330076 VO Qualitätskontrollsysteme, Prinzipien von GLP (2007W)

NUTRITIONAL METROLOGY: IN THEORY AND PRACTICE (G.V. Iyengar)

Metrology is a specialized discipline related to the science of measurements and permits us to understand the intricacies governing the accuracy of an analytical pursuit. Measures introduced to improve precision of a measurement and accuracy of an analytical result increase the reliability of the finding. However, confidence in any analytical result depends upon the overall validity of the result that is expressed as the "uncertainty". Uncertainty means doubt about the validity of the result of a measurement. Therefore, the pursuit of identifying the accuracy of analytical results with a clear insight into the sources and nature of analytical errors to be able to reflect on the measurement uncertainties is an essential component of metrology.

Several institutions play key roles in promoting good metrological practices. For example, the International Standards Organization (ISO) sets "normative" standards, while institutions such as the International Bureau of Weights and Measures (BIPM), create, maintain and disseminate physical and chemical standards that are linked to the SI (International Standards expressed in French!)) units. In this context it has to be mentioned that in contrast to metrology in physical measurements (length, weight, volume, etc) where the measurement and calibration infrastructure is well established for most measurement areas already for some time, the metrology in chemistry received appropriate attention only in the last decade. There are ongoing efforts of international organizations and groups (BIPM, CCQM, IUPAC, CITAC, EURACHEM, etc.) to clarify issues on traceability of chemical measurement results, including definition of appropriate stated reference and selections of suitable calibration tools. It may be mentioned here that the issue of measurement uncertainty of chemical analytical results is still evolving. It is only now, that the current revision of the International Vocabulary of Basic and General Terms in Metrology (VIM) accounts also for chemical measurements.

A host of measurements are carried out under physical and chemical metrology with some specific segments focused on areas such as environmental and nutritional metrology. Chemical measurements in particular are carried out in almost every sphere of our daily life leading to important decisions related to health, commerce, environment and legal issues. When decisions arise based on analytical results, it is important to be assured of the analytical quality of the results. In this context, analytical chemists are required to diligently document the integrity of their analytical findings. This is accomplished by providing for traceability by benchmarking the measurement process to a common reference point (referred to as stated reference) such as a certified reference material (CRM) by reputable institutions (Institute of Reference Materials and Measurements (IRMM), a reference method or an SI unit. Further it is expected that the analyst evaluates all possible sources of errors during the sampling and measurement process for a meaningful prediction of the uncertainty of the measurement.

The BCR/EC document on Metrology in Chemistry and Biology has attempted to extend the concept of traceability practised in physical and chemical metrology to the measurements undertaken for different biological parameters. While acknowledging the complexity in defining chemical measurements in biological systems as complex, this document identifies traceability as the heart of the matter in measurement providing the basis for reliable measurements, which are comparable. Then, taking a functional point of view, the report focuses on comparability to be considered as the primary requirement and concludes that traceability is a tool to help achieve comparability.
Nutritional and environmental metrology in practice

Measurement processes are adopted in field studies under practical condit