Accreditation and Quality Assurance (v.13, #10)

Certified reference materials for organic contaminants in sewage sludge: a feasibility study by Mikael Krysell; Roland Becker; Jan Willem Wegener; Hans-Gerhard Buge (553-562).
As an answer to the re-evaluation of the European sludge directive, two novel reference materials for organic components in municipal sludge have been produced and test certified for AOX (absorbable organic halogens), PAH (polycyclic aromatic hydrocarbons), NPE (nonylphenol and nonylphenolethoxylates), DEHP (di(2-ethylhexyl)phthalate), bisphenol A, and BFR (brominated flame retardants). The materials were prepared from raw sludge without spiking or mixing of different sludges. The study demonstrated that the technique for the preparation of a suitable reference material, sufficiently homogenous and stable, and with concentration levels that answer the needs of the laboratories and the relevant authorities, can now be presented in detail. The results, furthermore, show there is a need for method validation and standardisation of the measurements of NPEs and certain BFR congeners, and that the development of the laboratory structure in Europe, with fewer and more specialised laboratories, might become a major obstacle when trying to find a sufficient number of laboratories being appropriately proficient for this kind of study in the future.
Keywords: Organic pollutants; Municipal sludge; CRM; AOX; BPA; PAH; DEHP; NPE; BFR

Measurement uncertainty is widely recognized among physicists and chemists, but is a relatively new concept to many microbiologists. Generally, available documents about measurement uncertainty in microbiological testing are applicable to food and water microbiology. Two quantitative methods for evaluation of antimicrobial activity of textile materials are used commonly in textile laboratories. Methodology and expression of results for the two methods are similar; thus, calculation and expression of measurement uncertainty for results obtained by these test methods are also similar. This contribution describes the way in which measurement uncetainty for these methods can be evaluated and reported.
Keywords: Antimicrobial activity; Textile microbiology; Measurement uncertainty

Design and model of calibration for chemical measurements by L. Brüggemann; R. Wennrich (567-573).
The international standard ISO 11843 specifies basic methods to design experiments for estimation of critical values referring to the capability of detection. The detection capability depends on the experimental design, the calibration model used, and the errors of the measurement process. This study reports how the specification of the calibration points within the calibration range can be used as a-priori information for evaluation of calibration uncertainty without any consideration of the response variables of the calibration. As result of investigation of the experimental designs, calibration points within the calibration range can be omitted without significant changes of calibration uncertainty. The approach is demonstrated at a practical example, the determination of arsenic in surface water samples taken from a river in Germany.
Keywords: Linear calibration; Calibration design; Calibration model; Calibration uncertainty

On the accuracy of micro Winkler titration procedures: a case study by Lauri Jalukse; Irja Helm; Olev Saks; Ivo Leito (575-579).
Accuracy data (expressed as precision and trueness) presented by the authors of three different micro modifications of the Winkler titration procedure for dissolved oxygen concentration determination are critically evaluated. Tentative uncertainty estimates are extracted from the data based on the single-laboratory validation approach (originally published in the Nordtest Technical Report 537) and they lead to expanded uncertainty (k = 2) estimates in the range from 0.13 to 0.27 mg l−1 for the three procedures. It is demonstrated that, in all cases, the authors have presented the accuracy and/or precision estimates of the procedures in a way that can lead to too optimistic conclusions about the uncertainty of their procedures. This case study demonstrates the usefulness and flexibility of the single-laboratory validation approach to uncertainty estimation, even in the case of insufficient data, and can be of interest to laboratory workers dealing with measurement procedures from the literature. It is also expected to be of interest to university instructors of analytical chemistry and metrology in chemistry as a real-life example of the critical evaluation of the literature data.
Keywords: Winkler method; Titration; Dissolved oxygen; Measurement uncertainty; Nordtest method

The outcome of routine testing for instability and heterogeneity of materials used in proficiency tests tends to be weakly informative because of the unacceptable cost of obtaining enough data to support a suitably powerful statistical test. A better strategy would be to spend considerably more on a large initial validation of a type of material or a procedure for preparing it, and then to monitor all subsequent batches of material of that type by a much cheaper quality control system. The expense of a powerful validation would be quickly recouped by savings in the longer term.
Keywords: Validation; Quality control; Proficiency test; Reference material; Stability; Homogeneity; Statistical power

Absolute measurements of water density with very small uncertainties on the order of 0.001 kg/m3 have previously been a metrological challenge, as is shown by measurements of the density of pure water performed in recent decades with different methods. However, using water as a reference liquid with a well-known density, it is possible to perform density measurements relative to this reference liquid by means of an oscillation-type density meter. Using this so-called substitution method, it is possible to obtain uncertainties of about 0.002 kg/m3 or a relative uncertainty of 2 × 10−6. The conversion from relative to absolute measurements is performed using a water density table. The uncertainty of this absolute measurement is given by the combination of the uncertainty of the relative measurement and the uncertainty given for the density table.
Keywords: Water density; Hydrostatic measurement; Magnetic flotation method; Oscillation-type density meter; Substitution method

The SCOR/IAPSO Working Group 127 was founded in 2005 and charged with the development of a new, comprehensive, highly accurate, and consistent standard formulation of seawater thermodynamics for oceanography. This task was approached in cooperation with the International Association for the Properties of Water and Steam (IAPWS) for a joint standard with industrial applications. In addition to the available standard for fluid water, IAPWS-95, new formulations for ice, IAPWS-06, and the thermodynamics of seawater, IAPWS-08, have been developed, supplemented by a redefinition of salinity, the Reference-Composition Salinity Scale 2008. In this paper the starting situation is described, the requirements to be met during the development process are studied, and the properties of the final formulations are briefly characterized.
Keywords: Thermodynamics; Seawater; Water; Vapor; Ice; Salinity; Seasalt

Measurements of practical salinity are based on conductivity measurements, which are traceable to the conductivity of a defined KCl reference solution. The conductivity of this reference solution must be traceable to the SI in order to state a reliable uncertainty for practical salinity measurements and to guarantee their comparability. Currently, the conductivity ratios from various old standard seawater batches are compared instead. However this method is inadequate on a long time-scale, which is important for oceanography studies, because it lacks a time-invariant reference. Therefore the equivalence of conductivity measurements traceable to primary conductivity standards has been investigated in Euromet project 918. Several European metrology institutes measured the conductivity of a 5 S/m KCl solution and the practical salinity of IAPSO standard seawater. The study estimates relative standard uncertainties to be of the order of 10−4 to 10−3 for such measurements.
Keywords: Practical salinity; Seawater; Conductivity; Traceability

The measurement of temperatures accompanies almost every determination of physical quantities or material properties. This paper gives an outline of the concept of the traceability of temperature measurements according to the International Temperature Scale of 1990 (ITS-90) and the determination of measurement uncertainties. Furthermore, differences between ITS-90 and thermodynamic temperatures are discussed.
Keywords: Uncertainty; Temperature measurements; ITS-90