Definition of assigned values for proficiency tests in water analysis

The definition of an assigned value is usually achieved by calculating mean values from the data (with different methods) or by designating reference laboratories. Neither method is completely satisfactory. In this paper a new method is presented for the definition of the assigned value for spiked samples with an unknown content of the analyte in the matrix. The method consists of two parts. The first is the estimation of the assigned values from the spiked amounts and the content in the matrix, based on the results of reference laboratories. The other is the designation of these reference laboratories by comparing their results with the assigned values. Because each of these parts requires the other, an iterative procedure is necessary. As an example, the results of a proficiency test for the analysis of copper in wastewater are used to compare the calculated values with those from other methods, e.g., the Huber estimation. Received: 25 September 2000 Accepted: 9 December 2000     

Use of uncertainty estimates as reported by participants in proficiency testing for the evaluation of their results: pros and cons

The aim of this paper is to study the measurement uncertainties reported in proficiency tests (PTs) using examples from PTs in the environmental sector and to compare the obtained measurement uncertainty estimates using different approaches. In addition, the paper focusses on the differences between the z-score and the zeta score. Since the year 2000, the Finnish Environment Institute has asked participants to report analytical methods as well as measurement uncertainties in connection with PT results. The measurement uncertainties of the assigned value have also been evaluated. On the basis of the results, the measurement uncertainties reported by the participants varied greatly. Participants often reported underestimated measurement uncertainties, but overestimated uncertainties were also reported. At the moment, it seems as if performance assessment should be based on the z-score because of a number of significant over- and underestimated measurement uncertainties. The zeta score should be used for information and educational purposes mainly.     

A new approach in assessing microbiological results in water analysis proficiency testing

Due to the relatively large spread in the results of microbiological proficiency tests, the z-scores are often not able to detect zero or low results as being ‘bad’ results. This paper describes an adapted z-score based on the average or the standard deviation of the 50% ‘highest’ results. The combination of the adapted z-scores of four samples enables a better judgement of the performance of each laboratory. Papers published in this section do not necessarily reflect the opinion of the Editors, the Editorial Board or the Publisher.     

The application of data from proficiency testing to laboratory accreditation according to ISO 17025

Current methods of testing laboratories for their proficiency in reporting correct measurement results are liable to substantial errors of the second kind. This means that laboratories with deflated uncertainties are accepted as proficient, even though their reported measurement results pave the way for erroneous conclusions. Only by using E _n numbers based on an accepted reference value with the lowest possible uncertainty can the risk of recognising incorrect measurement results be kept at an acceptable level. Based on an actual set of proficiency test (PT)-data for the concentration of Pb in water, this paper compares PT results obtained by methods using E _n numbers with methods based on z-scores. Kaj Heydorn is a technical assessor at The Danish Accreditation and Metrology Fund (DANAK).     

Chlorophyll-a determination: results of an interlaboratory comparison

A chlorophyll-a interlaboratory comparison was carried out to compare three different analytical chlorophyll-a determination methods: a German standard DIN 38412-16, a method of the HELCOM-Combine-Manual and the different “in-house” methods of participating laboratories. Eleven laboratories took part. They had to determinate the chlorophyll-a and phaeopigment content in two seawater samples taken from the Baltic Sea. Furthermore, for the assessment three different statistical evaluation methods were applied: the German standard DIN 38402-42, the Q-method combined with an estimator according to Huber and the Cofino-method. All analytical methods were appropriate to determine the chlorophyll-a content. The relative standard deviations of reproducibility for chlorophyll-a varied between 12 and 31%. None of the analytical methods was appropriate to determine the phaeopigment content quantitatively. The relative standard deviations of reproducibility for phaeopigments ranged between 87 and 158%. The applied statistical evaluation methods resulted in different assessments. The Q-method combined with an estimator according to Huber proved to be the best qualified method.Presented at the Eurachem PT Workshop September 2005, Portorož, Slovenia     

Evaluation of laboratory performance in IMEP water interlaboratory comparisons

The aim of International Evaluation Programme (IMEP) is to present objectively the quality of chemical measurements. Participants in IMEP compare their reported measurement results with independent external certified reference values with demonstrated traceability and uncertainty, as evaluated according to international guidelines. Three major interlaboratory comparisons (ILCs), IMEP-6, IMEP-9 and IMEP-12, on trace elements in water were carried out from 1994 to 2000. Participants' results for Cd, Fe and Pb concentrations from these three different IMEP water ILCs were compared by means of suitable performance indicators. The performance evaluation criteria were set according to the requirements stated in the Council Directive 98/83/EC of 3 November 1998 on the quality of water intended for human consumption. Two different performance indicators were selected for evaluation of the individual participants' results: the commonly used z-score and the not so well-known E _n number. The group performance indicator is based on the variation of z-scores. To assess the individual measurement performance, not only the deviation of the reported measurement values from the certified reference value, but also criteria for maximum and minimum acceptable uncertainties were taken into account. The participants' performance is also reviewed by means of using a simple graphical tool, called “Naji-plots”.     

The determination of an accepted reference value from proficiency data with stated uncertainties

Proficiency data with stated uncertainties represent a unique opportunity for testing that the reported uncertainties are consistent with the Guide to the expression of uncertainty in measurement (GUM). In most proficiency tests, however, this opportunity is forfeited, because proficiency data are processed without regard to their uncertainties. In this paper we present alternative approaches for determining a reference value as the weighted mean of all mutually consistent results and their stated uncertainties. Using an accepted reference value each reported uncertainty estimate can be expressed as an E _n number, but a value of confirms its validity only if the uncertainty of the reference value is negligible in comparison.Reference values calculated for results from an International Measurement Evaluation Programme (IMEP-9) by “bottom up” as well as “top down” methods were practically identical, although the first strategy yielded the lowest uncertainty. A plot of individual coefficients of variation (CV) versus E _n numbers helps interpretation of the proficiency data, which could be used to validate relative uncertainties down to <1%.     

QUASIMEME results of laboratories involved in the German Marine Monitoring Programme for the North and Baltic Seas

This case study is written for laboratories in the German Marine Monitoring Program (GMMP) for the North and Baltic Seas in order to give support to the assessment of proficiency testing results for their external quality assurance. A second issue is to provide a practical, understandable and common way for the calculation of laboratory performance parameters, so-called combination scores. In this paper, the GMMP laboratory proficiency testing results of QUASIMEME (Quality Assurance of Information for Marine Environmental Monitoring in Europe) Laboratory Performance Studies for the last 5 years were reviewed. Combination scores for organochlorine pesticides and chlorinated biphenyls in marine sediment are presented and explained. However, the assessment approach is very general and should be applicable to most testing laboratories in the chemical field.Presented at the Eurachem PT Workshop September 2005, Portorož, Slovenia     

External quality assessment in microbiology: comparison of results from Belgian and Canadian laboratories with regard to their ability to identify <Emphasis Type="Italic">Streptococcus pyogenes</Emphasis>

Clinical Microbiology Proficiency Testing (CMPT; Canada) and the Institute of Public Health (IPH; Belgium) set up a joint external quality assessment (EQA) project in which they sent identical simulated clinical samples to their respective participants. Samples were sent out as throat swabs containing Streptococcus pyogenes (as pathogen) and viridans streptococci (as commensal). Results from identification by the Belgian laboratories were excellent: 99.5% detected the group A streptococci. About 10% of the Canadian laboratories reported the absence of reportable β-haemolytic streptococci, although for most of them the issue was one of non-examination of small or pin-point streptococci. Overall, Canadian and Belgian laboratories performed well in this EQA; the joint project clearly showed that although many similarities exist, there are differences between laboratories in treating EQA samples and reporting results.     

Interpretation of analysis of variance models using principal component analysis to assess the effect of a maternal anticancer treatment on the mineralization of rat bones

The aim of this paper is to show how it is possible to extract analytical information from images acquired with a flatbed scanner and make use of this information for real time control of a nickel plating process. Digital images of plated steel sheets in a nickel bath are used to follow the process under degradation of specific additives. Dedicated software has been developed for making the obtained results accessible to process operators. This includes obtaining the RGB image, to select the red channel data exclusively, to calculate the histogram of the red channel data and to calculate the mean colour value (MCV) and the standard deviation of the red channel data. MCV is then used by the software to determine the concentration of the additives Supreme Plus Brightner (SPB) and SA-1 (for confidentiality reasons, the chemical contents cannot be further detailed) present in the bath (these two additives degrade and their concentration changes during the process). Finally, the software informs the operator when the bath is generating unsuitable quality plating and suggests the amount of SPB and SA-1 to be added in order to recover the original plating quality.     

Rheological properties of viscoelastic biofilm extracellular polymeric substances and comparison to the behavior of calcium alginate gels

In this publication we present a detailed study of viscoelastic biofilms of Pseudomonas aeruginosa. Sample solutions were extracted from biofilm layers grown on Pseudomonas isolation agar. This aqueous solutions of extracellular polymeric substances exhibit weak elastic effects caused by entanglements and a small number of permanent junction points formed by calcium ions. The cross-linking mechanisms are confirmed by the Cox–Merz rule and dynamic frequency sweep tests, which result in an average lifetime of junction points of the order of 17 ms. The experimental data reveal 3.4×10¹⁷ elastically effective chains per liter of solution and no significant temperature effects in the regime between 2 and 24 °C. This result coincides pretty well with the concentration of dissolved polymer chains (2.9×10¹⁷ molecules/l). Upon addition of calcium ions, one observes the formation of stable supermolecular networks with permanent junction points. These cross-linking points did not show thermal fluctuations in time zones between 10 ms and several hours. The entanglement density of these gels is of the same order as observed in the non-cross-linked sol state (entrapped entanglements). In spite of the different molecular composition alginate gels show the same type of cross-linking mechanism as gels of extracellular polymeric substances.     

Conformational flexibility of six-membered 1,2-dihydrocycles based on the results of AM1 calculations

The conformational flexibility of 2H-pyran, 1,2-dihydropyridine, 2H-thiopyran, their oxo, imino, and methylene derivatives, and the 5-oxo, imino, and methylene derivatives of 1,3-cyclohexadiene was studied by the semiempirical quantum-chemical AM1 method. All compounds have planar equilibrium conformations and similar electronic structures. The transition to a distorted sofa conformation with the =C-X-C(=Y)-C= torsion angle (X = CH₂, NH, O, or S; Y = H₂, O, NH, or CH₂) of ±20° increases the energy of the molecule by less than 1.5 kcal mol^–1. The reasons for the high conformational flexibility of these dihydrocycles were analyzed. The nonaromatic character of the cyclic -system is the factor that determines the conformational flexibility of the dihydrocycle in ylide derivatives.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2650–2652, November, 1996.     

A Rapid Tandem Mass Spectrometric Assay for Determination of Ursolic Acid – Application to Analysis of Ursolic Acid in Four Species of Staphylea L. and Leaves of Staphylea Pinnata L. Gathered During Ontogenesis

Plant extracts of Staphylea L. exhibit a number of biological activities which are presumably attributed to ursolic acid. A rapid and specific tandem mass spectrometric (MS-MS) assay for the quantification of ursolic acid in the leaves of four species of Staphylea L. (Bladdernut) and in the leaves of S. pinnata L. during ontogenesis, was developed and validated. The samples were analyzed by flow injection analysis without chromatographic separation using a transport liquid of methanol/water/formic acid (80:20:0.1 v/v/v) at a flow-rate of 0.2 mL min⁻¹. The run cycle time was ~2-3 min injection-to-injection. Quantification was achieved using multiple reaction monitoring at MRM transition m/z 439 > 203. Calibration curves were linear over the concentration range of 2–20 μg mL⁻¹ with a lower limit of quantification of 2 μg mL⁻¹ (1.8 ± 0.297, RSD: 0.165). Validation data showed that the RSD% values were in the range of 1.8 to 6.8%, whereas the % DEVs ranged from −18 to −2% indicating reasonable and acceptable precision and accuracy, respectively. A recovery percent of 106.8 ± 10.3 of ursolic acid from spiked extracts samples, indicated the specificity and reliability of tandem mass procedure for determination of ursolic acid in the plant extracts. The derived data of sample analysis showed different contents of ursolic acid among various Staphylea species. The highest content of ursolic acid was found in the leaves extract of S. pinnata L. Additionally, the highest amount of ursolic acid accumulated in the leaves of S. pinnata L. was in the August /September period of the year. Smaller amounts of ursolic acid were found in samples collected before and after that time. The results obtained serve as a justification of determining the most appropriate time for collecting plant material as a source of ursolic acid.     

Improved Iterative Version of the Coats-Redfern Method to Evaluate Non-Isothermal Kinetic Parameters

An improved version of the Coats-Redfern method of evaluating non-isothermal kinetic parameters is presented. The Coats-Redfern approximation of the temperature integral is replaced by a third-degree rational approximation, which is much more accurate. The kinetic parameters are evaluated iteratively by linear regression and, besides the correlation coefficient, the F test is suggested as a supplementary statistical criterion for selecting the most probable mechanism function. For applications, both non-isothermal data obtained by theoretical simulation and experimental data taken from the literature for the non-isothermal dehydration of Mg(OH)₂ have been processed.This revised version was published online in November 2005 with corrections to the Cover Date.     

Influence of alkali and alkaline earth ions on the O -alkylation of the lower rim phenolic-OH groups of p-tert -butyl-calix[4]arene to result in amide-pendants: Template action of K+ and the structure of K+ bound tetra-amide derivative crystallized with a p-tert -butyl-calix[4]arene anion

Role of alkali and alkaline earth ions on the formation of calix[4]arene-amide derivatives through O-alkylation of the lower rim phenolic-OH groups in general and template action of K⁺ in particular have been explored. Na⁺ and K⁺ ions among alkali, and Ca²⁺ and Sr²⁺ ions among alkaline earth have shown tetra-amide derivatives bound to metal ion species. Among all these, potassium salts act as template and yields a K⁺ bound tetra-amide derivative where the charge is counter balanced by a calix[4] arene-monoanion and the product is crystallographically characterized. Change in the amide precursor used in these O-alkylation reactions has no effect on the type of the amide derivative formed. Also demonstrated is a direct one-step reaction for the preparation of 1,3-di-amide derivative in high yield and low reaction period using CsHCO₃.     

Metal ion incorporation into n-methyl-5,10,15,20-tetrakis (4-sulfonatophenyl) porphine and its differential rates as applied to the kinetic determination of copper(II) and zinc(II) in serum

Metal ion incorporation intoN-methyl-5,10,15,20-tetrakis(4-sulfonatophenyl)porphine (N-CH₃TSPP) has been shown to be much faster than that for non-methylated porphyrins such as 5,10,15,20-tetrakis(4-sulfonatophenyl)porphine (TSPP). We have proposed a kinetic method, utilizing differential rate of metal ion incorporation into N-CH₃TSPP, for the determination of submicrogram amounts of copper(II) and zinc(II) in serum.     

Nonequilibrium differential capacitance corresponding to the generalized model of the surface layer and its regression analysis

Equations that describe the nonequilibrium differential capacitance C of an electrode subjected to adsorption of organic molecules under the conditions where this adsorption strictly follows the generalized model of the surface layer (GMSL) are derived. Using the developed programs, the dependences of C _ω on the electrode potential φ at different concentrations of an organic substance are calculated. A regression analysis procedure is developed that allows one to obtain a set of GMSL parameters describing the dense layer capacitance in NaF solutions with different contents of n-C₄H₉OH     

Characterization of 1,3-alternate calix[4]arene-silica bonded stationary phases and their comparison to selected commercial columns by using principal component analysis

Twelve calix[4]arene stationary phases in 1,3-alternate conformation, synthesized in the authors’ laboratory, were characterized in terms of their surface coverage, hydrophobic selectivity, aromatic selectivity, shape selectivity, hydrogen bonding capacity and ion-exchange capacity. The set of tests commonly used for evaluation of commercially available stationary phases was applied to assess fundamental chromatographic properties of the calixarene phases. The new calixarene phases were compared to each other, to Caltrex and LiChrosorb C-18 columns. Principal component analysis has been used to provide comparison between 1,3-alternate calix[4]arene phases and commercially available phenyl, fluorophenyl and fluoroalkyl columns.     

Solute-Solvent Interaction Effects on Protonation Equilibrium of Substituted N-Benzylidene-2-hydroxyanilines in Aqueous Ethanol: The Application of Factor Analysis to Solvatochromic Parameters and Protonation Equilibria

Summary. In this study, the stoichiometric protonation constants, logK_OH and logK_NH, of sixteen substituted N-benzylidene-2-hydroxyanilines have been determined potentiometrically in ethanol-water mixtures of varying composition (10–80% ethanol by volume) at 25.0±0.1°C. The values of the constants, logK, were submitted to factor analysis in order to obtain the number of factors which affect the variation of the whole data sets of protonation constants and, afterwards, to target factor analysis to identify these factors. The influence of solvatochromic parameters in the interactions between Schiff bases derivatives and the solvent studied was identified and quantified. Kamlet and Taft general equations allow calculation of the logK values of Schiff bases studied in any ethanol-water mixtures up to 80% (v/v) and thus provide the knowledge of the acid-base behaviour in these solvent media. Further, the quasi-lattice quasi-chemical (QLQC) theory of preferential solvation has been applied to quantify the preferential solvation by water of electrolytes in ethanol-water mixtures.     

Sensitive and specific detection of potentially allergenic almond (Prunus dulcis) in complex food matrices by Taqman real-time polymerase chain reaction in comparison to commercially available protein-based enzyme-linked immunosorbent assay

Currently, causative immunotherapies are lacking in food allergy. The only option to prevent allergic reactions in susceptible individuals is to strictly avoid the offending food. Thus, reliable labelling of allergenic constituents is of major importance, but can only be achieved if appropriate specific and sensitive detection techniques for foods with allergenic potential are available. Almond is an allergenic food that requires mandatory labelling on prepackaged foods and belongs to the genus Prunus. Species of this genus are phylogenetically closely related. We observed commercially available almond specific ELISA being highly cross-reactive with other foods of the Prunoideae family, resulting in a false-positive detection of up to 500,000 mg kg⁻¹ almond. Previously published PCR methods were reported to be cross-reactive with false positive results >1200 mg kg⁻¹.We describe the development of a novel almond specific real-time PCR, based on mutated mismatch primers and sequence specific Taqman^® probe detection, in comparison with two quantitative commercially available ELISA. PCR sensitivity was investigated with chocolate, chocolate coating and cookies spiked between 5 and 100,000 mg kg⁻¹ almond. In all matrices almond was reproducibly detected by real-time PCR at the lowest spike level of 5 mg kg⁻¹. Further, between 100 and 100,000 mg kg⁻¹ spiked almond, the method featured good correlation between quantified copy numbers and the amount of spiked almond. Within this range a similar relation between detectable signal and amount of almond was observed for both PCR and ELISA. In contrast to ELISA the Taqman^® real-time PCR method was highly specific in 59 food items with negligible cross-reactivity for a very limited number of Prunoideae foods. The real-time PCR analysis of 24 retail samples was in concordance with ELISA results: 21% (n = 5) contained undeclared almond. This is the first completely disclosed real-time PCR method for a specific and potentially quantitative almond detection. This PCR method detects almond at a level where severe allergic reactions should not be expected for the majority of the almond allergic individuals.