Ad-hoc blocked design for the robustness study in the determination of dichlobenil and 2,6-dichlorobenzamide in onions by programmed temperature vaporization-gas chromatography-mass spectrometry.

An 'ad-hoc' experimental design to handle the robustness study for the simultaneous determination of dichlobenil and its main metabolite (2,6-dichlorobenzamide) in onions by programmed temperature vaporization-gas chromatography-mass spectrometry (PTV-GC-MS) is performed. Eighteen experimental factors were considered; 7 related with the extraction and clean up step, 8 with the PTV injection step and 3 factors related with the derivatization step. Therefore, a high number of experiments must be carried out that cannot be conducted in one experimental session and, as a consequence, the experiments of the robustness study must be performed in several sessions or blocks. The procedure to obtain an experimental design suitable for this task works by simultaneously minimizing the joint confidence region for the coefficient estimates and the correlation among them and with the block. In this way, the effect of the factors is not aliased with the block avoiding possible misinterpretations of the effects of the experimental factors on the analytical responses. The developed experimental design is coupled to the PARAFAC2 method, which allows solving some specific problems in chromatography when working with complex matrix such as co-elution of interferents (including silylation artifacts from the derivatization step) and small shifts in the retention time and, besides, the unequivocal identification of the target compounds according to document SANCO/12571/2013.

Experimental design for the optimization of the derivatization reaction in determining chlorophenols and chloroanisoles by headspace-solid-phase microextraction-gas chromatography/mass spectrometry.

The paper shows some tools (its interpretation and usefulness) to optimize a derivatization reaction and to more easily interpret and visualize the effect that some experimental factors exert on several analytical responses of interest when these responses are in conflict. The entire proposed procedure has been applied in the optimization of equilibrium/extraction temperature and extraction time in the acetylation reaction of 2,4,6-trichlorophenol; 2,3,4,6-tetrachlorophenol, pentachlorophenol and 2,4,6-tribromophenol as internal standard (IS) in presence of 2,4,6-trichloroanisole, 2,3,5,6-tetrachloroanisole, pentachloroanisole and 2,4,6-trichloroanisole-d5 as IS. The procedure relies on the second order advantage of PARAFAC (parallel factor analysis) that allows the unequivocal identification and quantification, mandatory according international regulations (in this paper the EU document SANCO/12495/2011), of the acetyl-chlorophenols and chloroanisoles that are determined by means of a HS-SPME-GC/MS automated device. The joint use of a PARAFAC decomposition and a Doehlert design provides the data to fit a response surface for each analyte. With the fitted surfaces, the overall desirability function and the Pareto-optimal front are used to describe the relation between the conditions of the derivatization reaction and the quantity extracted of each analyte. The visualization by using a parallel coordinates plot allows a deeper knowledge about the problem at hand as well as the wise selection of the conditions of the experimental factors for achieving specific goals about the responses. In the optimal experimental conditions (45°C and 25min) the determination by means of an automated HS-SPME-GC/MS system is carried out. By using the regression line fitted between calculated and true concentrations, it has been checked that the procedure has neither proportional nor constant bias. The decision limits, CCa, for probability a of false positive set to 0.05, vary between 0.221 and 0.420µgL(-1).

D-optimal designs and N-way techniques to determine sulfathiazole in milk by molecular fluorescence spectroscopy.

The present work proposes an analytical procedure to determine sulfathiazole in milk by using molecular fluorescence spectroscopy. For this sulfonamide the European Union in Regulation 37/2010 has established a maximum residue limit in milk of 100 µg kg(-1). The study includes the effect of six factors on the recovery of sulfathiazole. The factors are: (i) The one related to the matrix depending on the heat treatment of the milk (UHT, pasteurized); (ii) Those related to the protein precipitation step, namely the ratio between the volume of trichloroacetic acid (TCA) and milk, centrifugation speed and temperature; (iii) Those affecting the derivatization reaction: derivatization time and volume of fluorescamine. To do this, two chemometric tools are used together: a D-optimal design for studying the effect of the factors on the recovery of sulfathiazole, considerably reducing the number of needed experiments; and the second-order property of the PARAFAC (Parallel Factor Analysis) decomposition that avoids the need of fitting a new calibration model each time that the experimental conditions change. It has been found that the type of milk, the TCA:milk ratio and the volume of fluorescamine have significant effect on the response. The rest of factors and interactions are not significant. The best recovery is obtained with UHT milk, 4:6 rate for TCA:milk volumes and 40 µL of fluorescamine. In UHT milk, the mean recovery (n=5) in the optimal conditions is 88.7% (RSD=12.4%). As some non-linear behaviour may occur when using fluorescence spectroscopy, the calibration model that relates the fluorescence spectra with the concentration is computed by a partial least squares regression and a multi-layer feed-forward neural network. In both cases, the proposed procedures have been validated according to Decision 2002/657/EC, concluding that the two are accurate although the calibration model built with the neural network has better figures of merit, the decision limit (CCα) for x(0)=100 µg L(-1) is 103.3 µg L(-1) and the detection capability (CCß) is 106.5 µg L(-1), with the probabilities of false noncompliance (α) and false compliance (ß) equal to 5%.