Two bottles sit on the same shelf. Both are labelled with the same compound name, both assay at ninety-nine per cent or better, both come with paperwork. One can be used to calibrate a method whose result will end up in a regulatory submission. The other cannot. The difference between a reference standard and a reagent is not grade, price or purity figure; it is what the accompanying documentation actually asserts, and who stands behind that assertion.
This guide maps the vocabulary that governs that difference: reference material and certified reference material as defined in international metrology, metrological traceability and the calibration chain, measurement uncertainty and coverage factors, what the word primary means when a metrologist uses it, what ISO 17034 is for, and why a laboratory would deliberately buy a small quantity of an impurity.
What is the difference between a reference standard and a reagent?
Start with the definition that most other definitions hang from. The International Vocabulary of Metrology, published as JCGM 200:2012 and freely downloadable from the BIPM, defines a reference material at entry 5.13 as a material, sufficiently homogeneous and stable with reference to specified properties, which has been established to be fit for its intended use in measurement or in examination of nominal properties.
Three conditions are doing the work there: homogeneity, stability, and demonstrated fitness for a stated purpose. A reagent is bought for what it does in a flask. A reference material is bought for what it lets an analyst claim about a number. Purity is necessary for the second role and insufficient on its own, because a value without a documented basis cannot be defended.
The same vocabulary adds a note that laboratories routinely trip over: reference materials with or without assigned quantity values can be used for measurement precision control, whereas only reference materials with assigned quantity values can be used for calibration or for measurement trueness control. A material with no assigned value can still tell an analyst whether a method is repeatable. It cannot tell them whether the method is right.
| Material | What the paperwork asserts | What it supports |
|---|---|---|
| Reagent or technical grade | Identity and a purity figure from the producer, by the producer’s own method | Synthesis, cleaning, non-critical preparation |
| Reference material (RM) | Homogeneity and stability for specified properties, fitness for an intended use | Precision control, system suitability, qualitative identity |
| Certified reference material (CRM) | One or more property values with associated uncertainties and traceabilities, in documentation issued by an authoritative body using valid procedures | Calibration, trueness control, method validation, disputed results |
What a certified reference material adds that a reference material does not
JCGM 200:2012 defines a certified reference material at entry 5.14 as a reference material, accompanied by documentation issued by an authoritative body and providing one or more specified property values with associated uncertainties and traceabilities, using valid procedures. The vocabulary notes that this documentation is given in the form of a certificate.
Every clause of that definition is a requirement, and each one is a question worth asking a supplier. Which property value is certified, and is it the one being used? What is the stated uncertainty on it? To what is that value traceable? Which body issued the document, and on what basis is it authoritative? A certificate of analysis that reports a chromatographic purity figure with no uncertainty and no traceability statement is a test report. It is a useful document, but it is not a certificate in this sense, and treating the two as equivalent is the single most common category error in this area.
Scale helps here. The National Institute of Standards and Technology states that it certifies and provides over 1200 Standard Reference Materials with well-characterised composition or properties, or both. That is the size of the top of the pyramid for one national metrology institute, across every measurement field, which tells an analyst something useful: for most specific organic analytes, no national-institute CRM exists, and the practical question becomes how well the available material is characterised and documented.
What metrological traceability means in chemical analysis
Traceability is the property that connects a laboratory result to something outside the laboratory. JCGM 200:2012 entry 2.41 defines metrological traceability as a 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. The notes add that the reference can be a measurement unit realised in practice, a measurement procedure, or a measurement standard, and that traceability requires an established calibration hierarchy.
Entry 2.40 defines that hierarchy as a sequence of calibrations from a reference to the final measuring system, where the outcome of each calibration depends on the outcome of the previous one. Its first note is the sentence every purchaser of standards should have memorised:
Measurement uncertainty necessarily increases along the sequence of calibrations.
Uncertainty grows monotonically down the chain. It never shrinks. A working standard calibrated against a secondary standard calibrated against a national standard carries all three contributions. This is why a traceability claim with no accompanying uncertainty is not a claim at all, and why a short, documented chain is worth more than an impressive-sounding but undocumented one.
What is the difference between primary and secondary reference standards?
In metrology, primary is a technical term rather than a marketing adjective. JCGM 200:2012 entry 5.4 defines a primary measurement standard as a measurement standard established using a primary reference measurement procedure, or created as an artefact chosen by convention. Entry 2.8 defines a primary reference measurement procedure as a reference measurement procedure used to obtain a measurement result without relation to a measurement standard for a quantity of the same kind.
Read that last phrase slowly, because it is the whole distinction. A primary procedure does not need a standard of the same quantity to produce its value; it stands on independently measured quantities. The vocabulary’s own example is a primary standard of amount-of-substance concentration prepared by dissolving a known amount of substance of a chemical component into a known volume of solution. Mass and volume are measured; concentration is derived.
A secondary standard, by contrast, gets its value by comparison against another standard of the same kind. Nothing is wrong with that, and most laboratories work with secondary and working standards every day. The point is that the word primary carries a defined meaning, and when a supplier uses it, the fair question is which definition is meant and what procedure assigned the value.
What ISO 17034 covers, described from secondary sources
A direct disclosure is owed here. ISO standards are sold, not published openly, and the text of ISO 17034:2016 was not opened while writing this guide. What follows is described from a peer-reviewed paper in Accreditation and Quality Assurance written by authors involved in the standard’s development. No provision of the standard is quoted, and nothing here should be treated as a substitute for the standard itself.
According to that paper, ISO 17034:2016 sets out general requirements for the competence of reference material producers and supersedes ISO Guide 34:2009, which had been developed by the ISO Committee on Reference Materials, known as REMCO. The move from a guide to a standard was driven by a practical problem: several accreditation bodies could not accredit against a guide at all, while in other countries ISO Guide 34 could serve as an accreditation standard only in combination with ISO/IEC 17025. The transformation was carried out by a joint working group of REMCO and the ISO Committee on Conformity Assessment, CASCO, which aligned the structure with the rest of the ISO/IEC 17000 series. The paper states that requirements for producing reference materials were elaborated more clearly, specifying requirements for non-certified reference materials and additional requirements for certified ones.
The operative consequence for a purchaser is narrow and worth stating exactly. Accreditation to ISO 17034 is a statement about a producer’s competence and consistency of operation, assessed by a third party. It is not a per-batch guarantee, and it does not replace reading the certificate for the specific lot in hand. A supplier’s own terms, including ours, describe conditions of supply; they are not a conformity assessment.
What coverage factor k equal to 2 means on a certificate
Uncertainty on a certificate is usually reported as an expanded uncertainty. The Guide to the Expression of Uncertainty in Measurement, JCGM 100:2008, describes this in clause 3.3.7: an expanded uncertainty is obtained by multiplying the combined standard uncertainty by a coverage factor, with the intended purpose of providing an interval expected to encompass a large fraction of the distribution of values that could reasonably be attributed to the measurand. The guide states that the coverage factor is usually in the range 2 to 3 and that it is always to be stated.
On the common case, the guide is precise. Where the distribution characterised by the result and its combined standard uncertainty is approximately normal and the effective degrees of freedom are of significant size, one can assume that a coverage factor of 2 produces an interval having a level of confidence of approximately 95 per cent, and a coverage factor of 3 gives approximately 99 per cent. Two conditions are attached to that sentence. A certificate that reports an interval without naming the coverage factor has left out information the guide considers mandatory to state.
Why laboratories buy impurity reference standards
Buying a milligram quantity of a compound nobody wants in their product looks eccentric until the regulatory arithmetic is on the table. The ICH guideline Q3A(R2) on impurities in new drug substances, current step 4 version dated 25 October 2006, sets three thresholds that scale with the maximum daily dose of the drug substance.
| Maximum daily dose | Reporting threshold | Identification threshold | Qualification threshold |
|---|---|---|---|
| At or below two grams per day | 0.05 per cent | 0.10 per cent, or 1.0 mg per day intake, whichever is lower | 0.15 per cent, or 1.0 mg per day intake, whichever is lower |
| Above two grams per day | 0.03 per cent | 0.05 per cent | 0.05 per cent |
The guideline defines an identification threshold as a limit above which an impurity should be identified, and a qualification threshold as a limit above which an impurity should be qualified, where qualification means acquiring and evaluating data that establishes the biological safety of that impurity or impurity profile at the specified level. It also states that the quantitation limit of the analytical procedure should be not more than the reporting threshold. Crossing a line at the level of a tenth of a per cent therefore triggers structural work, and crossing the next line triggers safety work.
Q3A(R2) is explicit about the role of standards in this. Organic impurity levels can be measured by techniques that compare the analytical response for an impurity either to an appropriate reference standard or to the response of the drug substance itself, and reference standards used for impurity control should be evaluated and characterised according to their intended uses. The guideline permits using the drug substance as the standard, but adds the condition that matters: where the response factors of the drug substance and the impurity are not close, the practice can still be appropriate provided a correction factor is applied, or the impurities are in fact being overestimated.
That is the entire commercial logic of impurity standards in one paragraph. Without the isolated impurity, an analyst is assuming equivalent detector response. With it, the response factor is measured rather than assumed, and the assumption disappears from the submission.
How purity is assigned, and why area percent is not mass fraction
A purity figure is an outcome of a method, not a property of a bottle. Chromatographic area normalisation reports the fraction of detected signal attributable to the main peak, which is a different quantity from the mass fraction of the compound in the material. Anything the detector does not see is missing from the denominator.
Quantitative nuclear magnetic resonance is the usual orthogonal answer. A 2014 review in the Journal of Medicinal Chemistry argues that a truthful description of chemical constitution requires coverage of both structure and purity, and that compared with chromatography and elemental analysis, quantitative NMR uses nearly universal detection and provides a versatile and orthogonal means of purity evaluation. The authors note specifically that absolute quantitative NMR with flexible calibration captures analytes that frequently escape detection, naming water and sorbents, and that the workflow gives simultaneous qualitative and quantitative capability.
The practical reading: two methods answering the same question by different physics is how a purity figure earns confidence. One chromatographic number on its own, however impressive the decimal places, cannot tell the difference between a pure compound and a compound accompanied by something the detector is blind to.
Frequently asked questions
What is the difference between a reference standard and a reagent?
A reagent is characterised for what it does in a reaction; a reference standard is characterised so that a measured value can be defended. International metrology defines a reference material as sufficiently homogeneous and stable with reference to specified properties and established as fit for its intended use in measurement. A purity figure alone does not meet that definition.
What does a certified reference material provide that a reference material does not?
Documentation issued by an authoritative body, giving one or more specified property values with associated uncertainties and traceabilities, produced using valid procedures. That is the definition in JCGM 200:2012. A certificate of analysis reporting a purity figure without an uncertainty and without a traceability statement is a test report rather than a certification in this sense.
Why does measurement uncertainty increase along a calibration chain?
Because each calibration in the hierarchy depends on the outcome of the previous one, and each contributes its own uncertainty component. The International Vocabulary of Metrology states directly that measurement uncertainty necessarily increases along the sequence of calibrations. Uncertainty can therefore never be smaller at a working standard than at the reference it descends from.
What does a coverage factor of k equal 2 mean on a certificate?
It means the reported expanded uncertainty is twice the combined standard uncertainty. Where the distribution is approximately normal and the effective degrees of freedom are of significant size, the guide on uncertainty states that this yields an interval with a level of confidence of approximately 95 per cent. The coverage factor is always to be stated on the certificate.
Is ISO 17034 accreditation a guarantee of the material in the bottle?
No. It is a third-party assessment of the producer’s competence and consistent operation as a reference material producer. Described from secondary literature, ISO 17034:2016 superseded ISO Guide 34:2009 and specifies requirements for reference materials with additional requirements for certified ones. The certificate for the specific lot still has to be read.
Why would a laboratory buy an impurity reference standard?
To measure a response factor instead of assuming it. ICH Q3A(R2) allows an impurity to be estimated against the drug substance itself, but requires a correction factor where the response factors are not close. Identification and qualification thresholds begin at a tenth of a per cent for many products, so a misassumed response factor can move a result across a regulatory line.
Does 99.9 per cent by HPLC mean 99.9 per cent by mass?
No. Chromatographic area normalisation describes the detected signal, so anything the detector cannot see, such as water, residual solvent or inorganic residue, is absent from the calculation. Quantitative NMR is frequently used as an orthogonal check because it offers nearly universal detection and, with absolute calibration, captures species that escape chromatographic detection.
What does primary mean in primary reference standard?
In metrology it means the value was established using a primary reference measurement procedure, defined as one that obtains a result without relation to a measurement standard for a quantity of the same kind, or that the standard is an artefact chosen by convention. A secondary standard takes its value by comparison against another standard of the same quantity.
References
- JCGM 200:2012, International Vocabulary of Metrology (VIM), 3rd edition. BIPM Joint Committee for Guides in Metrology
- JCGM 100:2008, Evaluation of measurement data: Guide to the Expression of Uncertainty in Measurement. BIPM Joint Committee for Guides in Metrology
- Trapmann S, Botha A, Linsinger TPJ, Mac Curtain S, Emons H. The new International Standard ISO 17034: general requirements for the competence of reference material producers. Accreditation and Quality Assurance, 2017;22:381-387
- ICH Harmonised Tripartite Guideline Q3A(R2), Impurities in New Drug Substances, step 4 version dated 25 October 2006
- Pauli GF, Chen SN, Simmler C, Lankin DC, Godecke T, Jaki BU. Importance of Purity Evaluation and the Potential of Quantitative 1H NMR as a Purity Assay. Journal of Medicinal Chemistry, 2014;57:9220-9231
- Standard Reference Materials. National Institute of Standards and Technology
Research use only. Nonsensia Lab supplies analytical reference standards for laboratory and research applications. This article is published for scientific and educational purposes. It is not medical advice, it does not describe any use in humans, and nothing in it should be read as a recommendation to administer any substance to a person or animal.
Filed under: Reference Standards
Nonsensia Lab supplies the compounds discussed in this guide as analytical reference standards for laboratory and research use.