Certified reference materials

Last updated
Green Tea standard reference Green tea Standard Reference Material (SRM) (5978206005).jpg
Green Tea standard reference

Certified reference materials (CRMs) are 'controls' or standards used to check the quality and metrological traceability of products, to validate analytical measurement methods, or for the calibration of instruments. [1] A certified reference material is a particular form of measurement standard.

Contents

Reference materials are particularly important for analytical chemistry and clinical analysis. [2] Since most analytical instrumentation is comparative, it requires a sample of known composition (reference material) for accurate calibration. These reference materials are produced under stringent manufacturing procedures and differ from laboratory reagents in their certification and the traceability of the data provided.

Quality management systems involving laboratory accreditation under national and international accreditation/certification standards such as ISO/IEC 17025 require metrological traceability to Certified Reference Materials (where possible) when using reference materials for calibration. [3]

Whilst Certified Reference Materials are preferred where available, [3] [4] their availability is limited. Reference Materials that do not meet all the criteria for certified reference materials are more widely available: the principal difference is the additional evidence of metrological traceability and statement of measurement uncertainty provided on the certificate for certified reference materials. [5]

Terminology

ISO REMCO definitions

ISO REMCO, the ISO committee responsible for guidance on reference materials within ISO, [6] defines the following classes of reference material: [7] [8]

Reference Material
Material, sufficiently homogeneous and stable with respect to one or more specified properties, which has been established to be fit for its intended use in a measurement process.
Certified Reference Material
Reference material characterized by a metrologically valid procedure for one or more specified properties, accompanied by a certificate that provides the value of the specified property, its associated uncertainty, and a statement of metrological traceability.

Alternative terminology

Other bodies may define classes of reference material differently. WHO guidelines for biological [nb 1] reference materials [9] provide the terms: [nb 2]

For chemical substances some pharmacopoeias [10] use the WHO terms [11]

The United States National Institute of Standards and Technology (NIST) uses the trade marked term [12] Standard Reference Material (SRM) to denote a certified reference material that satisfies additional NIST-specific criteria. In addition, commercial producers adhering to criteria and protocols defined by NIST may use the trademark [13] "NIST traceable reference material" to designate certified reference materials with a well-defined traceability linkage to existing NIST standards for chemical measurements. [14] [15]

Types of reference material

ILAC describes the following five types of reference material: [1]

  1. Pure substances; essentially pure chemicals, characterised for chemical purity and/or trace impurities.
  2. Standard solutions and gas mixtures, often prepared gravimetrically from pure substances.
  3. Matrix reference materials, characterised for the composition of specified major, minor or trace chemical constituents. Such materials may be prepared from matrices containing the components of interest, or by preparing synthetic mixtures.
  4. Physico-chemical reference materials, characterised for properties such as melting point, viscosity, or optical density.
  5. Reference objects or artifacts, characterised for functional properties such as taste, odour, octane number, flash point and hardness. This type also includes microscopy specimens characterised for properties ranging from fibre type to microbiological specimens.

Production

Principal steps in producing certified reference materials

Standard reference peanut butter. Standard reference material peanut butter.jpg
Standard reference peanut butter.

The preparation of certified reference materials is described in general in ISO Guide 17034 [16] and in more detail in ISO Guide 35. [17] Preparation of biological reference standards is described in WHO Guidance. [9] General steps required in production of a certified reference material typically include: [17]

In addition it may be important to assess the commutability of a reference material; this is especially important for biological materials. [9]

Sample preparation

Detailed sample preparation depends on the type of material. Pure standards are most likely to be prepared by chemical synthesis and purification and characterized by determination of remaining impurities. [1] This is often done by commercial producers. Natural matrix CRMs (often shortened to 'matrix CRMs') contain an analyte or analytes in a natural sample (for, example, lead in fish tissue). These are typically produced by homogenization of a naturally occurring material followed by measurement of each analyte. Due to the difficulty in production and value assignment, these are usually produced by national or transnational metrology institutes like NIST (USA), BAM (Germany), KRISS (Korea) and EC JRC ( European Commission Joint Research Centre).

For natural materials, homogenization is often critical; [19] natural materials are rarely homogeneous on the scale of grams so production of a solid natural matrix reference material typically involves processing to a fine powder or paste. [20] Homogenization can have adverse effects, for example on proteins, so producers must take care not to over-process materials. [9] [17] Stability of a certified reference material is also important, so a range of strategies may be used to prepare a reference material that is more stable than the natural material it is prepared from. [21] : 119–124  For example, stabilizing agents such as antioxidants or antimicrobial agents may be added to prevent degradation, liquids containing certified concentrations of trace metals may have pH adjusted to keep metals in solution, and clinical reference materials may be freeze-dried for long term storage if they can be reconstituted successfully. [9] : 96 

Homogeneity testing

Homogeneity testing for a candidate reference material typically involves replicated measurements on multiple units or subsamples of the material.

Homogeneity tests for CRMs follow planned experimental designs. Because the experiment is intended to test for (or estimate the size of) variation in value between different CRM units, the designs are chosen to allow separation of variation in results due to random measurement error and variation due to differences between units of the CRM. Among the simplest designs recommended for this purpose is a simple balanced nested design (see schematic).

Schematic of a balanced nested design for a CRM homogeneity test. Large bottles show packaged individual CRM units; small vials show subsamples prepared for measurement. Balanced nested design schematic for CRM homogeneity test.png
Schematic of a balanced nested design for a CRM homogeneity test. Large bottles show packaged individual CRM units; small vials show subsamples prepared for measurement.

Typically 10-30 CRM units are taken from the batch at random; stratified random sampling is recommended so that the selected units are spread across the batch. [22] An equal number of subsamples (usually two or three) is then taken from each CRM unit and measured. Subsamples are measured in random order. [17] [22] Other designs, such as randomized block designs, have also been used for CRM certification.[ citation needed ]

Data processing for homogeneity tests usually involves a statistical significance test for evidence of differences between units of the candidate CRM. For the simple balanced design above, this typically uses an F test following ANOVA. A check for trends with production order is also recommended. [22] This approach is not taken in ISO Guide 35:2017; rather, emphasis is placed on deciding whether the between-unit standard deviation is sufficiently small for the intended end use. If statistical tests are used, however, the homogeneity experiment should be capable of detecting important heterogeneity, ISO Guide 35:2017 in turn requiring a sufficient combination of precision of the measurement procedure, number of RM units and number of replicates per unit. Statistical power calculations can assist in ensuring a sufficiently effective test . [17]

In extreme cases, such as microanalysis, materials must be checked for homogeneity on sub-micron scales; this may involve much larger numbers of observations and adjustments to statistical analysis. [23]

Stability assessment

Stability assessment and testing strategies

Stability is among the essential properties of a CRM (see definitions above), and stability assessment is accordingly required for certified reference materials. [9] [16] Stability under long term storage and also under conditions of transport are both expected to be assessed. [16] "Assessment" is not synonymous with "testing"; some materials - for example, many minerals and metal alloys - may be so stable that experimental tests are not considered necessary. [24] Other reference materials will usually undergo experimental tests of stability at some point prior to the material being distributed for sale. Where reference materials are certified for more than one property, stability is expected to be demonstrated for every certified property. [16]

There are two important strategies for CRM stability testing; simple real-time studies and accelerated testing. Real-time studies simply keep units of the material at their planned storage temperature for a suitable period of time and observe the material at intervals. Accelerated studies use a range of more stringent conditions, most commonly increased temperature, to test whether the material is likely to be stable over longer time scales.

Real-time stability studies

Real-time stability studies simply hold a set of RM units at a proposed storage temperature and test a proportion of them at regular intervals. The results are usually assessed by inspection and by linear regression to determine whether there is a significant change in measured value over time. [17]

Accelerated stability studies

Accelerated studies have been in use since at least the mid-1950s, at least for biological reference materials. [25] [26] CRMs are typically monitored at a range of temperatures and the results are used to predict the rate of change at a proposed, usually low, storage temperature. Often, the prediction uses a well known degradation model such as an Arrhenius model. [9] The advantage over real-time studies is that results are available sooner and predictions of stability over a much longer period than the stability study can be defended. For some applications, accelerated studies have been described as the only practical approach: [27]

In the absence of a reference method or a higher order standard, ... accelerated studies under stress conditions provide the only approach for assessment of stability

— World Health Organization

The principal disadvantage of accelerated studies is that reference materials, like any other material, can degrade for unexpected reasons over time, or can degrade following different kinetic models; predictions can then become unreliable. [28]

Isochronous studies

In most stability studies, real-time or accelerated, a few units of the reference material are tested at intervals. If the measurement system used for testing the materials is not perfectly stable, this can generate imprecise data or can be mistaken for instability of the material. To overcome these difficulties, it is often possible to move RM units, at intervals, to some reference temperature where they remain stable, and then test all the accumulated units - which have undergone different exposure times - at the same time. This is referred to as an isochronous study. This strategy has the advantage of improving the precision of data used in assessing stability at the cost of delaying results until the end of the stability study period. [28]

See also

Notes

  1. ↑ The term "Biological" as used by WHO is discussed in WHO report 932 Annex 2 [9]
  2. ↑ Descriptions have been shortened from the original text

Related Research Articles

Standard temperature and pressure (STP) are various standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union of Pure and Applied Chemistry (IUPAC) and the National Institute of Standards and Technology (NIST), although these are not universally accepted standards. Other organizations have established a variety of alternative definitions for their standard reference conditions.

<span class="mw-page-title-main">Laboratory glassware</span> Variety of equipment usually made of glass used for scientific experiments

Laboratory glassware refers to a variety of equipment used in scientific work, and traditionally made of glass. Glass can be blown, bent, cut, molded, and formed into many sizes and shapes, and is therefore common in chemistry, biology, and analytical laboratories. Many laboratories have training programs to demonstrate how glassware is used and to alert first–time users to the safety hazards involved with using glassware.

In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of known accuracy, a device generating the quantity to be measured such as a voltage, a sound tone, or a physical artifact, such as a meter ruler.

<span class="mw-page-title-main">Metrology</span> Science of measurement and its application

Metrology is the scientific study of measurement. It establishes a common understanding of units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to standardise units in France when a length standard taken from a natural source was proposed. This led to the creation of the decimal-based metric system in 1795, establishing a set of standards for other types of measurements. Several other countries adopted the metric system between 1795 and 1875; to ensure conformity between the countries, the Bureau International des Poids et Mesures (BIPM) was established by the Metre Convention. This has evolved into the International System of Units (SI) as a result of a resolution at the 11th General Conference on Weights and Measures (CGPM) in 1960.

Traceability is the capability to trace something. In some cases, it is interpreted as the ability to verify the history, location, or application of an item by means of documented recorded identification.

ISO 31 is a superseded international standard concerning physical quantities, units of measurement, their interrelationships and their presentation. It was revised and replaced by ISO/IEC 80000.

<span class="mw-page-title-main">International unit</span> Pharmacological measurement unit for the amount of a biologically active substance

In pharmacology, the international unit (IU) is a unit of measurement for the effect or biological activity of a substance, for the purpose of easier comparison across similar forms of substances. International units are used to quantify vitamins, hormones, some medications, vaccines, blood products, and similar biologically active substances.

<span class="mw-page-title-main">International Organization of Legal Metrology</span>

The International Organization of Legal Metrology, is an intergovernmental organisation that was created in 1955 to promote the global harmonisation of the legal metrology procedures that underpin and facilitate international trade.

<span class="mw-page-title-main">National Physical Laboratory of India</span>

The CSIR- National Physical Laboratory of India, situated in New Delhi, is the measurement standards laboratory of India. It maintains standards of SI units in India and calibrates the national standards of weights and measures.

ISO/IEC 17025General requirements for the competence of testing and calibration laboratories is the main standard used by testing and calibration laboratories. In most countries, ISO/IEC 17025 is the standard for which most labs must hold accreditation in order to be deemed technically competent. In many cases, suppliers and regulatory authorities will not accept test or calibration results from a lab that is not accredited. Originally known as ISO/IEC Guide 25, ISO/IEC 17025 was initially issued by ISO/IEC in 1999. There are many commonalities with the ISO 9000 standard, but ISO/IEC 17025 is more specific in requirements for competence and applies directly to those organizations that produce testing and calibration results and is based on more technical principles. Laboratories use ISO/IEC 17025 to implement a quality system aimed at improving their ability to consistently produce valid results. Material in the standard also forms the basis for accreditation from an accreditation body.

In experimental methodology, a round-robin test is an interlaboratory test performed independently several times. This can involve multiple independent scientists performing the test with the use of the same method in different equipment, or a variety of methods and equipment. In reality it is often a combination of the two, for example if a sample is analysed, or one of its properties is measured by different laboratories using different methods, or even just by different units of equipment of identical construction.

The South African National Accreditation System (SANAS) is the official accreditation body for South Africa. Founded in 1996, SANAS is headquartered in Pretoria, South Africa. SANAS accreditation certificates are a formal recognition by the Government of South Africa that an organisation is competent to perform specific tasks.

<span class="mw-page-title-main">STEP-NC</span> Machine tool control language

STEP-NC is a machine tool control language that extends the ISO 10303 STEP standards with the machining model in ISO 14649, adding geometric dimension and tolerance data for inspection, and the STEP PDM model for integration into the wider enterprise. The combined result has been standardized as ISO 10303-238.

Forensic metrology is a branch of metrology applied to forensic sciences. Forensic laboratories and criminalistic laboratories perform numerous measurements and tests to support criminal prosecution and civil legal actions. Examples of forensic metrology include the measurement of alcohol content in blood using breathalyzers, quantification of controlled substances, and length measurements of firearm barrels. The results of forensic measurements are used to determine if a person is charged with a crime or may be used to determine a statutory sentencing enhancement. Other examples of forensic metrology includes tests that measure if there is a presence of a substance, latent print examination, questioned documents examination, and DNA analysis.

<span class="mw-page-title-main">Standard (metrology)</span> Object, system, or experiment which relates to a unit of measurement of a physical quantity

In metrology, a standard is an object, system, or experiment that bears a defined relationship to a unit of measurement of a physical quantity. Standards are the fundamental reference for a system of weights and measures, against which all other measuring devices are compared. Historical standards for length, volume, and mass were defined by many different authorities, which resulted in confusion and inaccuracy of measurements. Modern measurements are defined in relationship to internationally standardized reference objects, which are used under carefully controlled laboratory conditions to define the units of length, mass, electrical potential, and other physical quantities.

<span class="mw-page-title-main">Rosstandart</span>

Federal Technical Regulation and Metrology Agency (Rosstandart) is the Russian federal government agency that serves as a national standardization body of the Russian Federation. It was previously known as Gosstandart. It is subordinated to the Ministry of Industry and Trade.

<span class="mw-page-title-main">National Accreditation Board for Testing and Calibration Laboratories</span>

National Accreditation Board for Testing and Calibration Laboratories (NABL) provides accreditation to Conformity Assessment Bodies (Laboratories) in India. NABL Schemes include Accreditation (Recognition) of Technical competence of testing, calibration, medical testing laboratories, Proficiency testing providers (PTP) & Reference Material Producers (RMP) for a specific scope following ISO/IEC 17025, ISO 15189, ISO/IEC 17043 & ISO 17034:2016 Standards. It has Mutual Recognition Arrangement (MRA) with Asia Pacific Accreditation Cooperation (APAC), International Laboratory Accreditation Cooperation (ILAC).

The National Standardization Agency of Indonesia is the International Organization for Standardization (ISO) member body for Indonesia. BSN is a non-ministerial Indonesian government agency with the main task of carrying out governmental tasks in the field of standardization and conformity assessment in Indonesia.

Canadian Reference Materials (CRM) are certified reference materials of high-quality and reliability produced by the National Metrology Institute of Canada – the National Research Council Canada. The NRC Certified Reference Materials program is operated by the Measurement Science and Standards portfolio and provides CRMs for environmental, biotoxin, food, nutritional supplement, and stable isotope analysis. The program was established in 1976 to produce CRMs for inorganic and organic marine environmental analysis and remains internationally recognized producer of CRMs.

<span class="mw-page-title-main">VAMAS</span>

VAMAS stands for Versailles Project on Advanced Materials and Standards. It is a collaborative project that was initiated at the 1982 G7 Economic Summit in Versailles to develop and promote standards for the characterisation of advanced materials, including surfaces, interfaces, thin films, and nanostructures. Using interlaboratory studies, the VAMAS project has developed a number of standard test methods and reference materials for a wide range of materials. These standards have been widely adopted by industry and academic researchers, and have contributed to the development of new materials and technologies.

References

  1. 1 2 3 "ILAC G9:2005 - Guidelines for the Selection and Use of Reference Materials" (PDF). 2005. Archived from the original (PDF) on 8 July 2011. Retrieved 30 May 2013.
  2. ↑ Calli, J. Paul (1979). "The role of reference materials in the analytical laboratory". Fresenius' Zeitschrift für Analytische Chemie. 297 (1): 1–3. doi:10.1007/BF00487739. ISSN   1618-2650. S2CID   91310454.
  3. 1 2 "ILAC-P10:01/2013 - ILAC Policy on the Traceability of Measurement Results" (PDF). International Laboratory Accreditation Cooperation. 2013. pp. 8–9. Retrieved 1 June 2014.[ permanent dead link ]
  4. ↑ Ellison, Stephen L R; King, Bernard; Roesslein, Matthias; Salit, Marc; Williams, Alex, eds. (2003). "Traceability in Chemical Measurement" (PDF). Eurachem. Eurachem. pp. 21–22. Retrieved 1 June 2014.
  5. ↑ ISO Guide 31:2015 Reference materials — Contents of certificates, labels and accompanying documentation. Geneva: International Organization for Standardization. 2015.
  6. ↑ "ISO/REMCO Committee on reference materials". International Organization for Standardization. Retrieved 1 June 2014.
  7. ↑ ISO Guide 30:2015 Reference materials — Selected terms and definitions. Geneva: International Organization for Standardization. 2015.
  8. ↑ "Definitions". National Institute of Standards and Technology. National Institute of Standards and Technology. 2010-08-11. Retrieved 1 June 2014.
  9. 1 2 3 4 5 6 7 8 WHO Expert Committee on Biological Standardization (2004). "Annex 2: Recommendations for the preparation, characterization and establishment of international and other biological reference standards". WHO Expert Committee on Biological Standardization: Fifty-fifth report. Geneva: World Health Organization. ISBN   92-4-120932-1.
  10. ↑ "Reference standards". British Pharmacopoiea. Archived from the original on 5 June 2014. Retrieved 1 June 2014.
  11. ↑ "Reference Substances and Infrared Reference Spectra for Pharmacopoeial Analysis: Introduction". Archived from the original on January 19, 2012. Retrieved 1 June 2014.
  12. ↑ [http://tarr.uspto.gov/servlet/tarr?regser=serial&entry=74287627 Standard reference material ]
  13. ↑ [http://tarr.uspto.gov/servlet/tarr?regser=serial&entry=75939575 NTRM]
  14. ↑ "SRM definitions". National Institute of Standards and Technology. National Institute of Standards and Technology. 2010-08-11. Retrieved 3 June 2014.
  15. ↑ May, W.; Parris, R.; Beck, J. Fassett; Greenberg, R.; Guenther, F.; Kramer, G.; Wise, S.; Gills, T.; Colbert, J.; Gettings, R.; MacDonald, B. (January 2000). "NIST Special Publication 260-136: Definitions of Terms and Modes Used at NIST for Value-Assignment of Reference Materials for Chemical Measurements" (PDF). Natl. Inst. Stand. Technol. Spec. Publ. 260–136. Retrieved 3 June 2014.
  16. 1 2 3 4 ISO Guide 17034:2016 - General requirements for the competence of reference material producers. Geneva: International Organization for Standardization. 2016.
  17. 1 2 3 4 5 6 ISO Guide 35:2017 - Reference materials — Guidance for characterization and assessment of homogeneity and stability. Geneva: International Organization for Standardization. 2017.
  18. ↑ ISO Guide 30:2015 Reference materials — Selected terms and definitions. Geneva: International Organization for Standardization. 2015.
  19. ↑ Linsinger, Thomas P J; Pauwels, Jean; van der Veen, Adriaan M H; Schimmel, Heinz; Lamberty, Andre (January 2001). "Homogeneity and stability of reference materials". Accreditation and Quality Assurance. 6 (1): 20–25. doi:10.1007/s007690000261. ISSN   1432-0517. S2CID   16625720.
  20. ↑ "Guidelines for the production of BAM reference materials (Version 1, June 2006 Updated April 2010)" (PDF). BAM, Germany. Bundesanstalt für Materialforschung und -prüfung. 20 June 2006. pp. 7–8. Archived from the original (PDF) on 2 November 2012. Retrieved 13 June 2014.
  21. ↑ Quevauviller, Ph.; Maier, E.A. (1999). Interlaboratory studies and certified reference materials for environmental analysis the BCR approach. Amsterdam: Elsevier. ISBN   978-0444823892.
  22. 1 2 3 Schiller, Susan B (July 1996). "NIST Special Publication 260-125. Standard Reference Materials: Statistical aspects of the certification of chemical batch SRMs" (PDF). Natl. Inst. Stand. Technol. Spec. Publ. 260–125. Retrieved 25 July 2014.
  23. ↑ Mattiuzzi, Marco; Markowitz, Andrej. "A Modified Approach to Homogeneity Testing at Microscale" (PDF). International Centre for Diffraction Data. International Centre for Diffraction Data. Retrieved 1 July 2014.
  24. ↑ Bokhari, Syed; Meisel, Thomas (8 June 2014). "Stability of geological reference materials" (PDF). Goldschmidt2014. Archived from the original (PDF) on 9 January 2017. Retrieved 20 September 2016.
  25. ↑ Jerne, N. K.; Perry, W. L. M. (1956). "The stability of biological standards". Bulletin of the World Health Organization. 14 (1): 167–182. PMC   2538106 . PMID   13329844.
  26. ↑ Kirkwood, T. B. L (December 1977). "Predicting the Stability of Biological Standards and Products". Biometrics. 33 (4): 736–742. doi:10.2307/2529472. JSTOR   2529472. PMID   588659.
  27. ↑ "Meeting Report: WHO Working Group on Stability of Reference Materials for Biological Medicines and In Vitro Diagnostics" (PDF). World Health Organization. World Health Organization. 27 November 2006. Retrieved 21 September 2016.
  28. 1 2 Linsinger, Thomas; van der Veen, Adriaan; Gawlik, B; Pauwels, Jean; Lamberty, Andre (2004). "Planning and combining of isochronous stability studies of CRMs". Accreditation and Quality Assurance. 9 (8): 464–472. doi:10.1007/s00769-004-0818-x. S2CID   120110272.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.