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.

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. 

Parameter	NMI Standard	Compendial Standard	Certified Reference Material	Reference Material	Analytical Standard	Research Chemical
Purity	√	√	√	√	√	√
Identity	√	√	√	√	√	√
Content	√	√	√	√
Stability	√	√	√	√	√
Homogeneity	√	√	√	√
Uncertainity	√		√
Traceability	√		√	√
Type	Primary Measurement Standard or Primary Standard (Pharma)	Primary Measurement Standard or Primary Standard (Pharma)	Primary or Secondary Standard (Pharma)	Secondary Standard (Pharma)

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.

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.

	Neat Analyte	In Solution	In Matrix
Form	Vial/Lyophilized	Ampoules/Vials/Bottles	Ampoules/Vials/Bottles
Uses	Weigh daily/weekly to make stock levels & working solutions	Ready-made, certified and ready to use or dilute	Ready-made and certified at working level in matrix of choice
Pros	Widely available Flexible to use in a variety of applications Large unit sizes available	Convenient - Saves time Concentration is certified and traceable Stable - protected from evaporation, transpiration, O2	Convenient - Saves time Remove need to further dilute into matrix of choice Concentration and stability is certified and traceable
Cons	Maybe hard to handle: Hygroscopic, viscous, unstable Time consuming Potentially greater week-week variability in results	Need correct mix of analytes at right concentration and volumes Diluent compatibility with method	Long-term analyte stability in matrix vs. diluent Special handling considerations & storage of biological matrices

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.

Type of test	Use of Reference Material	Examples	Requirements of the Reference Material
Instrument qualification/Calibration	Establish system performance Measurement Accuracy	Annual qualifications Routine balance calibrations	Traceable
Routine calibration/System suitability	Daily / weekly System / method specific Establish routine performance	Pre-use Balance calibrations System performance checks for LC-UV/MS; GC-FID…	Qualify as suitable for use
Method validation	Accuracy Precision Specificity & interferences LOD/LOQ & Linearity	Pharma QC; Environmental testing Standards of the analyte(s), interferences, impurities	Accurate Traceable
Identity	Comparison of unknown to known	In-coming raw materials in pharma, food etc. Screening tests	Authenticity
Content or assay	Quantitation of analytes	Pesticide/toxin limits Pharma QC – API content	Certified content Traceable
Stability assessment	Monitor product stability	Pharma QC	Stable, homogenous
Internal Quality Control	Method accuracy	Routine quantitation of analytes- pharma/pesticides/diagnostics	Certified content Traceable

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.

Type of Test	NMI Standard	Compendial Standard	Certified Reference Material	Reference Material	Analytical Standard	Reagent Chemical	Attribute
Instrument qualification /Calibration	√	√	√				Traceability & Accuracy
Routine calibration/ System suitability	√	√	√	√			Qualified standard (Primary or secondary)
Method validation	√	√	√	√			Accuracy, Precision, Bias
Identity	√	√	√	√	√	√	Authenticity
Content or assay	√	√	√	√			Qualified standard
Stability assessment	√	√	√	√	maybe		Qualified standard
Internal Quality Control	√	√	√	√	maybe		Qualified standard
Regulatory/ Accreditation	√	√	√	√			Qualified standard

Table 4. Fit for Purpose Guidance in Standard Selection