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How to Choose the Correct Reference Material Quality Grade
Quality Grades, Certificates of Analysis, and Metrological Traceability and What They Mean for You
Who Uses Reference Materials?
Reference material is an important part of the analytical testing workflow. Through the calibration of the test system, validation of the method, and quality control procedures, the reference material can ensure the accuracy of the test. From Certified reference materials (CRMs) and other quality grades to analytical certificates, metrological traceability and other related concepts, the world of reference materials is large and sometimes confusing.
The chart shows some important reference material related parameters, provides information on metrological traceability, reference material hierarchy, analysis certificates, reference material format and use, and selection considerations for the desired purpose. It is important to choose the right reference material for your laboratory's test application because the results will be as accurate as your reference material.
Figure 1. Metrological Traceability
Metrological Traceability and SI Units of Your Reference Materials
Metrological traceability is an important concept in the field of reference materials. A basic term for metrological traceability is the SI unit of measurement. The International System of Units (SI) defines seven units of measurement as the base set from which all other SI units can be derived. The two most common SI units of measurement used for reference material traceability are kilograms and moles.
Metrological traceability means that measurement results can be meaningfully compared at different locations, at different times, by different people, and using different equipment. The measurement results must be related to the reference material through a documented, complete calibration chain, traceable to the SI unit of measurement.
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ISO 17034 and Quality Grades of Standards, Reference Materials and Certified Reference Materials
1. National Metrology Standard (e.g. NIST, JRC, NMI Australia) /?Compendial Standard (e.g. USP, EP, BP, JP, IP)
??Issued by an authorized authority
??Consider providing the highest standards of accuracy and traceability
2. Certified Reference Material (CRM) (ISO 17034, 17025)
??Being identified as an SI measurement unit provides the highest level of accuracy, uncertainty, and traceability
??Manufactured by an accredited Reference Material Producer
3. Reference Material (RM) (ISO 17034)
??Meet ISO requirements, lower than CRM standards
??Manufactured by an accredited Reference Material Producer
4. Analytical Standard (ISO 9001)
??Certificate of Analysis available
??Level of certification varies
5. Reagent Grade / Research Chemical
??May come with a Certificate of Analysis
??Are not characterized for use as reference materials
Figure 2. The Hierarchy of Reference Materials - What are the Different Types?
The classification of reference materials includes five main quality levels, ranging from national metrological and other primary standards to certified reference materials (CRMs), reference materials (RMs), analytical standards, and research grade or research chemicals, with certification levels and traceability requirements increasing as the level increases. Where governments give the highest level of standardization, specific ISO guidelines provide standards for CRMs and RMs. These ISO requirements include ISO 17034, ISO/IEC 17025, and ISO 31 guidelines.
Reference material manufacturers must meet these ISO standard requirements to manufacture CRMs or RMs and, for both quality classes, must provide an analytical certificate containing information as defined by the above ISO guidelines. The last two levels of quality specification are defined by each manufacturing firm rather than by national governments or ISO certification specific to CRMs and RMs.
What is Measured in each Grade of Reference Material?
The purity and origin of the material are usually included in each analytical certificate for each quality level, of which there are five, and the completeness of the CRM and RM content and the stability indicators of the standard are mainly defined by ISO. Products in the two classes of analytical standards and research chemicals may or may not include these two parameters, as their inclusion depends on the product information given by the manufacturer. Analytical standards can also in some cases be quality control materials in accordance with ISO 80 guidelines.
For major reference material grades, such as CRM and RM, homogeneity is required, but for lower quality grades, this parameter is not required. Uncertainty and traceability information is limited to primary reference materials and CRMS. In the pharmaceutical field, secondary standards can be CRMs or RMs, but here, there are two different types of traceability -traceability of SI units of measurement for equivalency defined CRMS, and traceability to major pharmacopeia standards, which are specific requirements for pharmaceutical secondary standards.?
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Table 1.?The Hierarchy of Reference Materials – What’s the Difference?
Understanding Your Reference Material Certificate of Analysis
Certified Reference Materials (CRMS) or reference materials will contain Certificates of Analysis (CoA) at different levels. In CoA, the understanding of several quality parameters is crucial: accuracy, consistency, uniformity, purity, and stability. In addition, it is important to know what is being certificate-that is, the value of the property being certificated, whether it is concentration, potency, or content.
Ensure that CoA verification is consistent with the producer's quality system, certification process for reference materials, and needs to support the traceability of CRM information. CoA is important because it can provide laboratory information to ensure that the reference material is certified in accordance with the test method or in a way that can be used in the relevant application.
??Accuracy
Comparison to a primary source or certified second source – curve/calibration standard. Comparison of multiple independent preparations.
??Consistency
Lot-to-lot consistency verified by comparing to the previous lot.
??Stability
Expiration date established though real-time stability studie.
??Homogeneity
Across the batch of ampoules/vial.
??Purity
Consistent with the neat material. No contamination or degradation.
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Reference Material Formats – Do You Need a Neat, Solution or Matrix Material?
Reference materials can be used in different forms in testing laboratories depending on the use scenario and method requirements of the product. The three forms of reference materials are pure material or powder form, solution form, or matrix form. The Aladdin??family of standards includes CRM, RM, or analytical standards, with the choice of each form depending on the laboratory's needs and specific application scenario.
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Table 2.?Different Formats – How Reference Materials are Used in the Testing Laboratory
Choose the Correct Reference Material for Your Testing Purpose
For the identification and calibration of instruments, it is essential to establish and maintain traceability, and the reference material chosen should help the laboratory achieve this goal. It may be important to identify something practical and easy to use in a daily workflow, but for daily use, it must be reliable and cost effective. In method validation, the use of highly precise substances to demonstrate the precision of laboratory methods is essential. For identification and screening purposes, important attributes of reference materials include proof of their authenticity and traceability. Quantitative, qualitative or stability assessments require stable and accurate reference materials to serve as a basis.
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Table 3.?Use of Reference Materials – Type of Test
Which Quality Grade is the Best Fit for Your Purpose?
Fit for purpose decisions in selection of reference materials can depend on several factors, from regulatory requirements, availability, and type of testing application to level of accuracy and sample matrix.
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Table 4.?Fit for Purpose Guidance in Standard Selection
危险品化学品经营许可证（带存储）