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Accreditation and Quality Assurance: Journal for Quality, Comparability and Reliability in Chemical Measurement (v.12, #2)
Keeping the measurement units independent from arbitrary decisions (of worldly powers)
by Paul De Bièvre (pp. 57-58).
A comparison of performance statistics for proficiency testing programmes by Siu Kay Wong (pp. 59-66).
Recently, ISO issued a new document on statistical methods for use in proficiency-testing schemes. The document presented a number of new performance statistics that involve the use of measurement uncertainties of the participants’ results and/or the assigned values for the performance evaluation. Making use of the Monte Carlo simulation technique, this paper attempts to compare selected performance statistics of different approaches. In addition, the simulation programme was extended to study the effect of multi-modality in participants’ results on the performance evaluation when different performance statistics were applied.
Keywords: Proficiency testing; Performance statistics; Monte Carlo simulation; Measurement uncertainty
Modifying uncertainty from sampling to achieve fitness for purpose: a case study on nitrate in lettuce by Jennifer A. Lyn; Ilaria M. Palestra; Michael H. Ramsey; Andrew P. Damant; Roger Wood (pp. 67-74).
Existing methods have been applied to estimate the uncertainty of measurement, caused by both sampling and analysis, and fitness-for-purpose of these measurements. A new approach has been taken to modify the measurement uncertainty by changing the contribution made by the sampling process. A case study on nitrate in lettuce has been used to demonstrate the applicability of this new generic approach. The sampling theory of Gy was used to predict the alterations in the sampling protocol required to achieve the necessary change in sampling uncertainty. An experimental application of this altered sampling protocol demonstrated that the predicted change in sampling uncertainty was achieved in practice. For the lettuce case study, this approach showed that composite samples containing 40 heads, rather than the usual ten heads, produced measurements of nitrate that where more fit-for-purpose.
Comparison of shelf-life appraisal techniques for the estimation of the limiting date of stability for agrifood reference materials by Julien A. Sarembaud; Max Feinberg (pp. 75-83).
Reference materials play a major role for the verification of the quality control of biological and chemical analysis. Due to the poor availability of certified reference materials, external reference materials (ERM) are intensively developing in complement and as a consequence of proficiency testing schemes (PTS). However, the use of any reference material must be time-limited, whereas the assigned reference value may change with time and it is under the responsibility of the material producer to indicate the date until this can be used for calibration purpose with some guarantee. This article compares different methods used to determine the limiting date of stability (LDS) in food and food-related reference materials, using some specific analytes (moisture, protein and oil contents) as stability-monitoring analytes. LDS is defined as the moment when the assigned values of one or several analytes can no longer be guaranteed by the producer. For most of the ERMs used as examples (i.e., rapeseed, bread wheat, durum wheat) the calculated periods of stability were longer than the present storage times used in BIPEA. However, for sunflower, the rapid degradation of the monitoring analytes indicates a shorter time than forecast.
Keywords: External reference material; Stability study; Agrifood; Moisture content; Limiting date of stability
The role of different soil sample digestion methods on trace elements analysis: a comparison of ICP-MS and INAA measurement results by Stefania Gaudino; Chiara Galas; Maria Belli; Sabrina Barbizzi; Paolo de Zorzi; Radojko Jaćimović; Zvonka Jeran; Alessandra Pati; Umberto Sansone (pp. 84-93).
The measurement of trace-element concentration in soil, sediment and waste, is generally a combination of a digestion procedure for dissolution of elements and a subsequent measurement of the dissolved elements. “Partial” and “total” digestion methods can be used in environmental monitoring activities. To compare measurement results obtained by different methods, it is crucial to determine and to maintain control of the bias of the results obtained by these methods. In this paper, ICP-MS results obtained after matrix digestion with modified aqua regia (HCl+HNO3+H2O2) method and two “total” digestion methods (microwave aqua regia+HF and HNO3+HF) are compared with those obtained by instrumental neutron activation analysis, a non-destructive analytical method for the determination of the total
Keywords: mass concentrations of inorganic components in environmental matrices. The comparison was carried out on eight agricultural soil samples collected in one test area and measured by k0-INAA and ICP-MS to determine As, Co, Cr, Sb and Zn mass concentration. The bias of results for As, Cd, Co, Cr, Cu, Ni, Pb, Sb and Zn of the three digestion methods were assessed using selected measurement standards. This paper highlights that the digestion procedure is an integral part of the measurement and can affect the measurement result in environmental analysis.
Validation of SPME-GC and HS-GC procedures for the determination of selected solvent residues in edible oil matrices by Magdalena Michulec; Waldemar Wardencki (pp. 94-104).
The validation process – in accordance with the recommendation of the International Conference on Harmonization – was performed in order to define and determine the application of the developed procedures for the determination of solvent residues (hexane, benzene, toluene and chlorinated hydrocarbons – trichloromethane, 1,1,1-trichloroethane, tetrachloromethane, trichloroethene, tetrachloroethene) in oil samples. For extraction and preconcentration of analytes, two simple sample preparation techniques – static headspace analysis (HSA) and solid phase microextraction (SPME) – have been used. Gas chromatography with a flame ionization detector (FID) and electron capture detector (ECD) was applied for the final determination. A critical comparison of developed procedures was conducted considering the values of: limits of detection, concentration ranges, repeatability and uncertainty. The linearity issue was described in details due to the broad measurement ranges of the proposed procedures.
Keywords: Validation parameters; Gas chromatography; Solid phase microextraction; Headspace analysis; Procedure development
IMEP-12: trace elements in water; objective evaluation of the performance of the laboratories when measuring quality parameters prescribed in the European Directive 98/83/EC by Ioannis Papadakis; Lutgart Van Nevel; Caroline Harper; Yetunde Aregbe; Philip D. P. Taylor (pp. 105-111).
The International Measurement Evaluation Programme (IMEP) is an interlaboratory comparison scheme, founded, owned and co-ordinated by the Institute for Reference Materials and Measurements (IRMM) since 1988. IMEP-12, for the fourth time in the series, focused on trace elements in water and it was designed specifically to support European Commission directive 98/83/EC. Reference values for the concentration of ten elements were established with expanded uncertainties according to GUM. In total, 348 laboratories from 46 countries in five continents participated in the comparison and the degree of equivalence between the results of the laboratories and the reference values is presented graphically. Samples from the same batch were distributed to ten laboratories from European Countries, which represented their country in the framework of the EUROMET project 528. Participation in this comparison was offered to the European Co-operation for Accreditation (EA) for participation of accredited laboratories from all over Europe in the framework of the collaboration between IRMM and EA and to laboratories from the EU new member states and acceding countries in the frame of IRMM’s ‘Metrology in Chemistry support program for EU new member states and acceding countries.’
Keywords: Interlaboratory comparison; Uncertainty; Traceability; Trace elements; Water
CITAC nomination of the most interesting/important papers on metrology in chemistry
by Ilya Kuselman (pp. 112-112).