Purity determination of reference materials is the cornerstone of metrological traceability, ensuring that certified values used for instrument calibration, method validation, and quality control are accurate and defensible. This article provides a comprehensive technical overview of purity testing samples, testing items, characterization methods such as the mass balance approach, quantitative NMR, and DSC purity analysis, in line with ISO 17034, ISO Guide 35, and the GB/T 15000 series of standards.
Technical Overview
Reference materials (RMs) and certified reference materials (CRMs) are measurement standards with sufficiently homogeneous and well-characterized property values used to calibrate instruments, validate analytical methods, assign values to other materials, and support metrological traceability. Among all certified properties, purity is the most fundamental: every quantitative result derived from a calibration curve or standard solution inherits any error embedded in the assigned purity value of the reference material.
Purity determination, therefore, refers to the systematic characterization of the amount-of-substance fraction or mass fraction of the main component in a candidate reference material, together with a rigorous evaluation of all impurity contributions and their associated measurement uncertainty. The internationally recognized framework for this work is defined by ISO 17034 (general requirements for the competence of reference material producers), ISO Guide 35 (guidance for characterization and assessment of homogeneity and stability), and, in the Chinese national system, the GB/T 15000 series of standards, which is technically aligned with the ISO guides.
A key conceptual point is the distinction between chromatographic purity (the area percentage obtained by a single chromatographic method, which only sees impurities detectable by that method) and absolute purity (the true mass fraction of the main component, accounting for all impurities including water, residual solvents, and inorganic residues). Only the latter, supported by a complete uncertainty budget, can be certified as a reference material property value.
Testing Samples
Organic Pure Substances
This category represents the majority of candidate reference materials submitted for purity certification and includes:
- Pharmaceutical active ingredients and their related impurities (degradation products, process impurities, isomers)
- Pesticide and veterinary drug reference substances (insecticides, herbicides, antibiotics, hormones)
- Environmental pollutants such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), dioxins, and volatile organic compounds (VOCs)
- Food additives, natural pigments, mycotoxins, and nutrition-related markers
- Petrochemical and industrial chemicals, monomers, and specialty solvents
Inorganic Substances and Salts
High-purity metals, metal oxides, inorganic salts, and gravimetric primary standards (such as substances used for titrimetric standardization) require characterization of metallic and non-metallic impurity profiles in addition to the main component assay.
Chiral and Stereochemically Defined Compounds
Enantiomerically pure substances used in pharmaceutical and agrochemical analysis require independent determination of both chemical purity and enantiomeric excess, since these two dimensions of purity are orthogonal.
Biomolecules
Amino acids, peptides, steroids, nucleotides, and other biomolecular standards demand special handling because they may contain bound water, counterions, and conformational or degradation-related impurities.
Testing Items
A complete purity certification program typically encompasses the following measurement items, which together allow the certified value and its uncertainty to be established:
- Identity confirmation — structural verification by mass spectrometry (MS), nuclear magnetic resonance spectroscopy (NMR), infrared spectroscopy (IR), and comparison of chromatographic retention behavior with authoritative references
- Organic impurity profile — related substances and organic impurities determined by HPLC-UV/DAD, LC-MS, GC-FID, or GC-MS, expressed as mass fractions
- Water content — determined by Karl Fischer titration (volumetric or coulometric) or thermogravimetric analysis
- Residual solvents — volatile organic impurities from synthesis and crystallization, quantified by static or dynamic headspace GC
- Inorganic impurities — non-volatile residue (ash), elemental impurities, and heavy metals determined by ICP-MS, ICP-OES, or XRF
- Enantiomeric purity — enantiomeric excess measured by chiral HPLC, chiral GC, or polarimetry where applicable
- Isomeric and isotopic composition — positional isomers or isotopic enrichment determined by NMR, IR, or isotope-ratio mass spectrometry
- Homogeneity assessment — between-bottle and within-bottle homogeneity testing under ISO Guide 35, using a precision method with adequate repeatability
- Stability assessment — short-term (transport) and long-term (storage) stability studies, often designed as classical or isochronous studies
- Measurement uncertainty of the certified value — a combined uncertainty budget incorporating characterization, homogeneity, and stability contributions
Testing Methods
Mass Balance Approach
The mass balance method is the most widely applied primary method for organic reference materials. Absolute purity is calculated by subtracting all quantified impurity contributions from 100%:
Purity (%) = 100% − (organic impurities) − (water content) − (residual solvents) − (inorganic residue)
- Organic impurities are quantified with response-factor-corrected chromatographic methods, ideally against isolated impurity standards or by mass-detector response factors
- Water is determined by coulometric or volumetric Karl Fischer titration
- Residual solvents are measured by headspace GC-FID or GC-MS
- Inorganic residue is obtained from sulfated ash testing, TGA, or elemental analysis
The strength of the mass balance approach lies in its traceability chain to the SI through individual impurity measurements, but its validity depends entirely on the completeness of the impurity screening.
Quantitative Nuclear Magnetic Resonance (qNMR)
qNMR has become a leading orthogonal method because the NMR response is directly proportional to the number of nuclei, making it a primary ratio method that does not require substance-specific response factors for each impurity. Using a certified internal standard with gravimetrically prepared sample solutions, qNMR can deliver absolute purity values with uncertainties typically below 1% (relative), provided that adequate relaxation delays, validated integration regions, and suitable solvents are used.
Differential Scanning Calorimetry (DSC) Purity Method
The DSC purity method exploits melting point depression described by the van't Hoff equation. By analyzing the shape of the melting endotherm, the mole-fraction purity of highly pure crystalline substances (typically above 98.5 mol%) can be determined without any impurity reference standards. The method is rapid and absolute, but it is applicable only to pure, crystalline, thermally stable compounds that do not decompose or form solid solutions.
Titrimetric and Coulometric Methods
For substances with well-defined stoichiometric reactions, potentiometric titration, coulometric titration, and gravimetric analysis provide classical high-accuracy assays. These methods underpin many primary chemical standards, delivering relative uncertainties at the 0.05% level when performed under metrologically controlled conditions.
Orthogonal Confirmation and Statistics
Best practice under ISO Guide 35 requires purity values to be established by at least one primary method or by an interlaboratory characterization study combining multiple independent methods. Disagreement between mass balance and qNMR results frequently reveals overlooked impurity classes, such as non-detectable structural analogues or bound water. The certified value, its expanded uncertainty (coverage factor k = 2), expiry date, and minimum sample intake are then documented on the certificate in accordance with ISO Guide 31.
Applications
Certified purity values of reference materials directly support measurement quality across nearly every regulated and industrial sector:
- Pharmaceutical analysis — assay of active pharmaceutical ingredients, quantification of related substances, system suitability testing, and establishment of secondary working standards traceable to pharmacopoeial primary standards
- Environmental monitoring — calibration of GC-MS, LC-MS, and ICP-MS systems for pollutant quantification in water, air, soil, and waste streams
- Food safety and nutrition — determination of pesticide and veterinary drug residues, food additives, contaminants, and nutritional labeling parameters
- Instrument calibration and method validation — linearity, accuracy, precision, limit of quantification, and recovery studies in accredited laboratories operating under ISO 17025
- Industrial and petrochemical quality control — process control, product specification release, and proficiency testing programs
- Regulatory compliance and trade — metrologically traceable results that are mutually recognized in conformity assessment and international trade
FAQ
What is the difference between chromatographic purity and absolute purity?
Chromatographic purity is the peak-area percentage observed by a specific chromatographic method and ignores impurities that are not eluted, not detected, or not resolved. Absolute purity is the true mass fraction of the main component determined by primary methods such as mass balance or qNMR, accounting for water, residual solvents, and inorganic impurities. Only absolute purity with a stated uncertainty is certifiable.
Which standards govern reference material purity certification?
ISO 17034 defines the competence requirements for reference material producers; ISO Guide 35 provides guidance on characterization, homogeneity, and stability assessment; ISO Guide 31 specifies certificate content; and the GB/T 15000 series is the corresponding Chinese national framework. Testing laboratories involved in characterization typically operate under ISO 17025.
Why must homogeneity and stability be studied in addition to purity?
The certified value applies to every bottle of the batch over its entire shelf life. Homogeneity testing quantifies bottle-to-bottle variation, while stability studies quantify degradation during storage and transport. Both contributions are combined with characterization uncertainty to produce the final expanded uncertainty of the certified purity value.
What purity level is required for a reference material?
There is no universal threshold, but organic CRMs are commonly certified at ≥ 99% mass fraction with fully characterized impurity profiles. For high-accuracy applications such as primary standards, purities above 99.5% to 99.9% with correspondingly smaller uncertainties are typically expected.
How often should reference materials be re-certified?
The monitoring period defined during stability studies determines the expiry date. Ongoing stability monitoring under ISO Guide 35 supports periodic re-verification or re-certification, and accredited third-party laboratories and national metrology institutes typically apply post-certification monitoring to confirm that the certified purity remains valid throughout the stated shelf life.
Can a single analytical method certify purity?
Generally, no. A single chromatographic method is blind to undetectable impurity classes. Credible purity certification requires either one primary method (qNMR, DSC, mass balance, or titrimetry) applied with full metrological rigor, or preferably the agreement of at least two orthogonal methods, with the uncertainty budget assembled according to the ISO/IEC Guide 98 series (GUM).