Discover the important features of CRMs, how they should be used and understand the various associated terminologies.
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.
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.
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.
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.
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.
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.
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.
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.
The following terms are used here and in literature elsewhere to discuss reference materials.
4-Acid Digestion – Also known as "multi-acid digestion" or “near-total digestion”, this method uses a combination of HNO3 (nitric acid), HF (hydrofluoric acid), HClO4 (perchloric acid) and HCl (hydrochloric acid). Some laboratories differ in the specific acids used but the most important acid is HF because it dissolves silicate minerals. Four acid digestion quantitatively dissolves nearly all minerals in the majority of geological samples however, some refractory minerals may only be partially digested (eg. garnet, barite, rare earth oxides, columbite-tantalite, titanium, tin and tungsten). Some elements can also precipitate or volatilize during digestion. Fusion techniques are generally preferred for accurate quantification of Al, Ba, Cr, Hf, Mo, Mn, Nb, Pb, Si, Sn, Ti, Ta, W, Zr, As, Sb, Se and Te.
95% Confidence Interval – An interval for which we can assign a 95% probability that the Recommended Value lies within; alternatively, we can say that 95% of all confidence intervals determined in a like manner (from different samples of the population) will include the Recommended Value (it is important to note that this is not equivalent to two standard deviations of the Recommended Value).
95% Tolerance Interval – (where r = 0.95 & 1- a = 0.99) may be defined as meaning that 99% of the time at least 95% of samples will have values lying between the specified interval. Put more precisely, this means that if the same number of samples were taken and analysed in the same manner repeatedly, 99% of the tolerance intervals so constructed would cover at least 95% of the total population, and 1% of the tolerance intervals would cover less than 95% of the total population.
Accuracy – Accuracy (ISO 1575) refers to a combination of trueness and precision. It encompasses both systematic error (trueness) and random error (precision). It should not be confused with Trueness.
Analytical Outlier – A single result or entire set of results deviating in either trueness or precision from others in the set or from other sets, respectively, due to errors associated with measurement.
Assay – In exploration and mining rock and ore samples are tested (assayed) in Assay Laboratories. The word assay comes from the French word essai, which means "trial," an appropriate sense for a word that means to examine for analysis. As a noun, assay means a test or appraisal to determine the components of a substance or object. As a verb, it refers to the act of analyzing, or of conducting that test. It is usually used in chemistry-related fields like metallurgy and pharmaceuticals, but you can also assay a poem.
Best Practice – A set of working methods that have been found, through experience and research, to be the best available to use in a particular business or industry. The methods should be described formally and in detail. Mineral assay laboratory competence and documentation is accredited and tested through the National Accreditation Authority (as per ILAC system ISO/IEC 17025). Laboratories in general strive for accreditation for the analytical methods of key commercial importance. For example a laboratory might be 17025 accredited for its iron ore analysis methods but not for platinum analytical methods or may be accredited for Au analysis by Pb collection fire assay with ICP finish at grades between 5 and 10 ppm but not for grades between 0.2 and 1.0 ppm.
Over and above the ISO accreditation system and relevant to mineral laboratories are the Best Practice Laboratory QA/QC systems developed for pathology laboratories through the work of Dr James O. Westgard. “Westgard Rules”. These are multi-rule QC rules that use Control Samples (CRM's) to help analyse whether an analytical run is in-control or out-of-control and should be used by both the laboratory and the customer to monitor that the laboratory’s “Best Practice” procedures are actually working.
Ideally, in the mining and exploration industries, a cost effective "Best Practice" sampling and assay program should at least be recording the following statistics for its internal Quality Management Program:
Certified Reference Materials (CRMs) – Reference materials that are characterised by metrologically valid procedures for one or more specified properties and which are accompanied by a certificate providing the value of the specified property, its associated uncertainty, and a statement of metrological traceability. ISO Guide 30:2015 recommends terms and definitions that should be assigned to them when used in connection with reference materials, with particular attention to terms that are used in reference material certificates and corresponding certification reports. The certificate must contain a statement of traceability indicating the principles and procedures on which the property values (together with their measurement uncertainties) are based. In the case of mineral CRM's this will be method specific, by inter-laboratory testing, with operationally defined property values using a network of competent laboratories employing methods which have been independently validated (ISO 17034:2016).
CIM Code – The Canadian Institute of Mining, Metallurgy and Petroleum (CIM). The CIM Definition Standards for Mineral Resources and Mineral Reserves are one of the CRIRSCO-style reporting Codes. It was originally based on the Australasian JORC Code . It is the document underlying the Canadian Stock Exchanges NI 43-101.
Commutability – Reference material producers should ensure that a reference material is suited for its intended use. For calibrators and quality control materials this usually includes verification that the raw material selection and processing procedures result in a material with the same behaviour as routine samples in the relevant measurement procedures. The assessment of commutability is part of the demonstration that such a reference material is fit for the intended use.
Confidence Interval – A range of values within which the Recommended Value is expected to lie. The magnitude of the confidence interval is inversely proportional to the number of participating laboratories and inter-laboratory agreement. It is a measure of the reliability of the recommended value; the narrower the confidence interval the greater the certainty in the recommended value.
Control Charts – Schewart or Levey-Jennings control charts are used to monitor analytical processes. In the context of monitoring QC data for CRMs, these charts contain a centreline and the CRM’s ±2 and ±3 SD window control limits are plotted. The user’s own data obtained for the CRM by a laboratory being monitored is then progressively plotted and the data should follow a normal distribution. By definition, 4.5% of data falls outside the 2SD window and 0.3% of data will fall outside the 3SD window. This means approx. 1 in 22 analyses will naturally fall outside 2SDs and approx. 1 in 333 analyses will naturally fall outside 3SDs. Westgard Rules can be used to determine when intervention should be instigated due to QC failures. A CRMs statistics quoted in certs is linked to the round robin program and is at best, a first principle guide to what a lab may be able to perform within. Each lab has its own unique operators, equipment, reagents and processes all of which contribute to a repeatable and reproducible level of variability. This means each lab has its own inherent SD linked to the particular method carried out and this may or may not be a good match to the SD quoted in a CRM’s certificate. For this reason some CRMP’s prefer not to provide SD’s in certificates and recommends that users monitor the precision over time and attribute their own empircally derived SD. The obvious weakness of this is that it takes a while to accumulate a critical mass of analyses and if the analytical process has poor precision or bias the laboratory won’t be held accountable.
Control Limits – A window of acceptability for results obtained by a laboratory for a reference material and generally calculated from multiples of the standard deviation (SD) of the certification data. The SD for each analyte’s certified value reported in OREAS’ certificates is calculated from the same filtered data set used to determine the certified value, i.e. after removal of any individual, lab dataset (batch) and 3SD outliers (single iteration). These outliers can only be removed after the absolute homogeneity of the CRM has been independently established, i.e. the outliers must be confidently deemed to be analytical rather than arising from inhomogeneity of the CRM. The standard deviation is then calculated for each analyte from the pooled accepted analyses generated from the certification program.
In the application of SD’s in monitoring performance it is important to note that not all laboratories function at the same level of proficiency and that different methods in use at a particular laboratory have differing levels of precision. Each laboratory has its own inherent SD (for a specific concentration level and analyte-method pair) based on the analytical process and this SD is not directly related to the round robin program.
The majority of data generated in the round robin program was produced by a selection of world class laboratories. The SD’s thus generated are more constrained than those that would be produced across a randomly selected group of laboratories. To produce more generally achievable SD’s the ‘pooled’ SD’s provided in this report include inter-lab bias. This ‘one size fits all’ approach may require revision at the discretion of the QC manager concerned following careful scrutiny of QC control charts.
CRIRSCO – The International Reporting Template, first published in 2006, is a document that represents the best of the CRIRSCO-style codes, previously referred to as JORC-style codes; reporting standards that are recognised and adopted world-wide for market-related reporting and financial investment.
Custom Reference Materials – Synonymous with Matrix-Matched CRMs (MMCRMs also mine-matched or site-specific CRMs) manufactured out of source materials supplied by clients; usually for mines and advanced reserve-definition stage projects but can also be made from materials sourced from exploration projects or downstream metallurgical products (feed, tails and concentrate). Matrix-Matched Reference Materials provide the highest degree of assurance of the entire analytical process being in control and completely avoid issues of commutability.
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.
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/REMCO – REMCO 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:
JORC – The father of the most widely accepted modern Mineral Resource and Ore Reserves reporting codes is the Australasian Code for Reporting of Exploration Results, Mineral Resources and Ore Reserves (‘the JORC Code’). The original JORC Code was put together by the AusIMM after the calamatous Nickel Boom in the 1970's when the blameless bean counters suggested that the mining types should get their house in order with respect to how they described mineral deposits. To give them credit the mining professional societies involved did an exceptionally fine job. The resulting code described minimum standards for reports issued to investors and was so good that it was quickly adapted for use by some other countries, notably Canada and South Africa, whose stock exchanges quickly made their respective national JORC type codes (the CIM Code and the SAMREC Code) mandatory for listed mining company reporting. CRIRSCO was subsequently set up to oversee future development of the "JORC based" codes and to encourage the growth of their use internationally.
Matrix Reference Materials – Synonymous with Custom Reference Materials.
Metrological Traceability – Property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty.
Mineral Reference Materials – Reference Materials made from soils, rocks or ores for use to check assays of soil, rock and ore samples with similar grades or chemical characteristics. As opposed to, for example, other types of Reference Materials such as for those made to check tests for thermal conductivity, electrical resistivity, peanut butter, whale blubber, cigarette ignition etc.
NI 43-101 – The National Instrument 43-101 - Standards of Disclosure for Mineral Projects was developed by the Canadian Securities Administrators (CSA) and came into force in early 2001, establishing standards for all public disclosure an Issuer makes of scientific and technical information concerning mineral properties/projects.
OREAS SuperCRMs® – The proprietary name for OREAS CRMs which have been certified for complete ICP-OES and ICP-MS element suites for multiple digestion methods within a single CRM. This can result in a single CRM being certified for up to 179 individual analyte-method pairs. OREAS SuperCRMs® are useful for a diverse range of applications including pathfinder and litho-geochemical programs.
Physical Outlier – (as opposed to analytical outlier) - A single result or entire set of results deviating in either accuracy or precision from others in the set or from other sets, respectively, due to inhomogeneity of the reference material.
Pigeon Pair, Paired Offsets, Bracketed Standards, Variance Standards – Terms used to describe a pair of standards of almost identical matrix that differ in concentration of one or more certified elements by an amount approximating the typical measurement error of methods in common use at commercial analytical laboratories. All geochemists and geologists involved in QC are mindful of the shortcomings of insertion of the same standard or standards in batch after batch of samples submitted to their laboratory for analysis. After several months the laboratory’s QC manager soon becomes familiar with the control limits they are expected to report within for a particular standard. This knowledge can severely compromise the value of the standard. One strategy of overcoming this problem of familiarity is the production of paired offset standards (also referred to as pigeon-paired or bracketed standards). Instead of preparing one standard of a specific grade two are prepared at concentrations bracketing this. The concentration offset between the two is generally chosen to be of a magnitude comparable to measurement error of the analytical method employed (typically 1-2% for fusion XRF, 5% for ore grade precious and base metals and 10% for geochem grade precious and base metals). Obviously the preparation of standards from end-member components (e.g. barren and high grade material) lends itself well to the incorporation of paired offsets as they have compositional characteristics (both chemical and mineralogical) almost identical to one another.
ppm – parts per million (equivalent to grams per tonne). SI unit equivalents ≡ mg/kg ≡ µg/g ≡ 0.0001 wt.% ≡ 1000 ppb, parts per billion. 1 ppm ≡ 0.029 opt (troy ounces per short ton). Alternatively, 1 opt ≡ 34.2857 ppm.
QA – Quality Assurance in testing is the management process that makes sure you are doing the right things, the right way, all of the time. It includes documentation of the methods and standard operating procedures.
QC – Quality Control in analytical science is checking that the results you have obtained are distributed normally within expectations. It includes adherence to the QA procedures to ensure that a product or process is acceptable or under control. The QC process may encompass the use of Control Charts or Westgard Rules.
Quality Control Materials – Samples intended for internal laboratory quality control, without formally assigned property values or uncertainties (see ISO Guide 80). Sufficiently homogeneous and stable with one or more property value that can be used for maintaining or monitoring measurement processes. Known colloquially in some jurisdictions as IRM's (Internal Reference Materials).
Reference Materials (RMs) – Samples sufficiently homogeneous and stable with respect to one or more specified properties, which have been established to be fit for its intended use to check results for specific methods in a measurement process. RM’s may be chemicals, solutions, gas mixtures, matrix reference materials, physico-chemical reference materials, objects or biological specimens. The essential difference between an RM and a CRM is that the CRM is characterised by a metrologically valid procedure for one or more specified properties, accompanied by an RM certificate that provides the value of the specified property, its associated uncertainty, and a statement of metrological traceability.
Repeatability – The closeness of agreement between results for the same parameter carried out by the same method, by the same operator, with the same instrument, in the same laboratory, over a short interval of time.
Reproducibility – The closeness of agreement between results for the same parameter carried out by a different method, or different operator, or different instrument, or in a different laboratory, or over an interval of time quite long in comparison to the duration of a single measurement.
SACNASP – The South African Council for Natural Scientific Professions (SACNASP) was set up by the Natural Scientific Professions Act of 2003 but is now adminstered by a new version of the Act passed in 2013. SACNASP registers all Natural Science practitioners within South Africa and in the mining industry. It is relevant to all geologists and chemists writing technical reports about mineral exploration or mining or otherwise or providing a service to the South African public. This will include all geologists acting as consultants and/or writing reports for publication by JSE listed companies and all chemists signing off assay results customers or public companies. According to the Act it is illegal within South Africa to practice and provide a service to the public as a geologist or a chemist unless registered with SACNASP. The 2003 Act punished miscreants with a fine or prison. This was watered down in the 2013 Act to just a heavy fine. Registration in essence is a process whereby the qualifications of an applicant are confirmed to be true, valid and relevant to the field of practice specified. Registered geologists and chemists will list Pr.Sci. Nat. on the report after their name.
Sampling Constant – The required mass of material to achieve a 1% RSD using the known relationship between sample mass and SD (see Ingamells, C. O. and Switzer, P. (1973), Talanta 20, 547-568). This parameter is important as it provides users with a quantitative indication of the CRM’s level of homogeneity. Users require irrefutable data on the magnitude of CRM sampling errors and their impact on QC protocols. An article published in the EXPLORE newsletter for geochemists shows that some manufacturers are showing micro-nuggets in their gold CRMs.
SRM – Standard reference material, synonymous with reference material or certified reference material. Note: in the past this acronym has also been used for “Secondary Reference Materials” – materials characterised to a lower level than and generally not conforming to all ISO criteria for a CRM.
Standard – Literally this refers to a recognised and approved method or procedure (such as the standards published by ISO) but its use to describe reference materials is also well entrenched in the mineral and chemical industries.
Statistical Outlier – A single result or entire set of results deviating in either accuracy or precision from others in the set or from other sets, respectively, to a degree greater than can be justified by statistical fluctuations associated with a given frequency distribution.
Tolerance Interval – A measure of homogeneity of one or more parameters in a reference material in which there is a fixed probability the specified interval will contain at least a specified proportion r of the population from which the sample is taken.
Westgard Rules – Borrowed from pathology, these are multi-rule, QC rules and utilise trend and bias to determine whether a process is in control. The advantages of multi-rule QC procedures are that false rejections can be kept low while at the same time maintaining high error detection.
Each quarter, we will keep you up to date with news on the latest OREAS CRMs and QA/QC best practices in the analytical industry.