M.Angeles Herrador

Intra-laboratory testing of method accuracy from recovery assays

A revision on intra-laboratory testing of accuracy of analytical methods from recovery assays is given. Procedures based on spiked matrices and spiked samples are presented and discussed.

Pattern recognition procedures for differentiation of Green, Black and Oolong teas according to their metal content from inductively coupled plasma atomic emission spectrometry.

Eight metals Zn, Mn, Mg, Cu, Al, Ca, Ba and K selected as chemical descriptors were analysed by inductively coupled plasma atomic emission spectrometry. The metal profile was studied in 48 samples of commercial tea - 26 black, 18 green, and four Oolong teas. Pattern recognition procedures such as display methods (Principal Component analysis and Cluster analysis) and supervised techniques (Linear Discriminant analysis and Artificial Neural Networks trained by back propagation) were applied to attain the differentiation of the three tea classes.

Evaluation of measurement uncertainty in analytical assays by means of Monte-Carlo simulation.

The main limitations of the Guide to the expression of Uncertainty Measurement (GUM) approach for evaluating the measurement uncertainty of analytical assays are presented and explained. The advantages of using Monte-Carlo simulation against the GUM approach are outlined and discussed and the principle of propagation of distributions is explained. The procedure of Monte-Carlo analysis is illustrated by two case studies. A first simple example quoted from the EURACHEM Guide and dealing with the preparation of a calibration standard is used to present the technique with detail in a step-by-step way. In this case the results obtained by both approaches are very similar. A second example deals with the calibration of mass according to a strong non-linear model. In this case, the Monte-Carlo analysis leads to better results.

Accuracy profiles from uncertainty measurements

Accuracy profiles of chemical assays are introduced and derived from the uncertainty of the analytical result. The calculation of accuracy profiles is based on the estimation of the measurement uncertainty of the analytical assay from validation data. For the sake of illustration, a case study dealing with the spectrofluorimetric determination of quinine in tonic water is explained in detail.

Practical digest for evaluating the uncertainty of analytical assays from validation data according to the LGC/VAM protocol.

The estimation of the measurement uncertainty of analytical assays based on the LGC/VAM protocol from validation data is fully revisited and discussed in the light of the study of precision, trueness and robustness.

Intra-laboratory assessment of method accuracy (trueness and precision) by using validation standards

Assessment of accuracy of analytical methods is a fundamental stage in method validation. The use of validation standards enables the assessment of both trueness and precision of analytical methods at the same time. Procedures of intra-laboratory testing of method accuracy using validation standards are outlined and discussed.

Solubility prediction of caffeine in aqueous N,N-dimethylformamide mixtures using the Extended Hildebrand Solubility Approach

Abstract The solubilities of caffeine in several water–N,N-dimethylformamide mixtures have been determined. The data were treated on the basis of the Extended Hildebrand Solubility Approach and the results were discussed according to association phenomena between solute and solvent blend. An equation has been obtained for predicting the mole fraction solubility of caffeine in the studied mixtures.

Ionization constants of water insoluble arylpropionic acids in aqueous N,N-dimethylformamide mixtures from potentiometric pH-titrations

Abstract The ionization constants of five water insoluble anti-inflammatory arylpropionic acids were evaluated in 80% V/V N , N -dimethylformamide aqueous mixture. From these values, data on proton dissociation constants in water have been computed and compared with the results previously reported by using different approaches.

Potentiometric titrations in acetonitrile-water mixtures: evaluation of aqueous ionisation constant of ketoprofen.

Non ideality of acetonitrile-water mixtures was studied from data on the excess of molar volumes and viscosities. pH and autoprotolisis constants were evaluated at the standard state of the mixed solvent from titrations of a strong acid with a strong base. In order to illustrate the evaluation of the aqueous ionisation constant of water insoluble compounds from pH titrations in ACN-water mixtures, a typical insoluble arylpropionic acid, ketoprofen, was chosen. Ketoprofen was titrated in mixtures from 10 to 70% w/w of acetonitrile against a strong base. From the titration data, the ionisation constant of ketoprofen was evaluated at the standard state of the solvent mixture (pK(a)(*)). Aqueous pK(a) was determined by extrapolation, as the intercept of the plot of pK(a)(*) versus ACN mole fraction.

Differentiation between microcystin contaminated and uncontaminated fish by determination of unconjugated MCs using an ELISA anti-adda test based on receiver-operating characteristic curves threshold values: Application to Tinca tinca from natural ponds

The aim of this study was to evaluate whether the enzyme‐linked immunosorbent assay (ELISA) anti‐Adda technique could be used to monitor free microcystins (MCs) in biological samples from fish naturally exposed to toxic cyanobacteria by using receiver operating characteristic (ROC) curve software to establish an optimal cut‐off value for MCs. The cut‐off value determined by ROC curve analysis in tench (Tinca tinca) exposed to MCs under laboratory conditions by ROC curve analysis was 5.90‐μg MCs/kg tissue dry weight (d.w.) with a sensitivity of 93.3%. This value was applied in fish samples from natural ponds (Extremadura, Spain) in order to asses its potential MCs bioaccumulation by classifying samples as either true positive (TP), false positive (FP), true negative (TN), or false negative (FN). In this work, it has been demonstrated that toxic cyanobacteria, mainly Microcystis aeruginosa, Aphanizomenon issatchenkoi, and Anabaena spiroides, were present in two of these ponds, Barruecos de Abajo (BDown) and Barruecos de Arriba (BUp). The MCs levels were detected in waters from both ponds with an anti‐MC‐LR ELISA immunoassay and were of similar values (between 3.8–6.5‐μg MC‐LR equivalent/L in BDown pond and 4.8–6.0‐μg MC‐LR equivalent/L in BUp). The MCs cut‐off values were applied in livers from fish collected from these two ponds using the ELISA anti‐Adda technique. A total of 83% of samples from BDown pond and only 42% from BUp were TP with values of free MCs higher than 8.8‐μg MCs/kg tissue (d.w.).