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.
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.
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.
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.
How far the measured value lies from the true (or reference) value it is estimating. Bias can also be known as systematic error (an estimate of the level of trueness).
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:
- The laboratory bias (for the particular method used). This should be very low for samples close to the mine cut-off grade and for very high grade samples. It can be higher for less revenue sensitive analyses, such as for tailings or for geochem programs.
- The relative standard deviation (RSD) or coefficient of variation (CV) for the control sample results. It is important to consider that the single laboratory’s Control Sample RSD should be lower than the RSD from inter-laboratory testing; but the laboratory's Control Limits must still be within the Control Limits derived from the inter-laboratory testing.
- The error detection rate (the critical systematic error). How many of the laboratory errors identified by Control Samples turn out to be real errors upon careful investigation; and not false alarms.
- The false error detection rate. How many of the laboratory errors identified turn out to be false alarms.
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.
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.
The quality of agreement in measurements of the same parameter in the same RM between different laboratories in a certification 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
Coefficient of Variation
Measures the spread of a set of results as a proportion to its mean (SD/mean). Also known as one relative standard deviation (1RSD).
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. Non-commutability of a reference material can be caused by a matrix alteration or by the presence of different analyte host minerals (eg. refractory minerals). A matrix effect or a matrix bias is often caused by differences in sample matrix between the reference material and the routine 'field' samples.
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.
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 certificates is linked to the round robin program and is at best, a first principle guide to what a laboratory may be able to perform within. Each laboratory has its own unique operators, equipment, reagents and processes all of which contribute to a repeatable and reproducible level of variability. This means each laboratory 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 CRM producers prefer not to provide SD’s in certificates and recommend that users monitor the precision over time and attribute their own empirically derived SD. The obvious weakness of this is that it takes a while to accumulate a critical mass of analyses (in order to calculate a meaningful SD) and if the analytical process has poor precision or bias the laboratory won’t be held accountable. At the very least, empirically derived SD's should be compared to those published in the certificate as in indication of the general performance of their laboratory and whether their data has sufficient process control.
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.
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.
The expanded uncertainty is obtained by multiplying “Standard Combined Uncertainty” by a “Coverage Factor” (Usually 2 in the context of the normal distribution; to equal approximately 95.5% of probability). The Expanded Uncertainty enables us to obtain an uncertainty measurement that will determine the interval or range, in which the true value of a measurement (the measurand) is believed to lie with a high level of confidence.
Internal reference material (generally an in-house RM/CRM/SRM).
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).
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.
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.
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.
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 9001:2015 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).
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 Measurement Error
Error attributed to the method of measurement of a particular parameter at a particular laboratory.
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.
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.
The range of CRMs produced by Ore Research & Exploration Pty Ltd (OREAS is an acronym for Ore Research & Exploration Assay Standards).
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.
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.
The closeness of agreement between results obtained by measurement of the parameter numerous times under prescribed conditions.
Synonymous with Control Limit
(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.
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.
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).
A statistically robust estimate of the true value of a reference material parameter (equivalent to the Certified Value for a CRM).
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.
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.
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.
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.
The South African Code for the Reporting of Exploration Results, Mineral Resources and Mineral Reserves (the SAMREC Code).
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.
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.
To be fit-for-purpose, RMs should be sufficiently stable so that the end user can rely on the assigned value at any point within the period of validity of the certificate. Stability should be considered under long-term storage conditions, under transport conditions and under the storage conditions of the end user. This can include consideration of stability after opening, if re-use is permitted.
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.
By virtue of differences in size and physical property (volume, density, shape, etc), particles will differentially move (segregate) from one location to another. This is particularly important for metal concentrations in Certified Reference Materials (CRMs) given that the sample provided is typically sub-sampled for analysis which may or may not be representative and true to its certification. Particle segregation is particularly ubiquitous during powder handling and transfer and is most pronounced in free-flowing powders. Powders that tend to agglomerate are inherently not free flowing and exhibit high levels of cohesion and adhesion. This ‘sticky’ manifestation can only be overcome by mixing devices that employ high shearing forces or subject the powder to impact. However, when these types of powders have been mixed (homogenised) they are far less susceptible to segregation due to the high inter-particulate forces that resist inter-particulate motion that leads to unmixing.
Standard Combined Uncertainty
It is the estimated standard deviation, equal to the positive square root of the total variance obtained by combining all uncertainty components. Standard Uncertainty is the uncertainty at 1 Standard Deviation level or at roughly 68.3% of probability in the normal distribution curve.
Standard Deviation (SD, sd or s)
A measure of the spread or dispersion of a set of results and calculated from the square root of the variance.
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.
The closeness of agreement between the average of an infinite number of replicate measured quantity values and a reference quantity value.
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.
All measurements have an uncertainty associated with it, meaning that the measurement result cannot be regarded as absolutely true or precise. This imprecision is defined as the uncertainty of measurement and is one of the most important quality characteristics of any measurement result. After adding a realistic uncertainty estimate to a measurement result, it makes possible the comparison between values to determine if in fact a particular measurement is different from another, or in the case of reference materials, differs significantly from a reference value. “Uncertainty of measurement does not imply doubt about the validity of measurement; on the contrary knowledge of the uncertainty implies increased confidence in the validity of the measurement result” EURACHEM / CITAC Guide CG4. The true value of a measurement will always remain unknown to the scientist. In other words, even though the scientist will always aim for the true value of a measurement, it is generally impossible to achieve it. This is due to the fact that real measurements are never made under perfect conditions. Errors and uncertainties can be derived from all types of sources: - The measuring instrument; including bias, ageing, wear, poor readability, etc. - The item being measured. - The measurement process. - Uncertainties such as calibration of the instrument and purity/accuracy of reagents. - Operator Skills. - Sampling Issues. - The environment; such as temperature, air pressure, humidity, etc. Where the size and effect of an error are known a correction can be applied to the measurement result. In general, uncertainties from each of these sources, and from other sources, would be individual “inputs” contributing to the overall uncertainty in the measurement.
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.