Estimating the uncertainty of binary test results to assess their compliance with regulatory limits

Passive Sampling with Active Carbon Fibres in the Determination of Organic Pollutants in Groundwater

Legislation addressing the quality of groundwater and increasing concerns over public health calls for the development of analytical methods that can produce accurate and precise results at the ppt level. Passive sampling has been recognised as a helpful tool in identifying various organic pollutants in groundwater, even when their presence had not yet been identified through conventional groundwater quality monitoring. The article presents an analytical method involving a simple and cost-effective passive sampling device using Zorflex® activated carbon fibres (ACFs) for the qualitative monitoring of a broad range of organic pollutants in water in a single run. The applicability of the method developed was tested in three hydrogeological studies. In the first case, we present a non-targeted qualitative screening and a list of 892 different contaminants detected in the groundwater in Slovenia. In the second case, we discuss the presence and origin of organic compounds in the groundwater from a pilot area of the urban aquifer, Ljubljansko polje. The third case presents a comparison of results between passive and grab sampling. Passive sampling with ACFs confirmed the presence of a pollutant, even when it had not been previously detected through a quantitative method.

Altering the substituents of salicylic acid to improve Berthelot reaction for ultrasensitive colorimetric detection of ammonium and atmospheric ammonia

The Berthelot reaction is a classic method for detection of ammonium (NH₄⁺) and atmospheric ammonia (NH₃) by using salicylic acid (SA) as the chromogenic substrate. However, there lacks a method for improving the activity of the Berthelot reaction to enhance the analytical performance for detection of NH₄⁺ and NH₃. Here, five SA analogues with electron-withdrawing groups (-F) and electron-donating groups (-CH₃ and -OCH₃) at different positions of the aromatic ring have been chosen as the alternative to SA for Berthelot reaction. Among these analogues, 4-methoxysalicylic acid (4-OCH₃-SA) shows the best colorimetric response and color change at a NH₄⁺ concentration of 30 μM, and the sensitivity of 4-OCH₃-SA-based colorimetric assay for NH₄⁺ increases 1.75-fold compared with that of SA-based colorimetric method. This enhancement effect is attributed to the strong electron-donating property of 4-OCH₃ group, activating the two-step electrophilic aromatic substitution reaction in the Berthelot reaction. Additionally, visual and sensitive detection of NH₃ is realized, along with a low limit of detection down to 0.037 ppm. Furthermore, we demonstrate that this assay is reliable and practical for detection of NH₄⁺ and NH₃ in real water and air samples with good accuracy.

Assessing the measurement uncertainty of qualitative analysis in the clinical laboratory

Measurement uncertainty is a parameter that is associated with the dispersion of measurements. Assessment of the measurement uncertainty is recommended in qualitative analyses in clinical laboratories; however, the measurement uncertainty of qualitative tests has been neglected despite the introduction of many adequate methods. We herein provide an overview of three reasonable statistical methods for quantifying the measurement uncertainties of qualitative assays, namely Bayes’ theorem, the normal distribution method, and the information theoretic approach. Unlike in quantitative analysis, the measurement uncertainty of qualitative analysis is expressed using a conditional probability, likelihood ratio, and entropy. With the necessary theoretical background, the practical applications for clinical laboratories are also provided using statistical calculations. Using statistical approaches, we hope that our review will contribute to the use of measurement uncertainty in qualitative analyses in the clinical laboratory environment.

Analytical model for calculating indeterminate results interval of screening tests, the effect on seroconversion window period: a brief evaluation of the impact of uncertain results on the blood establishment budget

The evaluation of measurement uncertainty is not required by the European Union regulation or blood establishments’ laboratory tests. However, it is required for tests accredited by ISO 15189. Also, the forthcoming ISO 9001 edition requires “risk based thinking” with risk described as “the effect of uncertainty on an expected result”. ISO recommends GUM models for determination of measurement uncertainty, but their application is not intended for ordinal value measurements, such as what happens with screening test binary results. This article reviews, discusses and proposes concepts intended for measurement uncertainty of screening test results. The precision model focuses on cutoff level allowing the evaluation of the indeterminate interval using analytical sources of variance. The intervalis considered in the estimation of the seroconversion window period. The delta-value of patients and healthy subjects’ samples allows ranking two tests according to the probability of the two classes of indeterminate results: chance of false negative results and chance of false positive results (waste on budget).

Development, validation and data quality assurance of screening methods: a case study

Despite the growing importance of qualitative screening tests in routine laboratories involved in the EU official control, their validation is not as deeply explained in Commission Decision 2002/657/EC as the validation of quantitative confirmatory methods. At the same time, the issue of quality assurance of screening assays defining internal quality control (IQC) procedures as required by accreditation bodies is undoubtedly less developed in this analytical field. As an example the present study describes the development, the validation and the IQC implemented for a commercial enzyme linked immunosorbent assay (ELISA) able to detect 17-α-19-nortestosterone (α-NT) and 17-β-19-nortestosterone (β-NT) isomers in bullock urine. In order to select a suitable sample treatment, two SPE purification protocols were preliminary compared. The chosen method was therefore fully validated determining the mandatory parameters required by Commission Decision 2002/657/EC: specificity, detection capability and robustness. An in-depth discussion was carried out illustrating the possible validation approaches and their implications especially in the assessment of the key performance characteristic: detection capability. Finally, the control charts implemented for continuous method verification during analyses of real samples were reported.

Direct screening of tetracyclines in water and bovine milk using room temperature phosphorescence detection

A fast and simple flow-through optosensor was designed and characterized for the direct screening of four tetracycline (TCC) antibiotics (tetracycline, oxytetracycline, chlortetracycline and doxycycline) in water and bovine milk samples. The proposed optosensor provides rapid binary yes/no overall responses, being appropriate for the screening of this family of antibiotics above or below a pre-set concentration threshold. The experimental set-up is based on a flow-injection manifold coupled on-line to a phosphorescence detector. Aliquots of the samples are pretreated with Eu(III) to form room temperature phosphorescent metal chelates and injected in the flow manifold. Those chelates are then on-line retained on a conventional flow-cell (packed with polymeric Amberlite XAD-4 particles) which is placed inside the cell holder of the phosphorimeter. After the emission is registered, the antibiotic-metal complexes are eluted from the packed resin with 1M HCl (for milk samples a second regeneration step, using methanol, should be performed). A sample throughput of about 20 samples per hour was obtained. Optimum experimental conditions include a pH 9, a Eu(III) concentration of 2 x 10(-4) M and 8 mM sodium sulphite as chemical deoxygenant. The phosphorescence emitted by the europium-TCC complexes was measured at 394 and 617 nm for excitation and emission wavelengths, respectively. The unreliability region, given by the probability of false positives and false negatives, respectively (set at 5% in both cases) was in the range between 0.2 and 11.6 nM for detection of tetracyclines in water samples (at a cut-off level of 4 nM) and in the range between 165 and 238 nM for detection of tetracyclines in milk (cut-off level fixed at the normative EU level of 200 nM). Finally, the applicability of the proposed screening optosensor was tested for the reliable control of tetracyclines in contaminated and uncontaminated water and milk samples.

Uncertainty of Measurement: Approaches and Open Problems

Despite of exhaustive literature information, analysts are still fronting serious problems in estimating the uncertainty of measurement (UOM), mainly because of the intrinsic difficulties in choosing an appropriate approach and/or of an inadequate background in statistics and metrology fundamentals. Sometimes, additional problems, at present under debate, make their efforts even more stressful. In this paper, attention is mainly paid to intra-laboratory estimation of UOM, the most frequent situation. After a short re-examination of the existing UOM approaches, some applicative problems are discussed in the light of the most representative literature information.

Validation of a screening method for the simultaneous identification of fat-soluble and water-soluble vitamins (A, E, B1, B2 and B6) in an aqueous micellar medium of hexadecyltrimethylammonium chloride

Simultaneous determination of the fat-soluble vitamins A and E and the water-soluble vitamins B1, B2 and B6 has been carried using a screening method from fluorescence contour graphs. These graphs show different colour zones in relation to the fluorescence intensity measured for the pair of excitation/emission wavelengths. The identification of the corresponding excitation/emission wavelength zones allows the detection of different vitamins in an aqueous medium regardless of the fat or water solubility of each vitamin, owing to the presence of a surfactant which forms micelles in water at the used concentration (over the critical micelle concentration). The micelles dissolve very water insoluble compounds, such as fat-soluble vitamins, inside the aggregates. This approach avoids the use of organic solvents in determining these vitamins and offers the possibility of analysing fat- and water-soluble vitamins simultaneously. The method has been validated in terms of detection limit, cut-off limit, sensitivity, number of false positives, number of false negatives and uncertainty range. The detection limit is about microg L(-1). The screening method was applied to different samples such as pharmaceuticals, juices and isotonic drinks.