L.M. Polo-Díez

Stability studies of carbamate pesticides and analysis by gas chromatography with flame ionization and nitrogen–phosphorus detection ☆

As a result of thermal stability studies of carbamate pesticides, a method has been proposed for their direct determination by gas chromatography in the ranges 1-20 and 0.1-1 mg l(-1), using flame ionization and nitrogen-phosphorus detection, respectively. The method allows the determination of propham, propoxur, carbofuran, carbaryl, methiocarb, isopropoxyphenol and naphthol in powdered potato samples. The analytes were previously extracted with a light petroleum-dichloromethane (1:1, v/v) mixture and preconcentred by solid-phase extraction through a C8 cartridge. The recoveries obtained from spiked potato samples (n=4 replicates) at two concentration levels, 10 and 0.5 mg of pesticide per kg of sample, were in the ranges 72-115 and 50-73%, with relative standard deviations of 2-7 and 5-8%, respectively. The detection limits were 50-210 and 41-53 microg kg(-1) with flame ionization and nitrogen-phosphorus detection, respectively, and reaching the maximum residue levels, 0.05 mg kg(-1) for methiocarb and propoxur, set by the Real Decreto 280/1994 (based on the European directive).

Rapid determination of polycyclic aromatic hydrocarbons in tea infusion samples by high-performance liquid chromatography and fluorimetric detection based on solid-phase extraction

This paper describes a method for the determination of PAHs in black, green and decaffeinated tea infusion samples. The method is based on the solid phase extraction of the PAHs using Sep-Pak vac tC-18 cartridges. The PAHs are then eluted from the cartridges with dichloromethane. Quantification and detection are carried out by HPLC with a fluorimetric detector using a program of excitation and emission wavelength pairs. Recoveries at concentration levels in the range 190-1790 ng l-1 were higher than 65% for all PAHs except dibenz[a,h]anthracene, for which it was around 54%. The mean content of PAHs was in the range 28.7-112 ng l-1 in the tea infusions, with relative standard deviations between 2 and 18% (n = 4).

Selection of calibration mixtures and wavelengths for different multivariate calibration methods

Abstract A comparative study to select calibration mixtures and wavelengths in multivariate calibration methods was made. The methods studied were classical least squares (CLS), inverse least squares (ILS), partial least squares (PLS), principal component regression (PCR) and Kaiman filter. For each method the calibration samples were selected from a total random population of 37 calibration standards, taking into account the standard error of prediction (SEP). The selection of analytical wavelengths for each method was carried out using different criteria: the condition number for CLS and Kalman filter methods, the signal-to-noise ( S N ) ratio and the condition number for ILS method, and all the previous criteria and the full spectrum for PCR and PLS methods. The best results were obtained using the condition number as criterion to select the analytical wavelengths. The study has been applied to the spectrophotometric determination of four priority pollutant chlorophenols.

Determination of diethylhexyl phtalate in water by solid phase microextraction coupled to high performance liquid chromatography

Difficulties detected in the determination of the diethylhexylphthalate (DEHP) at trace levels by gas chromatography-mass spectrometry (GC-MS) using SPME, due to its ubiquitous distribution in the environment has been overcome and a new method for the determination of DEHP in drinking water has been proposed. The method is based on solid phase microextraction (SPME) coupled to high-performance liquid chromatography (HPLC). Detection was carried out spectrophotometrically. Calibration graph was linear in the range 10-110 microg/L with a regression coefficient of r(2)=0.998 and a detection limit of 0.6 microg/L. The relative standard deviation was 5 and 2% (n=4) for chromatographic areas and retention times, respectively. The usefulness of the SPME-HPLC technique was confirmed.

A NEW METHOD FOR THE DETERMINATION OF SELECTED PAHs IN COFFEE BREW SAMPLES BY HPLC WITH FLUORIMETRIC DETECTION AND SOLID-PHASE EXTRACTION

A rapid analytical method is proposed for the determination of PAHs (fluoranthene, pyrene, benzo[a]anthracene, chrysene, benzo[e]pyrene, benzo[a]pyrene, dibenzo[a,h]anthracene and benzo[g,h,i]perylene) in coffee brew samples. The method is based on solid phase extraction with Sep-Pak Vac tC-18 cartridges and elution with ethyl ether. The extracts were analyzed by RP-HPLC with a Green PAH column and fluorescence detection under an acetonitrile-water mobile phase gradient. Some PAHs were detected in the samples studied and their mean contents were 1.65–2.87 ng/L coffee brew. The relative standard deviations were in the range 2–13% for four replicates.

Direct determination of monolinuron, linuron and chlorbromuron residues in potato samples by gas chromatography with nitrogen–phosphorus detection ☆

A gas chromatography with nitrogen-phosphorus detection direct method for methoxyurea herbicide determination in powdered potato and fresh potato samples has been developed. A previous study of the thermal stability of the phenylurea herbicides seems to confirm that the ones containing the methoxy radical, i.e. monolinuron, linuron and chlorbromuron, were stable. The herbicides were extracted from the sample through liquid-liquid extraction with dichloromethane-light petroleum (1:1), followed by solid-phase extraction in a C8 cartridge. The recoveries were in the range 84-95% for powdered potato and 86-101% for fresh potato. The RSD values were less than 10%, at 0.1 microg g(-1) concentration level (n = 4) for both types of samples. Detection limits of the method were 7.0-30 ng g(-1) for powdered potato and 6.0-50 ng g(-1) for fresh potato.

Determination of carbamate, phenylurea and phenoxy acid herbicide residues by gas chromatography after potassium tert-butoxide/dimethyl sulphoxide/ethyl iodide derivatization reaction.

The usefulness of the potassium tert-butoxide/dimethyl sulphoxide/ethyl iodide reaction with carbamate and phenylurea herbicides, and its application to phenoxy acids as a way to prevent hazards and toxicity of the sodium hydride/dimethyl sulphoxide/methyl iodide reaction was studied. Using factorial design optimization of this reaction was carried out. A solid-phase extraction method using dimethyl sulphoxide as eluent on-line with this reaction was developed to determine these herbicides in water samples by gas chromatography-mass spectrometry. Relative standard deviation values were lower than 10% for most of the herbicides in multicomponent trace determinations. Detection limits were in the 0.110-0.652 ng L(-1) concentration range. The validity of the method was confirmed by recovery studies from natural water samples.

DETERMINATION OF SELECTED POLYCYCLIC AROMATIC HYDROCARBONS IN TOASTED BREAD BY SUPERCRITICAL FLUID EXTRACTION AND HPLC WITH FLUORIMETRIC DETECTION

A new method for PAHs determination in toasted bread samples is proposed. The method is based on supercritical-fluid extraction, using CO2 and acetonitrile as modifier. The extracted PAHs were collected in 1 mL acetonitrile. Quantitation was carried out by HPLC with fluorimetric detection. A Hypersil Green PAH column, acetonitrile-water gradient mobile phase, and a program of 11 excitation and emission wavelength pairs for fluorimetric detection were used. Recoveries at concentration levels in the range 0.15–3.56 μg/kg bread were close to 100% for all PAHs except fluoranthene, chrysene, and benzo[ghi]perylene. Some PAHs were detected in these samples within the range 0.323–9.40 μg/kg toasted bread; the relative standard deviations were in the range 2 – 12% (n=4).

Preconcentration and flow-injection multivariate determination of priority pollutant chlorophenols

Abstract A combined flow-injection analysis and multivariate calibration method is developed for the simultaneous determination of priority pollutant chlorophenols (2-chlorophenol, 4-chloro-3-methylphenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol and pentachlorophenol) in water. Chlorophenols were preconcentrated in an XAD-4 adsorbent resin. The ion pairs formed with tetrabutylammonium at pH 9.1 were extracted in chloroform and detected in a diode-array spectrophotometer in the wavelength range 200–430 nm. Three multivariate calibration methods, classical least squares (CLS), Kalman filter and partial least squares were used for comparative purposes. Comparison of the standard error of prediction shows significant differences for the determination of 2,4,6-trichlorophenol only, the best results being obtained by the CLS method.

Determination of serotonin and its precursors in chocolate samples by capillary liquid chromatography with mass spectrometry detection

A method for the analysis of serotonin (5-HT) and its precursors, 5-hydroxytryptophan (5-HTP) and l-tryptophan (TP) in chocolate samples by capillary liquid chromatography-mass spectrometry (cLC-MS) has been developed. Optimum chromatographic conditions were established by using a personalized multifactorial experimental design. Finally the cLC separation was achieved through a mixture of acetonitrile and 5mM ammonium formate at pH 4 (3:97, v/v) as mobile phase in gradient elution, setting the injection volume at 10 μL and using pure water as injection solvent for focusing purposes on the head of the capillary column. For extraction of targets in chocolate samples a new, fast and simple procedure based on the use of acidic extraction medium and sonication was developed. Working in selected ion mode (m/z 177 for 5-HT, m/z 205 for l-tryptophan and m/z 221 for 5-HTP) detection limits were between 0.01 and 0.11 μg g(-1) and linearity was in the concentration range of 0.5-25 μg g(-1). Recoveries higher than 76% with RSDs lower than 8% were obtained from spiked samples for all analytes, showing the effectiveness of the proposed method. Serotonin and its precursors were determined in 5 kinds of commonly consumed chocolates with different cocoa contents (70-100%). The highest serotonin content was found in chocolate with a cocoa content of 85% (2.93 μg g(-1)). Regarding l-tryptophan, the highest content of this amino acid (13.27-13.34 μg g(-1)) was found in chocolate samples with the lowest cocoa content (70-85%). 5-Hydroxytryptophan was not detected in any chocolate samples.