L.M. Polo

HPLC-Electrochemical detection with graphite-poly (tetrafluoroethylene) electrode Determination of the fungicides thiram and disulfiram

The suitability of composite graphite-poly(tetrafluoroethylene) (Teflon) electrodes as amperometric indicator electrodes in HPLC detection is demonstrated. The determination of the fungicides thiram and disulfiram in the presence of ziram has been chosen as an analytical problem. The optimization of working conditions, such as the choice of the organic solvent used in the mobile phase as well as its percentage, the potential applied to the composite electrode, and the time elapsed between mixing the carbamates and the injection, has been accomplished by using the wall-jet flow-cell configuration. The effect of the acetonitrile percentage used in the mobile phase on the retention of thiram, disulfiram, ziram and phenol was evaluated. Resolution up to the baseline can be achieved with 45% acetonitrile. The sensitivity of the determination of thiram and disulfiram in the presence of a constant concentration of ziram is slightly better when using a wall-jet cell; however, the background current is higher, as well as the baseline noise and the time necessary to achieve stabilization of the baseline before the injection. Lower limits of detection for both fungicides, as well as a better repeatability, were obtained when using a thin-layer flow cell configuration. As an application, the determination of thiram in spiked apple samples, at a level of 0.5 mg thiram kg(-1) apple, has been carried out with a mean recovery of 97 +/- 3% for a significance level of 0.05.

Determination of priority pollutant phenols by isocratic HPLC

SummaryAn isocratic high performance liquid chromatographic method, able to determine ng/l of Priority Pollutant Phenols (PPP) in water, is proposed. A Nucleosil 5 C 18 column and an 30 mM ammonium acetate (pH 5.0): acetonitrile: methanol (56:34:10) eluent are used. Linearity ranges from 5–50 ppm of phenols; variation coefficients are about 4% (n=7) at the 30 ppm level with a 5µl injection. Dual UV detection helps to identify different phenols. The use of an anionic exchanger column and of methanol: hydrochloric acid (9:1) as eluent allows selective preconcentration of priority phenols and recoveries in the range 79–105%.

Determination of methoprotryne and terbutryn by adsorptive stripping voltammetry on the hanging mercury drop electrode

Sensitive methods for the determination of the herbicides methoprotryne and terbutryn at nanomolar levels, by adsorptive stripping voltammetry at a hanging mercury drop electrode, are described. The electrocapillary curves, and cyclic voltammograms, demonstrate the adsorption of these compounds on the mercury electrode. A systematic study of the various experimental parameters that affect the stripping response was carried out by differential pulse voltammetry. The working medium chosen was 0.1 mol l–1 perchloric acid. By using an accumulation potential of –0.70 V and a 180 s accumulation time, the limits of detection were 5.2 × 10–10 and 2.4 × 10–9 mol l–1 for terbutryn and methoprotryne, respectively, and the relative standard deviations (n= 10) were 3.1% and 5.0% at concentration levels of 5.0 × 10–9 mol l–1 terbutryn and 1.0 × 10–8 mol l–1 methoprotryne, respectively. The degree of interference from some other herbicides on the differential pulse stripping signal for methoprotryne and terbutryn was evaluated. Finally, the methods were applied to the determination of terbutryn and methoprotryne in spiked irrigation and tap waters.

Polarographic study of simazine in micellar and emulsified media

Abstract A polarographic study of the reduction of the herbicide simazine in micellar solutions and oil-water emulsions is reported. The anionic surfactant sodium pentanesulphonate was chosen as the most suitable agent for micellar solutions and emulsions. In micellar solutions two reduction peaks were observed in differential-pulse polarography (DPP) below pH 2.0, whereas only one peak was obtained above pH 2.0. Ethyl acetate was chosen as the organic solvent to form simazine emulsions. Unlike in micellar solutions, DP polarograms of simazine in oil-water emulsions showed only one peak even at pH values lower than 2.0, suggesting that pesticide hydrolysis is hindered in the emulsified medium. The limiting current is diffusion-controlled and the electrode process is irreversible. The diffusion coefficient of simazine in emulsified medium was 8.85 × 10 −6 cm 2 s −1 , and the α n a and K ° f values were 0.82 and 8.03 × 10 −14 cm s −1 respectively. Using DPP, simazine can be determined over the concentration ranges 8.0 × 10 −7 −4.0 × 10 −5 mol l −1 . The limit of detection was 2.2 × 10 −7 mol l −1 . The effect of the presence of other herbicides belonging to the s -triazine family on the simazine reduction peak was studied. The method was applied to the determination of simazine in spiked irrigation water with good recoveries.

Determination of organochlorine pesticides in apple samples by differential-pulse polarography in emulsified medium

Abstract The determination of the organochlorine pesticides dieldrin, heptachlor, α-endosulfan, β-endosulfan and endosulfan sulphate and their mixtures in spiked apple samples was carried out in oil-water emulsions formed with ethyl acetate and a mixture of Hyamine 2389 and Triton X-405. Extraction of the pesticides from the apples was accomplished using x-hexane. Clean-up of the extract and fractionation was carried out with a Florisil column, using n-hexane and n-hexane-ethyl acetate (20:1) as eluents. The recoveries for each pesticide individually were about 90% for heptachlor, endosulfan sulphate and both isomers of endosulfan and about 70% for dieldrin at concentration levels of 0.37, 0.93 and 1.27 μg g−1 of apple for heptachlor, dieldrin and endosulfan sulphate, respectively, and 1.22 μg g−1 for each of the endosulfan isomers. Similar results were obtained with binary and ternary mixtures and with a mixture containing all the pesticides studied, indicating the validity of the extraction and separation method developed.

Transient changs of mobile phase in the high-performance liquid chromatographic separation of priority pollutant phenols

A study of the effect of cetyltrimethylammonium bromide on the separation of the eleven priority pollutant phenols is presented. Transient changes in a CTAB mobile phase produced by a sodium laurylsulphate solution plug permit the elution of hydrophobic pentachlorophenol.

Wine yeasts identification by MALDI-TOF MS: Optimization of the preanalytical steps and development of an extensible open-source platform for processing and analysis of an in-house MS database

Saccharomyces cerevisiae is the most important yeast species for the production of wine and other beverages. In addition, nowadays, researchers and winemakers are aware of the influence of non-Saccharomyces in wine aroma complexity. Due to the high microbial diversity associated to several agro-food processes, such as winemaking, developing fast and accurate methods for microbial identification is demanded. In this context, MALDI-TOF MS mass fingerprint provides reliable tool for fast biotyping and classification of microorganisms. However, there is no versatile and standardized method for fungi currently available. In this study, an optimized sample preparation protocol was devised for the biotyping of yeasts of oenological origin. Taking into account that commercially available reference databases comprise almost exclusively clinical microorganisms, most of them bacteria, in the present study a database of yeasts isolated from vineyards and wineries was created, and its accuracy was tested using industrial and laboratory yeast strains. In addition, the implementation of a program for MALDI-TOF MS spectra analysis has been developed as an extensible open-source platform for MALDI data processing and analysis with statistical techniques that has arisen from our previous experience working with MALDI data. The software integrates two R packages for raw MALDI data preprocessing: Continuous Wavelet Transform (CWT)-based algorithm and MassSpecWavelet. One of the advantages of the CWT is that it can be directly applied to a raw spectrum, without prior baseline correction. Mass fingerprints of 109 S. cerevisiae strains and 107 non-Saccharomyces isolates were generated by MALDI-TOF MS upon optimized sample preparation and instrument settings and analyzed for strain, species, and genus-level differentiation. As a reference method, for S. cerevisiae differentiation at strain level, the analysis of the polymorphism in the inter-delta region was chosen. The data revealed that MALDI-TOF MS can be used for the rapid and accurate identification of S. cerevisiae and non-Saccharomyces isolates at genus and species level. However, S. cerevisiae differentiation at strain level was not successfully achieved, and the differentiation among Metschnikowia species was also difficult.

Electroanalytical study of diethyl and dibutyl phthalate in micellar and oil-in-water emulsified media

A polarographic study was carried out of the reduction processes of diethyl and dibutyl phthalate in micellar solutions with the cationic surfactant Hyamine 1622, and in an emulsified medium from aliquots of the phthalates dissolved in a diethyl ether: ethyl acetate (1:9) mixture and Hyamine 1622 as emulsifying agent. The characteristics of the reduction processes in both media were established. The number of electrons involved was higher for the base form of the electroactive species. Using dpp at ΔE=−50 mV, the detection limits obtained in the emulsified medium were 6.7×10−8 and 7.4×10−7 moll−1 for diethyl phthalate and dibutyl phthalate, respectively. Interferences between the two phthalates were studied, and the possibility of carrying out the overall determination of both phthalates was demonstrated. Good results were obtained when applying the polarographic method developed in the emulsified medium to determine diethyl phthalate by dpp in spiked milk after extraction of the analyte with diethyl ether: ethyl acetate (1:9).