This objective of this recent project supported by the IUPAC Analytical Chemistry Division is to prepare an international best practice guidance document for the SI-traceable value assignment of the purity (mass fraction content of the main component) of organic compounds intended to be used as primary standards and calibrators for establishing traceable measurement results in analytical chemistry.
Analytical chemistry laboratories are required to demonstrate the metrological traceability of their measurement results in order to operate in conformity with the requirements of ISO 17025. In the case of organic analysis, the metrological traceability to the SI of the result of an analysis for an organic compound is generally achieved through linkage to a pure primary calibrator material of the defined compound, the mass fraction purity of which has been established in a manner that is also traceable within a stated uncertainty to the SI. The primary calibrator material may be a Certified Reference Material (CRM), or when not available, a laboratory may choose to value assign a pure material with valid methods.
Agreement on what constitute valid methods and best practice to measure the content of the main component of a calibrator material within a stated measurement uncertainty is needed and not currently available. The purpose of this report is to describe the approach, performance and applicability of the main methods used to determine purity:
1. Mass balance
2. Thermal analysis
3. Assay techniques
4. Quantitative NMR
In addition, a statistical section will recommend how to combine data, deal with value assignment as the upper limit values corresponding to 100% purity are reached, including the use of asymmetric uncertainties, and how to report and propagate the standard uncertainty associated with the assigned values.
The resulting document will provide an international reference document and guidance for analytical laboratories, CRM manufacturers, and accreditors on valid methods for the assignment of purity of organic calibrator materials, thus providing the basis for metrological traceability of measurement results and their global metrological comparability.