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Certified reference materials

Certified reference materials (CRMs) are reference materials, sufficiently homogeneous and stable with respect to one or more specified properties, that are accompanied by a certificate providing the certified value of the property, its associated measurement uncertainty, and a statement of metrological traceability, typically to the International System of Units (SI).^[1]^[2] These materials are produced by accredited reference material producers following rigorous international standards, such as ISO 17034, to ensure competence in their creation and certification.^[1]^[2] CRMs play a critical role in metrology and analytical chemistry by supporting calibration of instruments, validation of measurement methods, assessment of measurement uncertainty, quality control in laboratories, and verification of result traceability.^[1]^[2] They are essential across diverse fields, including environmental monitoring, pharmaceuticals, food safety, materials science, and clinical diagnostics, where precise and comparable measurements are required to ensure reliability and compliance with regulatory standards.^[1]^[2] The certification process involves metrologically valid procedures for value assignment, often using multiple independent methods, statistical evaluation of homogeneity and stability, and documentation of uncertainties that account for characterization, between-unit homogeneity, and long-term stability.^[2] The development of CRMs traces back to early 20th-century efforts in standardization, with the National Institute of Standards and Technology (NIST) issuing its first Standard Reference Materials (SRMs)—a type of CRM—in 1906 as uniform iron chip samples to address inconsistencies in industrial testing that contributed to safety issues like train derailments.^[3] Key milestones include NIST's issuance of a cholesterol SRM in 1967, which significantly improved accuracy in clinical measurements,^[4] and the establishment of international guidelines through ISO standards like Guide 30 (1992) for terminology^[5] and ISO 33405 (2024, superseding Guide 35 from 2006 and 2017) for characterization and production of reference materials.^[6] Today, over 1,300 NIST SRMs are available, alongside CRMs from global producers accredited under ISO 17034, facilitating international comparability in measurements.^[3]^[1]

Introduction

Definition and Purpose

Certified reference materials (CRMs) are reference materials 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.^[7] These materials are sufficiently homogeneous and stable with respect to their specified properties and are produced by metrology institutes or bodies accredited according to ISO 17034 to ensure competence in production and certification.^[1] The certified values are established through rigorous processes that often involve multiple independent measurement methods to achieve high reliability and minimize bias. The primary purpose of CRMs is to ensure the accuracy, traceability, and comparability of measurements across laboratories, industries, and countries by linking results to the International System of Units (SI) or other recognized standards.^[8] They serve key roles in calibrating instruments, validating analytical methods, assigning values to other materials, and supporting quality control in measurement processes.^[1] By providing a benchmark with documented uncertainty, CRMs enable the verification of measurement procedures and facilitate international trade and regulatory compliance through standardized results.^[3] Key characteristics of CRMs include their intended use, which is explicitly stated—such as for calibration or method validation—and the inclusion of comprehensive documentation on uncertainty, typically expressed with a stated confidence level.^[7] These materials must demonstrate long-term stability and homogeneity suitable for their application, ensuring reliable performance over time.^[1] For instance, in environmental monitoring, CRMs like those containing polycyclic aromatic hydrocarbons (PAHs) in particulate matter are used to calibrate instruments for pollutant detection, while in food safety, CRMs certified for total milk allergen content in food matrices help validate testing methods to protect consumer health.^[9]^[10]

Historical Development

The development of certified reference materials (CRMs) traces its origins to the early 20th century, when the need for standardized samples in chemical analysis prompted the creation of reliable measurement aids. In 1906, the U.S. National Bureau of Standards (NBS, now the National Institute of Standards and Technology or NIST) issued its first standard reference materials, consisting of iron chips with varying chemical compositions to assist foundries in quality control and calibration. These early materials, initially called standard samples, addressed inconsistencies in industrial testing and marked the beginning of formalized reference standards in the United States.^[3]^[11] Following World War II, the expansion of international trade and the demand for consistent quality assurance in manufacturing and scientific measurements accelerated the growth of reference materials programs globally. This period saw increased collaboration among national metrology institutes to harmonize standards, driven by economic integration and regulatory needs in sectors like chemicals and metals. In 1975, these efforts culminated in the establishment of the International Organization for Standardization (ISO) Committee on Reference Materials (REMCO), which aimed to promote the development and use of reference materials on an international scale.^[12]^[13] Key milestones in the standardization of CRMs followed, including the publication of ISO Guide 35 in 1989, which outlined general and statistical principles for their certification and production to ensure reliability and traceability. The International Vocabulary of Metrology (VIM), in its third edition published in 2007 (ISO/IEC Guide 99), formalized core concepts related to CRMs, such as metrological traceability and uncertainty, providing a unified terminological framework for global metrology. Additionally, the first edition of ISO 17034 published in 2016 established comprehensive requirements for the competence of reference material producers, replacing earlier guides like ISO Guide 34 and emphasizing quality management in CRM development.^[14]^[15] In December 2020, the ISO Technical Management Board decided to transform ISO/REMCO into a full technical committee, ISO/TC 334 Reference materials, which held its first meeting in 2021 to continue advancing international standards for reference materials. Recent publications as of 2024 include ISO 33405 (formerly ISO Guide 35:2017) for guidance on characterization, homogeneity, and stability assessment, and ISO 33401 for reference material certificates, reflecting ongoing refinements in CRM production and use.^[16]^[17]^[18] Over time, CRMs evolved from ad-hoc national standards to a globally harmonized system, propelled by regulatory demands in international trade, environmental monitoring, and public health to ensure comparable and accurate measurements worldwide. This progression reflected broader advancements in metrology, shifting focus from basic chemical analysis to multifaceted applications requiring certified values with demonstrated uncertainty.^[13]^[19]

Terminology and Standards

ISO REMCO Definitions

The ISO Committee on Reference Materials (ISO/REMCO) was established in September 1975 by the International Organization for Standardization (ISO) to promote the harmonization of reference materials, their production, and use, while providing guidance to other ISO committees and addressing inconsistencies in international metrology practices.^[13] ISO/REMCO operated until 2021, when it was transformed into ISO Technical Committee 334 (ISO/TC 334) on Reference Materials, which continues this work and is developing the ISO 33400 series of standards to replace earlier ISO Guides, including Guide 30.^[20]^[21] This formation aimed to standardize terminology and practices globally, reducing confusion arising from varying definitions in fields like chemistry, physics, and environmental science.^[19] According to ISO Guide 30:2015, a reference material (RM) is defined as a 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 measurement or in examination of nominal properties.^[7] This definition emphasizes the material's homogeneity and stability with respect to specified properties, ensuring it is fit for its intended use in measurement processes.^[1] A certified reference material (CRM) is a specific type of RM 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.^[7] CRMs are produced in accordance with ISO 17034:2016, which sets general requirements for the competence of reference material producers to ensure reliability and quality.^[15] Metrological validity here refers to the procedure's ability to realize the property's definition accurately, providing confidence in the result and its uncertainty through traceable measurements.^[7] Key terms in these definitions include "property value," which denotes the value of a specified property (quantitative, ordinal, or nominal, such as identity or sequence) and may include an uncertainty expressed as a probability or other means for non-quantitative attributes.^[7] The "certified value" is the property value assigned to a CRM, explicitly stated with its uncertainty and traceability in the accompanying certificate to support high-confidence applications.^[7] "Intended use" specifies the purpose for which the RM or CRM is designed, such as calibration or quality control, and is declared by the producer to confirm its fitness and metrological suitability.^[7] These terms collectively ensure metrological validity, enabling CRMs to serve as benchmarks in precise measurements across scientific and industrial contexts.^[7]

Alternative Terminology

In various contexts, certified reference materials (CRMs) are referred to by alternative terms that reflect institutional or regional preferences. In the United States, the National Institute of Standards and Technology (NIST) employs the term "Standard Reference Materials" (SRMs) for their CRMs, which are highly characterized materials issued under a registered trademark to support measurement traceability.^[1] Additionally, in the pharmaceutical sector, "Reference Standards" is a common designation, as seen in the United States Pharmacopeia (USP) standards, which serve as authentic specimens for verifying drug identity, strength, purity, and quality.^[22] Field-specific terminology often emphasizes the material's composition, such as "pure substance CRMs" for essentially pure chemicals characterized for purity and trace impurities, versus "matrix CRMs" for complex, naturally occurring materials mimicking real samples.^[23] Terms like "calibrators" are generally avoided outside medical or assay contexts, as they imply a narrower role in instrument calibration rather than broader metrological validation. In quality control practices, "control materials" may appear as sector jargon for in-house or supplementary references, distinct from certified versions.^[24] These variations stem from historical national programs, such as the Community Bureau of Reference (BCR) in Europe, which produced CRMs from the 1970s until the early 2000s before transitioning to the Joint Research Centre's framework.^[25] Sector-specific jargon further contributes, arising from practical needs in industries like pharmaceuticals or environmental testing. Such terminological diversity can lead to confusion in international trade and collaboration, where mismatched terms might hinder recognition of material equivalence; this has been mitigated through harmonization efforts by ISO/REMCO and now ISO/TC 334, which provides unifying definitions in standards like ISO Guide 30.

Classification

Types of Reference Materials

Reference materials (RMs) are broadly classified into types based on their composition, production context, and metrological characteristics, serving as foundational tools for calibration, method validation, and quality assurance in measurements. These classifications emphasize properties such as homogeneity, stability, and traceability, which ensure the material's suitability for specific analytical purposes without necessarily involving formal certification.^[26] One primary distinction is between pure substance RMs and matrix RMs. Pure substance RMs consist of highly homogeneous materials, typically elemental standards or solutions with certified purity levels and minimal impurities, used for direct calibration in techniques like atomic absorption spectroscopy. For instance, the National Institute of Standards and Technology (NIST) SRM 3128 provides a lead (Pb) standard solution at 10 mg/g concentration in dilute nitric acid, enabling traceability for lead measurements in various matrices.^[27] In contrast, matrix RMs mimic the complex composition of real-world samples, incorporating target analytes within a representative matrix to account for interferences during analysis; examples include soil or blood simulants for environmental or clinical testing, where the matrix closely resembles the material under investigation.^[28] Classification criteria for RMs focus on homogeneity (uniform distribution of properties across the material), stability (resistance to degradation under storage and use conditions), and traceability (linkage to established measurement standards), all assessed without the rigorous certification protocols required for CRMs. These attributes determine an RM's fitness for purpose, with pure substances typically excelling in homogeneity and traceability, while matrix RMs demand thorough testing to ensure matrix-matched performance. Non-certified examples, such as in-house working standards for pH calibration, illustrate lower-tier RMs suitable for routine tasks, highlighting how traceability levels vary from basic internal references to those approaching CRM standards.^[29]

Certified Reference Materials

Certified reference materials (CRMs) represent a specialized subset of reference materials characterized by their rigorous certification process, which ensures metrological traceability, reliability, and enhanced quality for analytical measurements. Unlike general reference materials, CRMs must be produced in accordance with ISO 17034, the international standard for the competence of reference material producers, and their property values are certified using multiple independent measurement methods to validate accuracy and minimize bias. This certification includes comprehensive uncertainty budgeting, typically following the Guide to the Expression of Uncertainty in Measurement (GUM) framework, to quantify and express the associated measurement uncertainties at a specified confidence level, often 95% with a coverage factor of k=2.^[30]^[31] CRMs are categorized into primary and secondary subtypes based on their metrological hierarchy and traceability chain. Primary CRMs, also known as primary certified reference materials (PCRMs), achieve the highest level of metrological quality through direct traceability to the International System of Units (SI) via primary reference measurement procedures, such as gravimetry, without intermediate references, resulting in minimal uncertainty propagation. Examples include Standard Reference Materials (SRMs) from the National Institute of Standards and Technology (NIST) and materials from the International Bureau of Weights and Measures (BIPM), which serve as benchmarks for calibrating other standards. Secondary CRMs, in contrast, are derived by comparison to primary CRMs or SRMs, offering high but slightly lower traceability with an additional link in the metrological chain; these are commonly produced commercially for practical applications in laboratories.^[32]^[33]^[34] A defining feature of CRMs is the accompanying certificate, which details the certified values for specified properties, their associated uncertainties calculated per the GUM framework, metrological traceability statements, and instructions for proper storage, handling, and use to maintain stability and validity. These certificates ensure users can reliably apply CRMs for calibration, quality control, and method validation across diverse matrices. The Codex of Online Available Reference Materials (COMAR) database serves as an international registry, listing over 2,000 CRMs from accredited producers worldwide as of 2025.^[35]^[36] Key providers include NIST with more than 1,200 SRMs, the Joint Research Centre (JRC, formerly IRMM) with about 700 materials, and the Bundesanstalt für Materialforschung und -prüfung (BAM) offering specialized CRMs for industrial and regulatory needs.^[37]^[35]^[32]^[25]

Production Process

Principal Steps

The production of certified reference materials (CRMs) involves a systematic workflow designed to ensure metrological traceability, homogeneity, and stability while meeting user requirements. The process begins with a feasibility study to evaluate the technical and economic viability of developing the material, followed by material selection to identify suitable candidates that align with intended applications. Production planning then outlines the detailed procedures, resources, and quality controls needed, incorporating characterization to determine material properties, certification to assign certified values with associated uncertainties, and distribution to end users with accompanying documentation.^[38] Compliance with ISO 17034 is a foundational principle, specifying requirements for the competence of reference material producers to maintain impartiality, confidentiality, and consistent quality throughout the process. Multidisciplinary teams, including chemists for material handling, statisticians for data analysis, and metrologists for traceability, collaborate to address complex challenges and ensure robust outcomes. Risk analysis is conducted at key stages to identify and mitigate potential issues, such as contamination or variability in material properties.^[38]^[39] Decision points emphasize assessing user needs, often through market surveys or consultations to confirm demand for specific measurands, matrices, and concentration levels, ensuring the CRM addresses real-world analytical gaps. The overall development timeline typically spans several years from initial conception to release, reflecting the extensive validation required. Costs vary widely based on complexity, ranging from hundreds of thousands to over one million euros per material, covering synthesis, testing, and certification efforts.^[39]^[37]^[40] Homogeneity testing and stability assessment represent critical phases embedded in the production workflow to verify material uniformity and long-term reliability.

Sample Preparation and Homogenization

The production of certified reference materials (CRMs) begins with the careful selection of source materials to ensure they meet stringent criteria for purity, representativeness, and scalability. Source materials are chosen based on their ability to mimic real-world samples, such as natural matrices like sediments, soils, or biological tissues, to provide relevant calibration for analytical methods. Purity is assessed to minimize impurities that could affect property values, while representativeness ensures the material reflects the composition and variability of the intended application, such as environmental or food samples. Scalability is critical, aiming for production batches of 100 to 1000 units to meet global demand without compromising quality. Preparation techniques transform raw source materials into a form suitable for CRM production, emphasizing contamination avoidance through protocols like cleanroom environments or dedicated equipment. Common methods include grinding to reduce particle size, mixing to distribute components evenly, and sieving to achieve uniform granule sizes, particularly for solid matrices like biological tissues or geological samples. For synthetic CRMs, techniques such as doping pure substances with analytes allow precise control over composition. In biological samples, freeze-drying followed by grinding and sieving through meshes as fine as 200 μm is standard to preserve analyte integrity while breaking down heterogeneous structures. These steps are designed to prevent cross-contamination, often by processing in isolated batches or using inert atmospheres. Homogenization methods ensure uniformity at the micro-scale, essential for reliable sub-sampling in end-use applications. Mechanical approaches, such as milling, tumbling in large drums for extended periods (e.g., 16 hours across multiple axes), or cryogenic grinding, are widely used for complex matrices to achieve even distribution without altering chemical properties. Thermal methods like melting and recasting apply to simpler inorganic materials, while chemical techniques involving dissolution and reprecipitation are employed for solutions or to incorporate spikes in biological matrices. For instance, in preparing tissue-based CRMs, extensive mechanical mixing post-grinding ensures analyte distribution comparable to natural variability. These methods target homogeneity sufficient for subsequent verification, though full assessment follows in dedicated testing. Challenges in sample preparation and homogenization arise particularly from matrix effects in complex samples, such as biological tissues, where natural inhomogeneities at microscopic levels can persist despite processing. Achieving uniform particle sizes below 200 μm is often difficult due to material toughness, requiring specialized equipment like ball mills or irradiation for sterilization without introducing bias. Scale-up from laboratory trials to production batches introduces risks of inconsistent mixing or contamination, necessitating iterative optimization to maintain purity and representativeness across hundreds of units. In environmental matrices like sediments, adsorption or phase separation during preparation can exacerbate these issues, demanding tailored protocols to ensure micro-scale uniformity.

Homogeneity Testing

Homogeneity testing is a critical step in the production of certified reference materials (CRMs) to verify that the material is sufficiently uniform across and within units, ensuring that the certified values are valid for any portion used in measurements. The primary purpose is to confirm that between-unit and within-unit variances are negligible relative to the overall measurement uncertainty, thereby supporting the reliability and metrological traceability of the CRM. According to ISO Guide 35:2017, this assessment must demonstrate that inhomogeneities do not significantly affect the intended use of the material in calibration or quality control.^[41] Techniques for homogeneity testing typically involve robust analytical methods capable of high precision, such as inductively coupled plasma mass spectrometry (ICP-MS) for elemental analysis or gas chromatography (GC) for organic compounds, applied to multiple units and subsamples from a batch. Sampling plans follow statistically valid designs, often testing 10 to 20 units randomly selected from batches exceeding 100 units, with subsampling to evaluate within-unit uniformity; ISO Guide 35:2017 recommends a minimum of 10 units and the use of power analysis to optimize the design for detecting inhomogeneities. For instance, in the development of a mercury-in-fish CRM, homogeneity was assessed by analyzing 15 units via ICP-MS after microwave-assisted digestion, confirming uniformity across the batch. If inhomogeneity is detected, re-testing or process adjustments, such as improved homogenization during preparation, may be required.^[41]^[42] Statistical approaches primarily rely on analysis of variance (ANOVA) to partition variance components, including between-unit standard deviation (s_bb) and within-unit contributions, using nested or randomized block designs to account for analytical variability. Outliers are identified via tests like Grubbs' or t-Student at 5% significance, and trends are checked to ensure no systematic biases. Acceptance criteria require that the between-unit inhomogeneity uncertainty (u_bbw) is insignificant compared to the certified value's uncertainty, often with s_bb < 1% of the certified value as a practical threshold in cases like gas mixture CRMs where ANOVA F-statistics must not exceed critical values. These results contribute to the CRM's uncertainty budget without dominating it, as outlined in foundational guidance.^[41]^[43]^[44]

Stability Assessment

Stability assessment in the production of certified reference materials (CRMs) involves monitoring potential changes in property values over time due to environmental factors such as temperature, light, and humidity, ensuring that the material remains suitable for its intended use throughout its validity period.^[45] This process is essential to confirm that users can rely on the certified values without significant degradation, as instability could compromise calibration, quality control, or method validation applications.^[41] General strategies for stability assessment begin with pre-production predictions based on the material's chemistry to identify potential degradation risks, followed by experimental verification during development and ongoing monitoring after certification.^[45] These approaches integrate into the overall production process to manage instability risks proactively, allowing producers to design appropriate storage and handling conditions.^[41] Monitoring typically includes initial short-term tests conducted during production to establish baseline stability, with subsequent periodic re-evaluations to verify long-term performance, often scheduled every 1 to 5 years depending on the material's risk profile and intended shelf life.^[45] Key factors influencing CRM stability encompass the material's inherent chemical reactivity, which may lead to decomposition or interactions, as well as packaging choices like inert glass vials that minimize exposure to contaminants and environmental stressors.^[45] Effective packaging and controlled storage conditions enable shelf-life assignments ranging from 2 to 10 years, during which the certified properties are expected to remain valid under specified conditions.^[46]^[47]

Certification Methods

Value Assignment Techniques

Value assignment in certified reference materials (CRMs) involves the characterization of property values using rigorous analytical and statistical approaches to ensure metrological reliability and minimize bias.^[41] The primary goal is to establish certified values that are traceable to the International System of Units (SI) through validated methods, typically employing multiple independent techniques or interlaboratory comparisons.^[41] One key approach is the use of multiple independent (orthogonal) techniques by expert laboratories to determine values for non-operationally defined measurands, such as elemental concentrations. These techniques rely on different physical or chemical principles to cross-validate results and reduce systematic errors; common examples include isotope dilution mass spectrometry (ID-MS) for precise trace element quantification, nuclear magnetic resonance (NMR) spectroscopy for organic purity assessment, and gravimetry for compositional mixtures. For instance, ID-MS combined with liquid chromatography tandem mass spectrometry (LC-MS/MS) has been applied to certify veterinary drug residues in bovine muscle CRMs, achieving high accuracy through isotope-labeled standards.^[41]^[48] Similarly, quantitative NMR (qNMR) enables direct value assignment for pharmaceuticals and carbohydrates by integrating signal intensities without external calibration, as demonstrated in the certification of ginkgolide standards.^[49] Gravimetry, often paired with these, provides primary traceability for mass-based properties in solution CRMs.^[50] This multi-method strategy, recommended in ISO Guide 35, ensures consensus values from at least two to four orthogonal approaches, enhancing confidence in the assigned property.^[41] For operationally defined measurands or complex matrices, interlaboratory comparisons—such as pilot studies or proficiency testing schemes—involving 5 to 20 competent laboratories are employed to derive consensus values through data reconciliation. Participants apply their validated methods independently, submitting results for centralized evaluation to identify compatible datasets and reconcile discrepancies.^[41] Statistical treatment of these datasets includes calculating weighted means, where weights account for method uncertainties, and robust estimators to handle non-normal distributions. Outlier detection is critical, often using tests like the Grubbs test for individual results or Mandel's statistics to assess laboratory and method biases, ensuring only reliable data contribute to the final value.^[51] A representative case study is the certification of trace elements in open-ocean seawater CRMs NASS-7 and nearshore CASS-6, where an interlaboratory study coordinated by the National Research Council Canada involved expert labs worldwide. Values for elements like cadmium (Cd), copper (Cu), iron (Fe), and zinc (Zn) were assigned using 4 to 6 orthogonal methods, including high-resolution ICP-MS, double isotope dilution, standard addition calibration, and co-precipitation preconcentration. Data from multiple labs were reconciled via robust means and outlier rejection, yielding certified concentrations (e.g., Cd at 0.182 ± 0.015 nmol/kg in CASS-6) with compatibility checks confirming agreement within 5-10% across methods.^[52] This approach minimizes bias and supports global traceability for marine environmental monitoring.^[52]

Uncertainty Evaluation

The evaluation of uncertainty in certified reference materials (CRMs) follows the framework established by the ISO/IEC Guide 98-3 (GUM), which distinguishes between Type A uncertainties derived from statistical methods (e.g., repeated measurements) and Type B uncertainties based on other information such as instrument calibration or expert judgment. This approach ensures a comprehensive assessment of all potential error sources affecting the certified value, enabling users to propagate these uncertainties into their own measurements.^[41] Key sources of uncertainty in CRMs include contributions from sampling (if applicable), homogeneity testing, stability assessments, and analytical method biases during characterization.^[41] The combined standard uncertainty u_c is calculated by propagating these components using the law of propagation of uncertainty, given by:

u_c = \sqrt{\sum u_i^2}

where u_i represents the standard uncertainty from each source. For instance, between-unit heterogeneity u_{bb} and within-unit heterogeneity u_{wb} arise from homogeneity studies, while long-term stability u_{lts} and short-term (transport) stability u_{trn} are derived from stability monitoring; characterization uncertainty u_{char} accounts for variability in value assignment techniques.^[41] To provide a confidence interval, the expanded uncertainty U is computed as U = [k](/page/K) \cdot u_c, where the coverage factor [k](/page/K) is typically 2 for approximately 95% confidence assuming a normal distribution. CRM certificates must report the certified value alongside this expanded uncertainty and include a detailed uncertainty budget outlining each component's contribution.^[41] For example, in the CRM ERM®-CZ130 for cocoa powder, the certified cadmium content is 4.1 mg/kg with an expanded uncertainty of 0.4 mg/kg (relative 9.8%, [k](/page/K)=2), where the budget includes 2.12% from between-unit homogeneity, 1.66% from long-term stability, 0.21% from transport stability, and 2.92% from characterization.^[53]

Metrological Traceability

Metrological traceability is defined as the 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.^[54] This concept, as outlined in the International Vocabulary of Metrology (VIM), ensures that measurements are comparable worldwide by linking them to the International System of Units (SI) or to certified reference measurement procedures.^[54] In the context of certified reference materials (CRMs), traceability establishes the reliability of certified property values, enabling users to connect their analytical results to these international benchmarks. CRMs serve as key anchors in metrological traceability hierarchies, providing stable and verified property values that support the chain of calibrations. Primary CRMs are those whose certified values are determined directly by primary measurement methods, which realize SI units without reference to other standards, thus offering the highest level of direct traceability to the SI.^[55] Secondary CRMs, in contrast, are calibrated against primary CRMs or other traceable references, extending accessibility to a broader range of measurements while maintaining the unbroken chain.^[55] This hierarchical structure allows CRMs to disseminate traceability from national metrology institutes to routine laboratory practices, ensuring consistency across global analytical workflows.^[41] In practice, CRMs are implemented to establish traceability through calibration of analytical instruments and validation of measurement methods. For calibration, CRMs with known concentrations are incorporated into calibration graphs or curves, allowing the adjustment of instrument responses to align with traceable values and thereby linking subsequent measurements to SI units.^[37] Similarly, in method validation, CRMs are analyzed alongside test samples to verify that the method produces results consistent with certified values, confirming the method's accuracy and traceability.^[37] These applications ensure that end-user measurements inherit the metrological validity of the CRM's certification. On an international scale, metrological traceability for CRMs is supported by the Bureau International des Poids et Mesures (BIPM) through key comparisons and the Calibration and Measurement Capabilities (CMC) database. Key comparisons, conducted among national metrology institutes under the Mutual Recognition Arrangement (CIPM MRA), demonstrate equivalence in measurement capabilities, including those for CRM value assignment, thereby underpinning global confidence in traceability chains.^[56] The KCDB lists approved CMCs for CRM production and certification, providing a public registry that verifies the traceability of specific measurement services to SI units or reference procedures.^[56] This framework facilitates international comparability by allowing users to select CRMs from producers with demonstrated metrological competence.^[56]

Applications

Calibration and Quality Control

Certified reference materials (CRMs) play a crucial role in calibrating analytical instruments and ensuring the reliability of measurements in laboratory settings. In calibration, CRMs serve as primary standards to establish metrological traceability to the International System of Units (SI), enabling accurate value assignment to instruments or methods. Direct calibration involves using CRMs to directly calibrate equipment, such as in spectrometry where CRM solutions are spiked into samples to generate calibration curves for quantitative analysis. Indirect calibration, on the other hand, uses CRM values to bracket unknown samples or correct for matrix effects, ensuring comparability without direct instrument adjustment. These practices are outlined in international guidelines for measurement processes.^[57] For quality control, CRMs are routinely incorporated into laboratory workflows to monitor method performance and detect drifts or biases. Daily or periodic checks involve analyzing aliquots of CRMs alongside test samples, with results compared against certified values to verify ongoing accuracy. Acceptance criteria typically require that the absolute difference between the measured and certified value be less than twice the expanded uncertainty (e.g., |measured - certified| < 2U, where U is the combined standard uncertainty at a coverage factor k=2 for approximately 95% confidence). This approach helps maintain statistical control, as deviations trigger investigations or recalibrations. In industrial chemical metrology, such checks are essential for instruments like X-ray fluorescence (XRF) spectrometers, where CRMs condition the equipment and confirm stability.^[58]^[57] Practical applications highlight the versatility of CRMs in specific sectors. In petrochemical laboratories, CRMs certified for sulfur content in fuels or oils are used to calibrate methods compliant with ISO/IEC 17025, ensuring accurate low-level determinations critical for environmental regulations and product specifications. Similarly, in pharmaceutical analysis, CRMs for active pharmaceutical ingredient (API) purity support quality control under FDA Good Manufacturing Practice (GMP) guidelines, verifying assay results and impurity profiles to confirm drug safety and efficacy. These uses reduce systematic bias, enhance measurement precision, and facilitate interlaboratory comparability, ultimately supporting regulatory compliance such as ISO/IEC 17025 accreditation for testing labs.^[24]^[59]^[57] The metrological traceability of CRMs underpins their effectiveness in these roles, linking routine operations to higher-order references. By integrating CRMs into calibration and quality control, laboratories minimize errors and uphold the integrity of analytical data across diverse applications.^[57]

Validation of Analytical Methods

Certified reference materials (CRMs) play a pivotal role in the validation of analytical methods by providing an independent, traceable benchmark to assess the performance of new or modified procedures, ensuring they meet required criteria for reliability and accuracy before routine application.^[60] This verification is essential in fields such as environmental monitoring and food safety, where method validation confirms that analytical results are fit for purpose, minimizing errors in decision-making processes. Key validation parameters evaluated using CRMs include accuracy, precision, linearity, and limits of detection. Accuracy, often assessed through bias or recovery studies, involves comparing method results against CRM certified values to quantify systematic errors; for instance, recovery rates close to 100% indicate minimal bias.^[60] Precision is determined by the repeatability of measurements on CRM samples under varying conditions, such as different analysts or instruments, typically expressed as relative standard deviation.^[60] Linearity is verified by analyzing CRM-spiked samples across a concentration range, confirming proportional responses without deviations.^[61] Limits of detection are established using low-level CRM materials or spiked blanks to ensure the method can reliably identify analytes at regulatory thresholds.^[61] Common procedures for CRM-based validation include spike-and-recovery tests and direct comparison to certified values. In spike-and-recovery experiments, a known amount of analyte is added to a representative matrix, and the measured recovery is calculated to evaluate extraction efficiency and matrix effects; the relative recovery R is given by:

R (\%) = \frac{\bar{x}_{\text{spiked}} - \bar{x}_{\text{unspiked}}}{x_{\text{spike}}} \times 100

where \bar{x}_{\text{spiked}} and \bar{x}_{\text{unspiked}} are the means of replicate measurements, and x_{\text{spike}} is the added amount.^[60] For comparison, multiple analyses (e.g., 10 replicates) of a CRM are performed, and bias b is computed as:

b = \bar{x} - x_{\text{ref}}

or in relative terms, b (\%) = \frac{\bar{x} - x_{\text{ref}}}{x_{\text{ref}}} \times 100, where x_{\text{ref}} is the certified value; this provides an objective measure of trueness.^[60] These approaches ensure the method's results are comparable to international standards. In environmental analysis, CRMs for pesticides in plant matrices are used to validate high-performance liquid chromatography (HPLC) methods for pesticide residues, confirming accuracy and precision in complex samples. Similarly, in food safety, CRMs such as BCR-471 (ochratoxin A in wheat) support validation of methods for mycotoxins, enabling spike-recovery tests to assess detection limits and linearity for contaminants like aflatoxins.^[62] These practices align with established guidelines, such as ICH Q2(R1), which recommends using reference materials for accuracy verification in pharmaceutical analyses, and Eurachem's "The Fitness for Purpose of Analytical Methods," emphasizing CRMs for independent trueness assessment in broader applications.^[63]^[60] Calibration using CRMs forms a foundational subset of this validation process.^[63]

Quality Assurance

Accreditation of Producers

Accreditation of producers of certified reference materials (CRMs) is governed primarily by the international standard ISO/IEC 17034:2016, which specifies general requirements for the competence and consistent operation of reference material producers (RMPs).^[15] This standard ensures that producers demonstrate technical competence, impartiality, and the use of validated processes for producing reliable reference materials, including CRMs used in calibration, quality control, and method validation.^[15] Accreditation is typically granted by national or international accreditation bodies that are signatories to the International Laboratory Accreditation Cooperation (ILAC) Mutual Recognition Arrangement (MRA), which promotes global confidence in the competence of accredited RMPs through harmonized criteria.^[64] The accreditation process involves rigorous evaluations to verify compliance with ISO 17034, including document reviews, on-site audits to witness production activities such as material processing and homogeneity testing, and assessments of personnel competence.^[65] Proficiency demonstrations, often through participation in interlaboratory comparisons or proficiency testing schemes, are required to confirm the accuracy of value assignment and uncertainty evaluation.^[65] Peer reviews by technical experts, including those with expertise in statistics and metrology (e.g., per ISO Guide 35), evaluate aspects like stability studies and traceability.^[65] Examples include the U.S. National Institute of Standards and Technology (NIST) Standard Reference Materials (SRM) program, which conforms to ISO 17034 for CRM production, and the European Commission's Joint Research Centre (JRC) Reference Materials Unit, accredited under ISO 17034 as part of the European Reference Materials (ERM) framework since completing the transition from ISO Guide 34 in 2017.^[66]^[67] Key producers of CRMs include national metrology institutes such as NIST in the United States and the Bundesanstalt für Materialforschung und -prüfung (BAM) in Germany, both operating under ISO 17034 accreditation scopes for specific material types.^[66]^[68] Commercial entities like LGC Standards, accredited by the American Association for Laboratory Accreditation (A2LA) to ISO 17034 for a wide range of chemical and microbiological CRMs, and Sigma-Aldrich (part of MilliporeSigma), which manufactures CRMs in compliance with ISO 17034 and ISO/IEC 17025, also play significant roles.^[69]^[70] International organizations, such as the International Atomic Energy Agency (IAEA), hold ISO 17034 accreditation for radionuclide CRMs like IAEA-478 (agricultural soil) and IAEA-479 (milk powder), ensuring traceability in nuclear applications.^[71] The 2016 edition of ISO 17034, which replaced ISO Guide 34:2009, introduced structural alignment with ISO 9001:2015, incorporating risk-based thinking for process management and requirements for integrating user feedback to improve CRM production and relevance.^[72]^[73] These updates emphasize proactive risk assessment in areas like supply chain and stability monitoring, while mandating records of customer discussions and feedback to refine material specifications and certification.^[72]^[73]

Monitoring and Recalls

Producers of certified reference materials (CRMs) are required to implement ongoing monitoring programs to ensure the continued validity of certified property values after production and distribution. These programs typically involve periodic re-testing of retained samples for stability, with intervals often ranging from 2 to 5 years depending on the material's expected shelf life and risk assessment, as outlined in ISO 33405:2024. For instance, certain CRMs, such as those for genetically modified organisms, undergo additional stability monitoring every 3 years to confirm that property values remain within specified uncertainty limits.^[17]^[74] User feedback mechanisms are integrated into these programs, where producers respond to inquiries from users regarding observed deviations, allowing for rapid assessment and potential adjustments to usage recommendations on certificates.^[37] Recall procedures for CRMs are activated when post-certification monitoring or user reports reveal evidence of instability, contamination, or other compromises to the material's integrity, such as degradation beyond certified limits. These procedures follow established protocols from accrediting bodies and producers, including notification to users, withdrawal from distribution, and coordination for returns or disposal, as exemplified in the NIST Standard Reference Materials distribution guidelines. Notifications are disseminated through producer certificates, dedicated websites, and international databases like COMAR, which facilitate user access to updated CRM status information.^[75]^[76] Global coordination of CRM monitoring and recalls is supported by organizations such as the International Bureau of Weights and Measures (BIPM) and the International Organization for Standardization (ISO), which provide frameworks for traceability and quality management through initiatives like the Consultative Committee for Metrology in Chemistry and Biology (CCQM). The BIPM oversees key comparisons and calibration capabilities that indirectly support CRM surveillance by ensuring metrological consistency across national metrology institutes, while ISO standards like 33405:2024 mandate stability monitoring strategies to prevent widespread issues. Successful interventions, such as timely stability updates shared via international networks, have helped maintain user trust in CRM reliability by averting broader impacts on analytical measurements.^[77]^[17] Emerging trends in CRM quality assurance include the adoption of digital certificates, which enable real-time updates on stability and recall status without reprinting physical documents. Efforts by organizations like NIST are advancing machine-readable digital reference material certificates, potentially incorporating QR codes for instant verification and access to post-production data, addressing gaps in traditional post-certification communication.^[78]^[79]

References

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