The results of HbA1c measurement and its comparison with reference method values in an EQA programme

This article describes the effect of a change in reference method for HbA_1c measurement on results in our EQA programme. Important indicators of analytical quality, particularly reproducibility, bias and uncertainty, were assessed. Basic analytical quality indicators for the measurement of HbA_1c in an EQA survey during 2000–2008 were evaluated. Three EQA surveys per year were performed. Usually there were more than 200 participating laboratories in each survey. Reproducibility, systematic differences, bias, and methodology changes before and after IFCC reference method implementation were determined. In the EQA surveys, target values traceable to the IFCC reference method are used. Estimation of the combined uncertainty of the result was a voluntary part of the surveys and was reported by 50% of participating laboratories. Reproducibility of all participants’ measurements was close to the target value of CV = 5.0% (coefficient of variation). The group of participants using HPLC methodology had reproducibility lower than 5.0% in each survey, whilst the immunochemistry group usually had CV values more than 6.0%. The differences in individual results from target values ranged in the interval ±12% for HPLC methods but more (−8 to +30%) for immunochemistry methods. During the monitored time interval the number of participants using HPLC methods increased up to 70% whilst the number of participants using immunochemistry methods decreased approximately to 30%. The combined uncertainty (u _c ) reported by participants varies between 4.0 and 4.5% for HbA_1c values between 30 and 90 mmol/mol. No significant differences between precision and systematic errors (bias) after the implementation of IFCC method of measurement for HbA_1c to routine processes for HPLC were observed. Immunochemical methods do not fulfil requirements for analytical reproducibility.     

Assessment of the determination of heavy metals in organic soil improvers by ICP–OES

Directive 86/278/EEC sets maximum levels of heavy metals in sewage sludge used in agriculture to regulate the risk of using these organic soil improvers. The assessment of the compliance of soil improvers with this legislation should be supported on measurements with adequately low uncertainty. This work presents a strategy for assessing the performance of the determination of aqua regia extractable Cu, Zn and Cr in organic soil improvers and urban sewage sludges following EN 13650 standard. The measurement procedure validation involves checking the adequacy of the linear weighted regression model for ICP–OES calibration, the determination of the limit of quantification, the assessment of measurement repeatability, intermediate precision and trueness, and the evaluation of the measurement uncertainty using the differential approach. Routine tests quality control, including the estimated measurement uncertainty, is checked through the analysis of control standards, equivalent to calibrators, and reference materials from proficiency tests. Since the estimated relative expanded uncertainty is smaller than the defined target value (40 %), measurements are fit for assessing compliance of the mass fraction of heavy metals in organic soil improvers with the council directive.     

Reliability of measurement results in food microbiology: the contribution of reference laboratories in the European Union and of international/European standardization

The contribution of reference laboratories in the European Union and of European/international standardization to the reliability of food microbiology measurement results is discussed. A set of European Union reference laboratories has been established. Each of them coordinates a network of national reference laboratories which, in turn, coordinate networks of laboratories in charge of official testing and sometimes own checks in each European Union country. Their contribution to the reliability of food microbiology measurement results is illustrated by three food safety cases: Listeria monocytogenes, coagulase positive staphylococci and milk/milk products. The contribution of European/international standardization focuses on two topics: method validation and measurement uncertainty. The standards covering these topics—EN ISO 16140 and ISO/TS 19036—are briefly discussed, and an update given on their ongoing revision.     

Effect of insignificant bias and its uncertainty on the coverage probability of uncertainty intervals Part 1. Evaluation for a given value of the true bias

This paper investigates the coverage probability of the uncertainty intervals determined in compliance with the GUM and EURACHEM Guide, which are defined by expanded uncertainty U about the results uncorrected with the insignificant biases and corrected with the significant biases. This coverage probability can significantly fall below the chosen level of confidence in some cases as Maroto et al. discovered by using the Monte Carlo method. Their numerical results obtained provided that only the β errors have occurred in the test significance and findings that the coverage reduction depends on the mutual proportions of the magnitudes of the systematic error, overall uncertainty and bias uncertainty are confirmed in this paper by using probability calculus and numerical integration. This problem is also studied when all possible experimental biases, both significant and insignificant, are considered. From this point of view, the reduction of the coverage probability turns out to be less severe than from the previous one. The coverage probability is also investigated for some uncertainty intervals computed in different ways than the above mentioned documents recommend. The intervals defined by U about the results corrected with both significant and insignificant bias give always the same coverage probability equalling the chosen level of confidence. The intervals with some uncertainties modified or enlarged with the insignificant biases remove or moderate the coverage reduction.     

Proposals for new data quality objectives to underpin ambient air quality monitoring networks

The data completeness requirements for the production of defensible annual averages of air quality parameters are discussed. Data quality objectives concerning time coverage and data capture specified by current legislation are critically examined, and the improved, combined metric of data coverage is proposed. Best practice proposals to ensure the representativeness of air quality data across the calendar year are presented. These are based on minimising any bias between the annual average calculated using the data acquired and the annual average that would have been calculated if data coverage had been 100 %. In all cases, the optimum solution is for air quality network operators to aim for 100 % data coverage.     

An alternative method by pressurized DSC to evaluate biodiesel antioxidants efficiency

The purpose of this article is to present an alternative method developed using pressurized differential scanning calorimetry (P-DSC) to evaluate the oxidative behavior of biodiesel obtained from soybean oil, produced by ethylic route (BSET), with and without the addition of synthetic antioxidants, as well as to compare the results with those obtained by Rancimat Method, which were carried out according to the European Official Standard Method EN 14112. BSET samples were analyzed using four different types of synthetic antioxidants at concentrations varying between 250, 500, and 1000 mg kg⁻¹. The measurements done by P-DSC were performed using static air at 80 psi and isothermal conditions at 110 °C, the same operating temperature than the Rancimat method. The experimental results of the oxidative stability measured from both methods have shown good correlations, which depend on the antioxidant, showing a gradual improvement in the oxidative stability of BSET with increasing added concentration of antioxidants. Opposite result behavior occurred for the measurements done in the presence of one of the antioxidants, which was composed by a significant content of components volatile below 110 °C, which accelerate the induction of the biodiesel oxidation in P-DSC method, but apparently do not interfere in the electrical conductivity measurement method (Rancimat). The advantages of the developed method using P-DSC technique with respect to Rancimat method is that it allows one to use a smaller sample, increases sensitivity to antioxidant action, reduces the analysis time for the evaluation of the antioxidant efficiency and detects any occurring oxidative process, independently of the volatility of the formed products, which may form oxidated products that remain in the liquid phase and that are non detectable by Rancimat method.     

Comparative study of biodiesel oxidation stability using Rancimat,PetroOXY, and low P-DSC

Oxidation stability of biodiesel is one of the most important factors used to evaluate its quality, allowing its commercialization and ensuring its shelf life. In this context, several accelerated methods have been used to measure oxidative stability to predict the maximum storage time at which biodiesel can be submitted without compromising its quality. In the present study, the oxidation stability of two commercial biodiesel and of their blends with two antioxidants was evaluated using the standard methods described in the EN 14214 norm (Rancimat Method) and in ASTM D7545 (PetroOXY method), as well as by low pressurized differential scanning calorimetry (low P-DSC). Both biodiesel were obtained from soybean oil, produced by methylic and ethylic route. The antioxidants, which were used in different concentrations, have the following active components: bisphenol and phenol. The three assessed methodologies can be used to determine the effect of the commercial antioxidants on the oxidative stability of the studied biodiesel. As each method is based on the measurement of different parameters to obtain the respective oxidation induction time, their results cannot be compared directly. But when the results are expressed in terms of the percentage change of the respective oxidation induction times, there is a higher correlation between those obtained by Rancimat and PetroOXY methods than when compared with the results obtained by low P-DSC. Because of their characteristics, the bisphenolic antioxidant is more effective than the phenolic one and, at a same antioxidant concentration, the oxidation stability of the ethylic biodiesel is higher than that of the methylic one. Considering analysis time and sensitivity, the low P-DSC method is the one that better attends both parameters among the applied methods.     

The certification of a new paint reference material

The use of paint as a coating for toys intended for sale on the European market is controlled by a European Standard (EN 71-3: 1994-European Standard for the Safety of Toys, Part 3—Migration of Elements). This work is the result of a preliminary study organised by the European Commission to produce a new paint reference material which can be used to validate the test methods given in the European Standard. The migration of elements from painted panels and comminuted paint produced by the Laboratory of the Government Chemist, (U.K.) was studied. Several methods were used in this certification exercise and this work reports the recults of neutron activation analysis and inductively coupled plasma mass spectrometry to determine the migration of As, Ba, Cd, Cr, Hg, Pb, Se and Sb into a simulated stomach environment. Replicate extracts obtained from paint samples gave a precision of within 10% for most elements by both analytical techniques for the paint panels and for the comminuted paint sample. This preliminary study has shown that the standard method can provide reproducible results for each of the paint materials studied and indicates that the study should be continued to produce a fully certified paint reference material.     

Determination of metal components in marine sediments using energy-dispersive X-ray fluorescence (ED-XRF) spectrometry

A rapid energy-dispersive X-ray fluorescence (ED-XRF) spectrometric method for the analysis of metal components of marine sediments has been presented. Calibrations were made using synthetic matrix. The agreement of the results for sediment standard reference materials with reference values is satisfactory. Major advantages of the non-destructive ED-XRF technique over conventional chemical digestion methods include the applicability to analyzing the major oxide components, as well as to trace metals, and the avoidance of hazardous chemicals. The method has been applied to the routine analysis of Hong Kong marine sediment.     

Modelling European quality assurance procedures for analysers monitoring emissions under the EU’s Industrial Emissions Directive

Accreditation and Quality Assurance - A model has been created to investigate the uncertainty of a gas analyser operating according to the European Standard EN 14181, which ensures that the...     

Using uncertainty functions to predict and specify the performance of analytical methods

In both European legislation relating to the testing of food and the recommendations of the Codex Alimentarius Commission, there is a movement away from specifying particular analytical methods towards specifying performance criteria to which any methods used must adhere. This ‘criteria approach’ has hitherto been based on the features traditionally used to describe analytical performance. This paper proposes replacing the traditional features, namely accuracy, applicability, detection limit and limit of determination, linearity, precision, recovery, selectivity and sensitivity, with a single specification, the uncertainty function, which tells us how the uncertainty varies with concentration. The uncertainty function can be used in two ways, either as a ‘fitness function’, which describes the uncertainty that is fit for purpose, or as a ‘characteristic function’ that describes the performance of a defined method applied to a defined range of test materials. Analytical chemists reporting the outcome of method validations are encouraged to do so in future in terms of the uncertainty function. When no uncertainty function is available, existing traditional information can be used to define one that is suitable for ‘off-the-shelf’ method selection. Some illustrative examples of the use of these functions in methods selection are appended.     

Effect of insignificant bias and its uncertainty on the coverage probability of uncertainty intervals: Part 2. Evaluation for a found insignificant experimental bias

This paper continues in studying the coverage probability of uncertainty intervals. It particularly investigates the uncertainty intervals determined in compliance with the GUM and EURACHEM Guide in case of the results uncorrected for the systematic error, since the experimental bias has been found insignificant. The problem is solved for known values of the experimental bias, its standard uncertainty and the overall standard uncertainty. The obtained findings given in graphs and tables show that coverage probability of the uncertainty intervals defined by expanded uncertainty about the uncorrected results can considerably fall below the chosen level of confidence; this depression depends only on the ratio of the bias and the overall uncertainty. The bias uncertainty does not directly influence this depression, it only determines whether the bias is significant or not and thereby determines whether the results will be corrected or not. The paper proposes three methods how to remove this coverage probability reduction: to apply a higher level of confidence in the significance test, to correct the results with the insignificant but too high biases and to compute the uncertainty intervals defined by some type of the uncertainties enlarged with the insignificant bias.     

Appropriate rather than representative sampling, based on acceptable levels of uncertainty

Appropriate sampling, that includes the estimation of measurement uncertainty, is proposed in preference to representative sampling without estimation of overall measurement quality. To fulfil this purpose the uncertainty estimate must include contribution from all sources, including the primary sampling, sample preparation and chemical analysis. It must also include contributions from systematic errors, such as sampling bias, rather than from random errors alone. Case studies are used to illustrate the feasibility of this approach and to show its advantages for improved reliability of interpretation of the measurements. Measurements with a high level of uncertainty (e.g. 50%) can be shown to be fit for some specified purposes using this approach. Once reliable estimates of the uncertainty are available, then a probabilistic interpretation of results can be made. This allows financial aspects to be considered in deciding upon what constitutes an acceptable level of uncertainty. In many practical situations ”representative” sampling is never fully achieved. This approach recognises this and instead, provides reliable estimates of the uncertainty around the concentration values that imperfect appropriate sampling causes. Received: 28 December 2001 Accepted: 25 April 2002     

Needs for reliable analytical methods for monitoring chemical pollutants in surface water under the European Water Framework Directive

The state of the art in monitoring chemical pollutants to assess water quality status according to Water Framework Directive (WFD) and the challenges associated with it have been reviewed. The article includes information on environmental quality standards (EQSs) proposed to protect the aquatic environment and humans against hazardous substances and the resulting monitoring requirements. Furthermore, minimum performance criteria for analytical methods and quality assurance issues have been discussed. The result of a survey of existing standard methods with a focus on European (EN) and international standards (ISO) for the analysis of chemical pollutants in water is reported and the applicability of those methods for the purpose of compliance checking with EQSs is examined. Approximately 75% of the 41 hazardous substances for which Europe-wide EQSs have been proposed can be reliably monitored in water with acceptable uncertainty when applying existing standardised methods. Monitoring in water encounters difficulties for some substances, e.g., short-chain chlorinated paraffins (SCCPs), polybrominated diphenyl ethers (PBDEs), tributyltin compounds, certain organochlorine pesticides and six-ring PAHs, mainly due to a lack of validated, sufficiently sensitive methods that are applicable in routine laboratory conditions. As WFD requires monitoring of unfiltered samples for organic contaminants more attention needs to be paid to the distribution of chemical pollutants between suspended particulate matter and the liquid phase. Methods allowing complete extraction of organic contaminants from whole water samples are required. From a quality assurance point of view, there is a need to organise interlaboratory comparisons specifically designed to the requirements of WFD (concentrations around EQSs, representative water samples) as well as field trials to compare sampling methodologies. Additional analytical challenges may arise when Member States have identified their river basin specific pollutants and after revision of the list of priority substances.     

Attempts to include uncorrected bias in the measurement uncertainty

In ISO Guide it is strictly recommended to correct results for the recognised significant bias, but in special cases some analysts find out practical to omit the correction and to enlarge the expanded uncertainty for the uncorrected bias instead. In this paper, four alternatively used methods computing these modified expanded uncertainties are compared according to the levels of confidence, widths and layouts of the obtained uncertainty intervals. The method, which seems to be the best, because it provides the same uncertainty intervals as in the case of the bias correction, has not been applied very much, maybe since these modified uncertainty intervals are not symmetric about the results. The three remaining investigated methods maintain their intervals symmetric, but only two of them provide intervals of the kind, that their levels of confidence reach at least the required value (95%) or a larger one. The third method defines intervals with low levels of confidence (even for small biases). It is proposed a new method, which gives symmetric intervals just with the required level of confidence. These intervals are narrower than those symmetric intervals with the sufficient level of confidence obtained by the two mentioned methods. A mathematical background of the problem and an illustrative example of calculations applying all compared methods are attached.     

Measurement uncertainty arising from trueness of the analysis of two endocrine disruptors and their metabolites in environmental samples. Part I: ultrasonic extraction from diverse sediment matrices

The validation of preconcentration strategies for the simultaneous determination of two endocrine disrupting compounds (EDCs) and their metabolites present in the aquatic environment including natural waters and freshwater sediments as well as the estimation of uncertainty arising from trueness using fully nested experimental designs are presented in a series of two papers. In this work, we present Part I of our ongoing study, the validation of an analytical method based on ultrasonic extraction of the target analytes from various freshwater sediments and the estimation of the method measurement uncertainty. The selected endocrine disruptors included two widely used herbicides, diuron (1-(3,4-dichlorophenyl)-3,3-dimethylurea) and linuron (3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea) and their common degradation products namely, 3,4-dichloroaniline (3,4-DCA), 1-(3,4-dichlorophenyl) urea (DCPU) and 1-(3,4-dichlorophenyl)-3-methylurea (DCPMU). A high-performance liquid chromatography system coupled to UV-diode array detector (HPLC/UV-DAD) was used for the target analytes quantification. A fully nested experimental design was applied to study the measurement uncertainty arising from trueness by estimating proportional bias (in terms of recovery). The overall recoveries, that is, those determined by the nested experiments were in the range of 59.5-85.1%, except 3,4-DCA for which a low overall recovery of 29.0% was observed. The analytical method was shown to be linear over the studied range of concentrations (5-100 microg/kg), exhibiting satisfactory repeatability and reaching limits of detection usually in the 0.6-4.6 microg/kg range on dry sediment basis. The method used permitted the determination of the target EDCs and their metabolites in sediment samples collected from selected study stations in the region of Epirus (N.W. Greece) at the concentration levels demanded by current legislation.     

Calibration strategies for quantifying the Mn content of tooth and bone samples by LA-ICP-MS

There is a growing interest in using biomonitoring of tooth and bone specimens to assess human exposure to manganese (Mn). Information on historical exposure to Mn can be obtained through micro-spatial analysis of such specimens by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The principal aim of this study was to compare several calibration strategies for determining Mn content in tooth and bone by LA-ICP-MS including: (a) a six-point calibration curve based on synthesized hydroxyapatite (HA) materials, and single-point calibrations based on (b) NIST SRM 1400 Bone Ash, (c) NIST SRM 1486 Bone Meal, and (d) NIST SRM 612 Trace Elements in Glass. Performance was similar between different ICP-MS platforms (quadrupole ICP-MS, dynamic reaction cell ICP-MS, and sector field ICP-MS). Data based on calibration using the ⁵⁵Mn count rate were compared to that based on using the ⁵⁵Mn/⁴³Ca count rate ratio to obtain results as the Mn mass fraction. Reasonable performance was obtained by calibration using either SRM 612 or SRM 1400, in combination with the ⁵⁵Mn/⁴³Ca count rate ratio and using either the synthesized HA standards or SRM 1400 as calibrators, combined with ⁵⁵Mn count rate. By contrast, calibration based on SRM 1486 resulted in a systematic low bias. While there are several options for quantifying the Mn content of tooth and bone using LA-ICP-MS, users should be aware of the potential for strong matrix effects that may affect results. Overall, determining the ⁵⁵Mn/⁴³Ca count rate ratio, rather than the mass fraction, may represent a better approach for reporting the content of Mn in tooth and bone by LA-ICP-MS.     

Improved evaluation of measurement uncertainty from sampling by inclusion of between-sampler bias using sampling proficiency testing

A realistic estimate of the uncertainty of a measurement result is essential for its reliable interpretation. Recent methods for such estimation include the contribution to uncertainty from the sampling process, but they only include the random and not the systematic effects. Sampling Proficiency Tests (SPTs) have been used previously to assess the performance of samplers, but the results can also be used to evaluate measurement uncertainty, including the systematic effects. A new SPT conducted on the determination of moisture in fresh butter is used to exemplify how SPT results can be used not only to score samplers but also to estimate uncertainty. The comparison between uncertainty evaluated within- and between-samplers is used to demonstrate that sampling bias is causing the estimates of expanded relative uncertainty to rise by over a factor of two (from 0.39% to 0.87%) in this case. General criteria are given for the experimental design and the sampling target that are required to apply this approach to measurements on any material.     

Performance of electrolyte assays in the United Kingdom—reference target values

Wales External Quality Assessment Scheme (WEQAS) is one of the largest external quality assessment (EQA) providers in the United Kingdom, with over 600 participants. Over the last two and a half years, reference target values have been used for the electrolyte panel in the WEQAS General Chemistry scheme. Reference methods have a vital role in that they ensure the transfer of accuracy from definitive methods to routine methods. Validated reference methods, using flame emission/atomic absorption spectrometry were established for sodium, potassium, calcium, magnesium and lithium. These methods were validated by direct comparison with the definitive method via the use of primary reference material. Comparison against the all laboratory trimmed mean (ALTM) showed a small mixed systematic error for sodium and potassium, with a maximum bias from the reference value of 1% and 2%, respectively. Lithium and magnesium data showed predominant systematic constant errors of +0.03 and +0.02 mmol/l, respectively. For calcium, ALTM results showed a mixed systematic error with a cross over in the reference range (5% positive bias and constant error of –0.1 mmol/l). Over 40% of participants used the cresolpthalein complexone method, greatly influencing the ALTM value, (11% bias, constant error –0.26 mmol/l). Other methods such as the ion selective electrode (ISE) method were in closer agreement to the reference method (–4% bias, +0.05 mmol/l constant error). The study highlights the pitfalls of using the overall mean data as an accuracy target in EQA schemes.Presented at the 8th Conference on Quality in the Spotlight, 17–18 March 2003, Antwerp, Belgium