Case study of the effectiveness of a distributed metrology infrastructure for chemical measurements: supporting measurements for the regulated content of proteins in wheat in Slovenia
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- Majcen, N. & Taylor, P. Accred Qual Assur (2004) 9: 485. doi:10.1007/s00769-004-0819-9
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This paper describes a demonstrator case study of how a distributed metrology institute can bring value to the measurement infrastructure. The example is taken from the food measurement area, where the Metrology Institute of the Republic of Slovenia (MIRS) has set up a collaboration with the Slovenian National Food Reference Laboratory (NFRL) to act as the holder of the reference standard for the measurement of the content of protein in wheat. The responsibilities of the different parties are described; examples of the joint activities are given. The benefits of the co-operation are described in detail.
KeywordsMetrological infrastructureMeasurements in chemistryQuality of food
Introduction: an example illustrating the operation and potential of a distributed national measurement infrastructure for chemical measurements
For clarity, the following descriptions are used:
The National measurement infrastructure is the collection of all measurement services providers (e.g. field laboratories, reference laboratories and the state-financed national metrology institute).
The National metrology institute in this concept is the central, cross-sectoral responsible body (by law) that identifies needs, sets up the strategy and supports development and research in areas which are of economic or social interest to the country. It co-ordinates, leads and manages the national measurement infrastructure. It employs measurement scientists; therefore has in-house measurement expertise (e.g. mass, volume, validation software and some chemistry measurements), but also relies to a large extent on capabilities outside its own institution.
Reference laboratories are holders of national or reference standards. These must fulfil certain criteria defined in national legislation and other associated documents (e.g. regarding expertise and quality) to apply for being authorised/designated by the metrology institute (by law) to act as such laboratories. They have a contract with the national metrology institute.
The measurement problem
Wheat contains several kinds of protein. As these proteins are large, nitrogen-containing molecules, the total nitrogen content reflects protein content. “Protein content” as such is therefore an operationally defined measurand (i.e. what is measured depends on how it is measured), and therefore an internationally agreed measurement procedure (e.g. EN or an ISO procedure) must be used to obtain equivalent results. ISO-1871 is such a procedure, which is based on the use of a wet chemical digestion and a titration based on the Kjeldahl reaction.
Field laboratories measure protein content in wheat by using near-infrared (NIR) spectrophotometers, which are calibrated by using several tens of different kinds of wheat samples, so as to span as wide a range of protein concentration as possible. As the NIR technique is very matrix-sensitive, it is important that samples for calibration are from the same geographical region as samples to be measured.
Measurement traceability must be established between results obtained by the wet chemical reference procedure and values obtained by the NIR procedure.
The role of different parties in the national measurement infrastructure
Measurements are performed by an independent accredited laboratory, which has 16 mobile laboratories located at different places in Slovenia. Farmers decide whether or not to sell their harvest on the spot, on the basis of a price calculated according to the measured parameters of the wheat.
The “holder of the reference standard” for this particular measurement is the National Food Reference Laboratory of Slovenia (NFRL), which is accredited for this kind of measurement and which is a part of the national distributed measurement infrastructure. The various tasks it does in this context, or will be realised in the near future, are:
Produces Slovenian calibration samples, as otherwise reference materials (RMs) would have to be imported from other geographical regions
Assigns the value to such calibration samples by using Kjeldahl titration and the standardised procedure
Interacts and interfaces efficiently with the field laboratory in the local language
Organises proficiency tests (PTs) during the harvest season, partly financed by MIRS
Participates in international inter-laboratory comparisons (ILCs; e.g. EUROMET)
Acts as an expert in international organisations, for example, the International Legal Metrology Organisation (OIML)
Co-operates as an expert in the preparation of legislation
The Metrology Institute of the Republic of Slovenia (MIRS):
Provides traceability for NFRL by purchasing internationally recognised CRMs (BCR-563; BCR-382) in this way ensuring traceability to SI
Provides traceability to NFRL for mass and volume, and performs the software validation
Enables and co-finances participation of NFRL to the international ILCs (e.g. EUROMET)
Involves an NFRL expert in the Technical Committee of the OIML; the MIRS employee performs the correspondence and interaction but also ensures that a competent expert (e.g. from NFRL) participates and represents Slovenia in these meetings
Invites NFRL experts to participate in preparation of national legislation
Trains NFRL staff in issues related to traceability, uncertainty and validation
Situation before the distributed system
Due to market and legal requirements, the field laboratories must be accredited. At the beginning, the field laboratory found this very difficult to achieve, and the information available was very scattered. The field laboratory had serious problems with properly establishing traceability and demonstrating its capabilities via PTs. Initially, it struggled on its own (e.g. giving samples for NIR calibration to be analysed by NFRL), which was quite expensive for them. NFRL was not at all keen to address the issue, as there were not enough samples per year to cover all the cost of research and development work linked to obtaining reliable measurements.
As a result of the field laboratory’s complaints and owing to the close interaction of MIRS with the reality of chemical measurements in Slovenia (e.g. interfacing at national workshops and seminars organised by MIRS), MIRS pro-actively addressed the core issues. It organised meetings where both the field laboratory and the NFRL were present. Requirements and common interests were identified and in a combined effort, resulting in a proper solution being worked out.
A successful partnership in which all parties gain
Such an arrangement that links all involved in the process of measurement (i.e. field laboratory, reference laboratory, metrology institute, accreditation body, ministry and standardisation body) in a systematic and transparent way proves to be most efficient. It uses all available resources in the country (human and financial) to the maximum by forming a virtual—but recognisable and operational—network which assures both parties, farmers and purchasers, get independent, equivalent and reliable measurement results. Within such an organised distributed system, NFRL, which already has established expertise in this specific field, will (on behalf of MIRS) be able to support field laboratories by providing knowledge, PTs and calibration; on the other hand, it is encouraged (by MIRS) to participate in inter-laboratory comparisons at the international level and to be actively involved in the preparation of national legislation and/or relevant documents at the international level (e.g. OIML).