 and involved in the accreditation of conformity assessment bodies including calibration laboratories and testing laboratories (using ISO/IEC 17025), medical testing laboratories (using ISO 15189) and inspection bodies (using ISO/IEC 17020). The aim of ILAC is increased use and acceptance by industry and governments of the results from accredited laboratories, including results from laboratories in other countries. In this way, the free-trade goal of a 'product tested once and accepted everywhere' can be realised.

INAA – Instrumental Neutron Activation Analysis (also commonly referred to as NAA). It is used to determine the concentration of trace and major elements in a variety of matrices. Owing to its high precision, INAA is particularly unique as a highly effective method for determining the homogeneity of gold. Compared to other methods, INAA avoids complications due to incomplete dissolution, volatilisation, precipitation and inaccurate voluming. A sample is subjected to a neutron flux and radioactive nuclides are produced. As these radioactive nuclides decay, they emit gamma rays whose energies are characteristic for each nuclide. Comparison of the intensity of these gamma rays with those emitted by a standard permit a quantitative measure of the concentrations of the various nuclides. The SD from replicate analysis of a sample using INAA can be used to determine the Sampling Constant.

IRM – Internal reference material (generally an in-house RM/CRM/SRM).

ISO – The International Organisation for Standardisation (ISO) is an independent, non-governmental international organization set up to facilitate world trade by providing common standards between nations. It was founded in 1947 and is headquartered in Geneva, Switzerland. ISO has membership of 162 national standards bodies (2018).

ISO/CASCO – The ISO Subcommittee that works on issues relating to conformity assessment. Casco develops policy and publishes standards related to conformity assessment. ISO/CASCO is the body responsible for ISO 17034.

ISO/REMCOREMCO is the ISO Subcommittee tasked with looking after Reference Materials. It was set up in 1975 to carry out and encourage a broad international effort for the harmonization and promotion of certified reference materials, their production, and applications. Note that rock standards are a very small category of Reference Materials in the overall scheme of things. There are actually five broad categories of reference materials defined by ILAC (another ISO body, see above)- pure substances, solutions and gas mixtures, Matrix Reference Materials (including rock standards), physico-chemical reference materials (characterised for properties such as melting point, viscocity or optical density) and objects or artifacts (characterised for properties such as taste, octane number, hardness).

REMCO guides can be purchased from ISO. They are continuously being updated and changed (and are quite expensive). If an older version is being quoted it is very important to refer to the paragraph number and its year of publication. The current guides are:

  • ISO Guide 30:2015 - Recommends terms and definitions used in connection with reference materials (RM's), with particular attention to terms that are used in RM certificates and corresponding certification reports.
  • ISO Guide 31:2015 - Intended to help RM producers prepare clear and concise documentation to accompany an RM. It lists and explains mandatory, recommended and other categories of information to be used in the product information sheets and certificates. It also contains the minimum requirements for a label attached to the RM container.
  • ISO Guide 33:2015 - Describes good practice in using RM's and CRM's (previously ISO Guide 32:1997), particularly in measurement processes. These uses include the assessment of precision and trueness of measurement methods, quality control, assigning values to materials and calibration.
  • ISO Guide 35:2017 - Guidance for characterization and value assignment of properties of an RM, assessment of homogeneity and stability and the establishment of the metrological traceability of CRM's.
  • ISO/TR 79:2015 - Summarizes the state of the art of the production and certification or characterization of qualitative property reference materials (RMs). A document intended to bring together the diverse international wisdom on production and the nominal properties of RM's and to contribute to the ongoing general discussion.
  • ISO Guide 80:2014 - Guidance for the in-house preparation of Quality Control Materials (QCMs). The requirements for "in-house" QCMs are less demanding than those for an RM or CRM. Their preparation should involve homogeneity and stability assessments and a limited characterization of the material to provide an indication of its relevant property values and their variation. This document provides the quality criteria that a material should fulfill to be considered fit-for-purpose for demonstrating a measurement system is under statistical control.
  • ISO Guide 9001:2015 - The most widely utilized quality standard for quality management systems. Independently validated, ISO 9001:2008 is applicable to any manufacturing and service organization providing a framework for system development focusing on the customer, quality system performance and ongoing improvement.
  • ISO/TR 10989:2009 - Reference materials - Guidance on, and keywords used for, RM categorisation. The results of a study into existing classification and categorisation schemes for reference materials and proposals for a harmonised scheme that would meet the needs of producers, users and the needs of modern forms of communication such as internet based catalogues and databases.
  • ISO/TR 11773:2013 - Global distribution of reference materials. Contains an inventory of problems and recommendations related to the transport, import and export of non-nuclear, non-radioactive reference materials, specifically for the packaging, labelling, and documenting of the shipments in order to comply with legal requirements.
  • ISO/TR 16476:2016 - Reference materials - Establishing and expressing metrological traceability of quantity values assigned to reference materials.
  • ISO 17034:2016 - General requirements for the competence of reference material producers (previously ISO Guide 34:2000 and 2009).