L. Cuadros Rodríguez

ALAMIN: a chemometric program to check analytical method performance and to assess the trueness by standard addition methodology

Abstract ALAMIN is a DOS program that establishes the performance characteristics of the analytical method from the calibration data set. It allows an estimation of the maximum level of interferents that causes a bias equal to or greater than the random error of determination. A statistical protocol checks the trueness of the analytical method by standard addition methodology. This shareware program is available from the authors upon request.

Statistical estimation of linear calibration range

Abstract The “in-line” and “on-line” linearity terms of a linear calibration are defined. “On-line” linearity, which is calculated from the standard deviation of the slope, is proposed as a performance characteristic for the analytical method. The relation between the precision of an analytical result, and the analytical sensitivity and “on-line” linearity of the corresponding chemical calibration is established. The simplified equation that is obtained makes it possible to rapidly estimate the standard deviation with a sufficient degree of freedom, even when few analysis replicates are available. An indication is also given of the suitable equation for a more accurate calculation. The use of the two linearities, both “in-line” and “on-line”, is discussed, concerning the determination of the linear calibration range, for those techniques with which curvatures are obtained when the concentration of the standard analyte exceeds a particular value.

Sequential response surface methodology for multioptimization in analytical chemistry with three-variable Doehlert designs

Abstract In this paper, the use of Doehlert designs for the optimization of three experimental variables is proposed. Sequential response surface methodology is based on a strategy to obtain a quadratic function relationship between the response and the significant variables using a three and five-level experimental design. The Doehlert design structure allows us to study this reponse surface by estimating the parameters of the quadratic model and thus, by building sequential designs, it is possible to obtain the region in which the optimum values of such variables are simultaneously obtained. The mobility of these designs across the experimental domain of the variables permits the use of data from a previous design for a new design, obtained by response variation for the other two variables. The principle of this methodology is described and an example of its application in a spectrofluorimetric method is given.

Determination of organochlorine pesticides by GC-ECD and GC-MS-MS techniques including an evaluation of the uncertainty associated with the results

SummaryThe pesticides lindane, vinclozolin, aldrin, p,p′DDE, o,p′DDT, p,p′DDT, methoxychlor, kepone and mirex, which are classifed as endocrine disrupters, have been analysed. This was conducted within an extensive hospital-based epidemiological study that includes surgically treated breast cancer patients and a matching group of women with no sign of estrogenic-dependent disease. Patients were from the Hospital Torrecardenas and living in agricultural areas of Almería (Spain). The results from the analysis of 100 serum samples were performed by gas chromatography (GC) with electron capture detection (ECD) and tandem mass spectrometry (MS-MS) detection mode. Performance characteristics, such as linearity, detection limit (LOD) and quantitation limit (LOQ), together with an estimate of measurement uncertainty (using validation data) is presented for each pesticide, in both chromatographic methods. The results obtained were compared. The advantage of MS-MS over ECD to determine endocrine disrupting compounds in complex matrix was also shown. p,p′DDE was the most frequently encountered pesticide, as well as the one found at higher concentration level. p,p′DDT was found in eight samples. Lindane was detected at low concentration level. While vinclozolin, aldrin, methoxychlor, kepone, o,p′DDT and mirex were absent in all samples analysed.

Estimation and correction of matrix effects in gas chromatographic pesticide multiresidue analytical methods with a nitrogen–phosphorus detector

The assessment of matrix effects in the quantification of organophosphorus pesticides in fruit and vegetables by GC-NPD, were studied applying ANCOVA. Calibration curves prepared in solvent were compared with calibration curves prepared in a blank matrix extract for eight different commodities, establishing whether the matrix induces systematic or proportional errors in the quantification of the pesticides. In such cases correction functions were obtained and validated by quantifying spiked samples using solvent calibrations and applying the correction functions to the data obtained. The results were compared with those obtained by quantification using matrix-matching calibrations and with those from 100% recovery experiments. It was found that the matrix effects can be avoided using the correction functions. Finally the contribution of the correction functions to the uncertainty of the results was estimated as well as their stability during a four month period.

Estimation of the Tolerance Level of Interfering Substances in Instrumental Analytical Methods

Abstract A model to estimate the tolerance level of interfering substances for analytical procedures, applying the linear simple regression, is proposed. The confidence interval on the analytical signal for the selected concentration value to check the interference, is calculated using the calibration data set. The tolerance levels of molybdenum, aluminium and iron in the spectro fluorimetric determination of boron with Alizarin Red S by means of direct, synchronous and first- and second-derivative methods, are estimated. The adequate selection of an analyte concentration for the interferent test is discussed.

A first and straightforward approach to detection limits for narrow-bore HPLC and CE.

Two methodologies have been proposed for a simple statistical estimation of the detection limits in microbore-HPLC and CE techniques. Due to the impossibility to measure the blank, by using the extrapolated or approximated expressions it is possible to obtain a straightforward approach of the detection limits from the study of the residuals of the calibration data set, avoiding tedious treatment of the signals. The utility of these expressions has been checked on the data from an experimental microbore HPLC system.

Simultaneous quantification of chlorophenoxyacid herbicides based on time-resolved photochemical derivatization to induce fluorescence in micellar medium

In this paper a sensitive and simple method for the resolution of mixtures of chlorophenoxyacid herbicides using photochemical derivatization induced fluorescence has been described. These compounds do not show any fluorescence, hence photolysed to induce fluorescence after direct irradiation with ultraviolet light in presence of a cationic surfactant (cetyltrimethylammonium chloride). Critical variables such as the surfactant concentration and the irradiation time have been optimised for each compound using Sequential Response Surface Methodology (SRSM) by applying Doehlert designs in order to obtain maximum fluorescence intensity. The difference shown between the optimised irradiation times for the formation of the photoproducts allowed us to propose a time-resolved photoactivation method, for the simultaneous determination of binary mixtures, based on the use of different linear calibration curves established at various irradiation times depending on the mixture to be resolved. Satisfactory recoveries were obtained in the analysis of several mixtures of these herbicides at different ratios in spiked waters.

Specific Fluorescence Detection of Lithium Ion using Quinizarin and Extraction in TBP

Summary A simple and specific spot-test for lithium based on a fluorescent reaction betwen Li+ ion and Quinizarin in alkaline medium is described. The resulting compound is extracted in TBP, and fluorescence is observed in the organic phase under illumination with a UV lamp at 360 nm. The sensitivity of the test is 0.1 μg.cm−3. Applications of the test for lithium include the corresponding cation groups using the H2S and Na2CO3 systematic schemes of analysis; and directly in synthetic mixtures of inorganic ions, mineral water, drugs, dietetic products and blood serum.

Correction of the Matrix Effect in the Determination of Benzoylphenylurea Insecticides in Vegetables

Abstract The matrix effect in the analysis of benzoylphenylurea insecticides, in four vegetables by HPLC using continuous on‐line post‐elution photoirradiation with fluorescence detection, was investigated by applying the analysis of covariance (ANCOVA). Experimental results show that a matrix effect exists in the quantification of diflubenzuron and flufenoxuron in cucumber and flufenoxuron in green bean. In order to correct this effect, two different methods have been compared in the analysis of real samples: the first one was based on the use of calibration solutions prepared in blank matrix extracts for the quantification of the target analytes in the different matrices, and the second one consisted of the establishment of a correction function. The obtained results showed that the application of the correction function leads to results very similar to those obtained when the quantification was carried out using calibration curves prepared in blank matrix extract; the utility of this function is demonstrated to eliminate the matrix effect in the quantification of benzoylphenylurea insecticides in real samples with the advantages that this presents.