Certified reference materials
Certified reference materials (CRMs) are 'controls' or standards used to check the quality and metrological traceability of products, to validate analytical measurement methods, or for the calibration of instruments.
[2] Since most analytical instrumentation is comparative, it requires a sample of known composition (reference material) for accurate calibration.
These reference materials are produced under stringent manufacturing procedures and differ from laboratory reagents in their certification and the traceability of the data provided.
Pure standards are most likely to be prepared by chemical synthesis and purification and characterized by determination of remaining impurities.
Due to the difficulty in production and value assignment, these are usually produced by national or transnational metrology institutes like NIST (USA), BAM (Germany), KRISS (Korea) and EC JRC ( European Commission Joint Research Centre).
[21]: 119–124 For example, stabilizing agents such as antioxidants or antimicrobial agents may be added to prevent degradation, liquids containing certified concentrations of trace metals may have pH adjusted to keep metals in solution, and clinical reference materials may be freeze-dried for long term storage if they can be reconstituted successfully.
[22] This approach is not taken in ISO Guide 35:2017; rather, emphasis is placed on deciding whether the between-unit standard deviation is sufficiently small for the intended end use.
[17] In extreme cases, such as microanalysis, materials must be checked for homogeneity on sub-micron scales; this may involve much larger numbers of observations and adjustments to statistical analysis.
Accelerated studies use a range of more stringent conditions, most commonly increased temperature, to test whether the material is likely to be stable over longer time scales.
Real-time stability studies simply hold a set of RM units at a proposed storage temperature and test a proportion of them at regular intervals.
The results are usually assessed by inspection and by linear regression to determine whether there is a significant change in measured value over time.
For some applications, accelerated studies have been described as the only practical approach:[27] In the absence of a reference method or a higher order standard, ... accelerated studies under stress conditions provide the only approach for assessment of stabilityThe principal disadvantage of accelerated studies is that reference materials, like any other material, can degrade for unexpected reasons over time, or can degrade following different kinetic models; predictions can then become unreliable.
