Sensitive determination of herbicides in food samples by nonaqueous CE using pressurized liquid extraction

We have developed a method involving extraction with mixtures of solvents under pressure (pressurized liquid extraction (PLE)) for the determination of triazine herbicides in a series of samples from the food industry. The organic extracts obtained were subjected to a clean-up step with SPE, using Oasis MCX sorbents, after which they were analyzed by NACE. Potato was chosen as a representative matrix of horticultural products since it has a high water content. Spiked potato samples were used to optimize extraction conditions. In order to compare the results obtained with NACE, different studies were also conducted using HPLC. The detection limits in NACE were similar to those found with HPLC and were of the order of 10-15 microg/kg, depending on the analyte. Satisfactory results were obtained on applying the method proposed for the potato matrix (PLE with separation by electrophoresis) to other food matrices such as other tubercles, fruits, vegetables and cereals.

Determination of tocopherols and tocotrienols in cereals by pressurized liquid extraction–liquid chromatography–mass spectrometry

A rapid analytical method including pressurized liquid extraction (PLE) and liquid chromatography-electrospray ionisation-mass spectrometry (LC-ESI-MS) has been developed for the determination of tocopherols and tocotrienols in cereals. The pressurized liquid extraction parameters were optimized in order to maximize the extraction efficiency. The use of methanol as extraction solvent at a temperature of 50 degrees C and a pressure of 110 bar, using one cycle of extraction with a static time of 5 min, provided the best results. A good LC separation was achieved using a C(18) column and a solution of 6.0 mM ammonia in methanol/water (97:3, v/v) as the mobile phase at a flow rate of 0.2 mL min(-1). MS coupling with an ESI interface in the negative ion mode was used as the detection technique. In the present work, it is shown that the addition of a base to the mobile phase is required to enhance the ionization of tocopherols and tocotrienols in negative ion mode electrospray ionization. The applicability of the method to cereal samples was confirmed. The reproducibility of the procedure was good, with relative standard deviations in the 6-10% range. The recoveries of added tocopherols from cereal samples ranged from 91 to 109%.

Development of a chemometric correlation technique to estimate acid–base descriptors for cationic acids in non-aqueous media

We propose a procedure for estimating acid-base constants in organic solvents or mixture of solvents from the corresponding pK(a) values in aqueous medium and from certain properties of the organic solvents that characterize them. To accomplish this, we developed and validated a chemometric correlation for the calculation of the acid-base constants of different cationic acids in a broad variety of non-aqueous solvents. The parameters chosen for building the model were as follows: the acid-base constant of the compound in aqueous medium and those corresponding to the polarity-polarizability, basicity and acidity scales of the solvent. The results of the fitting were significant (p<0.01), with a root mean error in cross-validation of 18%, with no overfitting. The prediction of the acid-base constants for an external set of compounds had a mean absolute prediction error value of less than 0.8 pK(a) units.

Determination of endocrine-disrupting compounds in cereals by pressurized liquid extraction and liquid chromatography–mass spectrometry

A sensitive method based on pressurized liquid extraction (PLE) and liquid chromatography-electrospray ionization mass spectrometry (LC-ESI-MS) has been developed for the determination in cereal samples of seven endocrine-disrupting compounds: bisphenol A (BPA), 4-tert-butylbenzoic acid (BBA), 4-nonylphenol (NP), 4-tert-butylphenol (t-BP), 2,4-dichlorophenol (DCP), 2,4,5-trichlorophenol (TCP) and pentachlorophenol (PCP). For the PLE procedure, methanol was selected as the extraction solvent. An experimental design approach was applied to optimize other PLE parameters. The recoveries achieved for the all seven compounds were in the 81-104% range, with relative standard deviations of 4-9%. An additional preconcentration step, based on solid-phase extraction (SPE), after the PLE step proved to be a successful way for obtaining a more sensitive method. The detection limits achieved in corn breakfast cereals were in the 0.003-0.013 microg g(-1) range, except for BPA, with a detection limit of 0.043 microg g(-1), for a sample size of 2.5 g. These values are similar to or even lower than currently legislated limits for pesticides in cereals and cereal-based foodstuffs. We also investigated possible contamination during the experimental process by the target compounds released from purified water, plastics, syringes, peristaltic pump tubes, glassware and other laboratory materials in contact with the samples along the analytical process.

Use of a polar-embedded stationary phase for the separation of tocopherols by CEC

A polar-embedded stationary phase (ULTIMA C18) has been investigated for the separation of alpha-, beta-, gamma- and delta-tocopherols by CEC in comparison with commercially available C(18) and C(30) n-alkyl RPs. The behavior of this stationary phase was tested for different mobile phases based on methanol, ACN, or mixtures thereof and different separation parameters such as retention factors and resolution were evaluated. The main feature of this stationary phase is the improved selectivity for the separation of beta- and gamma-tocopherols (positional isomers) when compared with the pure n-alkyl C(18) material, which was unable to resolve these compounds. Additionally, it is possible to observe a reversal in the elution order of the beta- and gamma-tocopherol isomers with respect to that obtained on the C(30) column. The resulting data indicate that the enhanced selectivity obtained with the polar-embedded stationary phase, with respect to the conventional C(18) material, is due to the participation of both hydrophobic and polar interactions: these latter are of the hydrogen bridge type with the amide group of the polar-embedded stationary phase, which increases the retention of the tocopherols and facilitates the discrimination between the beta- and gamma-isomers. Adequate separation of the four tocopherols was obtained by CEC using the polar-embedded stationary phase and 95:5 v/v methanol/water (5 mM Tris, final concentration) as the mobile phase.

Microemulsion electrokinetic chromatography for the separation of retinol, cholecalciferol, δ-tocopherol and α-tocopherol

A microemulsion electrokinetic chromatographic method was used to separate fat-soluble vitamins. The separation of retinol, cholecalciferol, and delta- and alpha-tocopherol was performed using a microemulsion containing 0.75% (v/v) n-heptane, 30 mM bis(2-ethylhexyl)sodium sulfosuccinate (AOT), 5% (v/v) 1-butanol, 15% (v/v) 1-propanol and 15% (v/v) methanol in 20mM boric acid-sodium borate buffer. The effect of the different microemulsion constituents was studied, including the type and concentration of surfactant, buffer, oil and co-surfactants. The presence of methanol in the microemulsion was found to be necessary to achieve the separation of the tocopherols. Detection was carried out at 200, 265 and 325 nm for the tocopherols, cholecalciferol and retinol, respectively. Calibration curves and precision data were obtained for each analyte. Good linear relationships were found between the analytical signal and the analytes concentration in the 25-500 mg L(-1) for retinol and cholecalciferol, and 25-300 mg L(-1) for tocopherols ranges. The precision of the method afforded relative standard deviations in the 4.0-10% range.

Comparison of a non-aqueous capillary electrophoresis method with high performance liquid chromatography for the determination of herbicides and metabolites in water samples

A method of capillary electrophoresis (CE) for the determination of triazine herbicides and some of their main metabolites in water samples has been developed. The proposed CE method includes an off-line solid-phase extraction (SPE) procedure with LiChrolut EN sorbent coupled to a non-aqueous capillary electrophoresis (NACE) separation with UV detection. The target compounds were the chloro-s-triazines simazine, atrazine, propazine; the methyltio-s-triazines ametryn and prometryn and three main derivatives from the atrazine degradation products; namely, deethylatrazine, deethylhydroxyatrazine and deisopropylhydroxyatrazine. The analytical characteristics of the CE method are reported. The repeatability of the method was studied considering the different steps of the method separately in order to determine the contributions of each step to the total variability of the method. The NACE-UV results are compared with those obtained with a high performance liquid chromatography with UV detection (HPLC-UV) method. The same off-line SPE procedure was applied to both techniques. The results obtained show that both methods afford the same results in the analysis of surface and drinking water samples, with a level of significance regarding the F- and t-tests greater than 0.05 in all the cases. The detection limits in surface water samples were in the 0.04-0.32 microg l(-1) and 0.11-1.2 microg l(-1) ranges for the NACE-UV and HPLC-UV methods, respectively. The recoveries (spiked/found) were significantly 100% in all cases.

Ion-pair association and acid–base equilibria in nonaqueous capillary electrophoresis of weakly basic compounds

CE in nonaqueous media was used to study the migrating behavior of two weakly basic s-triazine pesticides and one of their metabolites. The target pesticides were selected to be representative for each of the two main groups: propazine and deethylatrazine for the chloro-s-triazines group and ametryn for the methylthio-s-triazines group. To elucidate the phenomena involved, systematic studies were carried out in the different organic media studied. Absolute mobilities were determined in 50% v/v methanol (MeOH)/ACN by extrapolation of the effective mobilities to zero ionic strength in the presence of different concentrations of perchloric acid. Conductivity measurements performed in MeOH and 50 and 20% v/v methanol/ACN permitted the evaluation of the associations of the components of the BGE. The effects of ionic strength on the actual mobilities of the compounds were determined in the presence of perchloric acid and SDS in different organic media. Two different ion-pair equilibria were considered: one due to the presence of perchlorate anions present in the BGE and second that from the added dodecyl sulfate anions. Bearing in mind that these weakly basic compounds can exhibit ion-pair and acid-base equilibria, the acid-base and ion-pair parasite reaction coefficients were determined. Finally, the effects of ionic strength, ion-pair interactions and acid-base properties on the effective electrophoretic mobilities of the analytes are discussed.

Pressurized liquid extraction in the analysis of food and biological samples

Originally, the use of the pressurized liquid extraction technique (PLE) was mainly focused on the extraction of environmental pollutants present in soil matrices, sediments, and sewage sludge. However, more recently the distinct advantages of this technique are being exploited in diverse areas, including biology, and the pharmaceutical and food industries. The aim of the present review is to explore recent analytical applications of this extraction technique (PLE) in the extraction of contaminant compounds and matrix components in food and biological samples, placing special emphasis on the strategies followed to obtain a rapid, selective, efficient and reliable extraction process.

Behaviour of carbamate pesticides in gas chromatography and their determination with solid-phase extraction and solid-phase microextraction as preconcentration steps

This work reports a study of the chromatographic behaviour of seven carbamate pesticides (aldicarb, carbetamide, propoxur, carbofuran, carbaryl, methiocarb, and pirimicarb) by gas chromatography-mass spectrometry (GC-MS). Variables such as injector temperature, solvent, injection mode, and the degree of ageing of the chromatographic column were studied. One of the aims of this work was to achieve a controlled decomposition of carbamates by a solid-phase microextraction (SPME) preconcentration step with a polyacrylate fibre in order to obtain reproducible chromatographic signals of the degradation products. Optimisation of the SPME process was accomplished by means of experimental design. Several methods using ultrapure water were developed with different preconcentration configurations: SPME-GC-MS, SPE followed by SPME-GC-MS, and SPE plus GC-MS. For all the pesticides studied, method detection limit (MDL) values below 0.1 microg L-1 were reached in at least one of the proposed configurations.

Determination of triazines and dealkylated and hydroxylated metabolites in river water using a propazine-imprinted polymer

A molecularly imprinted polymer (MIP) obtained by precipitation polymerisation using propazine as template has been employed as sorbent for the solid phase extraction of triazines and some of their hydroxylated and dealkylated metabolites from river water. Three configurations were studied: (a) use of the propazine-MIP as a selective sorbent for the extraction of triazines directly from water; (b) use of mixtures of LiChrolut EN (a polymeric sorbent of styrene divinylbenzene) and propazine-MIP as sorbent, and (c) use of propazine-MIP as a clean-up sorbent for organic extracts obtained in a prior SPE procedure with LiChrolut EN. The former two configurations imply that the analytes pass through the propazine-MIP in aqueous medium, whereas in the latter case the analytes percolate through the propazine-MIP in an organic medium coming from the previous SPE step. Different types of water were tested to assess matrix effects. The analytical characteristics of the three configurations were evaluated.

Development and characterisation of a molecularly imprinted polymer prepared by precipitation polymerisation for the determination of phenylurea herbicides

New materials based on molecularly imprinted polymers (MIPs) have been developed for use as sorbents in solid phase extraction to preconcentrate some urea herbicides. In the preconcentration step, different molecularly imprinted polymers were tested using methacrylic acid (MAA) and 2-(trifluoromethyl)acrylic acid (TFMAA) as functional monomers, and linuron and isoproturon as templates. The best results were obtained when the polymer was synthesised using MAA with isoproturon as template. Another parameter evaluated was the way in which the polymer was obtained. We observed that the imprinted polymers obtained by precipitation displayed a greater capacity to retain the phenylureas. Studies conducted using scanning electron microscopy (SEM) revealed that the bulk polymerisation method is far from ideal owing to the random shape and size distribution of the particles obtained, whereas when polymerisation was carried out in precipitation microspheres were obtained. In order to confirm the interaction between the functional monomer and the template, 1H NMR (CD2Cl2) analyses were conducted. The results obtained suggest that the hydrogen and/or nitrogen of the amino group of the template would be involved in the formation of hydrogen bonds with the functional monomer. The imprinted polymer obtained by precipitation polymerisation with MAA as functional monomer and isoproturon as template can be applied to preconcentrate phenylureas when the sample is dissolved in toluene. The proposed methodology was employed to evaluate polymer selectivity towards humic acids and towards other herbicides.

Determination of weakly acidic endocrine-disrupting compounds by liquid chromatography–mass spectrometry with post-column base addition

A sensitive analytical method based on liquid chromatography-electrospray ionisation mass spectrometry (LC-ESI-MS) has been developed for the determination of seven endocrine-disrupting compounds: 4-n-nonylphenol (NP), 4-tert-butylphenol (t-BP), bisphenol A (BPA), 2,4-dichlorophenol (DCP), 2,4,5-trichlorophenol (TCP), pentachlorophenol (PCP) and 4-tert-butylbenzoic acid (BBA) in water samples. To achieve a good LC separation, acidification of the LC mobile phase was necessary, but this led to MS signal suppression for the less acidic compounds. In order to enhance the sensitivity for these analytes, post-column addition of different bases such as ammonia, trimethylamine, and 1,8-diazabicyclo-(5,4,0)undec-7-en (DBU) was evaluated. The post-column addition of base is proposed here to raise effluent pH, helping in the ionisation process of the compounds with higher pKa values (t-BP, BPA, DCP and NP). The use of DBU, diluted in MeOH, proved to be the most efficient post-column reagent for enhancing the MS signal. The signal-to-noise ratios for t-BP and NP increased by more than 200-fold and 35-fold, respectively, whereas for DCP and BPA an increase of about 10-fold was achieved. This strategy permitted direct determination of the seven compounds at low ppb levels. For application to real water samples, an extraction and preconcentration step using the solid-phase extraction (SPE) technique was carried out. The applicability of three solid-phase materials--Bond Elut C18, and two polymeric sorbents: LiChrolut EN and Oasis HLB--and the optimization of other SPE parameters such as the elution solvent and sample volume used, were studied in order to maximize extraction efficiency. Oasis HLB provided the best results, obtaining--with the proposed SPE procedure--satisfactory percentage recoveries for all compounds (70-110%) with the exception of NP, for which a recovery of 54% was achieved. Application of the whole method, SPE-LC-(ESI)-MS, to natural waters permitted low nanogram-per-liter determination of all seven compounds.

Pressurized liquid extraction prior to liquid chromatography with electrochemical detection for the analysis of vitamin E isomers in seeds and nuts

Pressurized liquid extraction (PLE) was used to isolate tocopherols from seeds and nuts. Very clean extracts were obtained, which were injected directly into the chromatographic system. This enables rapid and simple control in food analysis. The PLE extraction temperature was set at 50 degrees C, with two cycles of extraction, a static time of 5 min, and acetonitrile as the extraction solvent. LC separation was accomplished on a Synergi Hydro-RP column with methanol-water (99.9:0.1, v/v) containing 2.5 mM acetic acid/sodium acetate buffer, as eluent. Coulometric detection, with a porous graphite electrode at +700 mV, was used. The method was successfully applied to the determination of alpha-, (beta + gamma)- and delta-tocopherols in almonds, sunflower seeds, hazelnuts and walnuts. The recoveries were in the 82-110% range. The results were validated with those obtained using sample treatment including alkaline hydrolysis.

Comparison of gas chromatography–mass spectrometry and capillary electrophoresis in analysis of phenolic compounds extracted from solid matrices with pressurized hot water

Self-constructed pressurized hot water extraction (PHWE) equipment was used in dynamic mode to extract spiked phenolic compounds (phenol, 3-methylphenol, 4-chloro-3-methylphenol and 3,4-dichlorophenol) from sea sand and soil. Phenols were analyzed by both gas chromatography-mass spectrometry (GC-MS) and capillary zone electrophoresis (CZE) to compare the techniques and to find out if CZE is a suitable tool for analysis of phenols extracted from environmental matrix. Good recoveries of phenols spiked in sea sand were achieved at all PHWE temperatures (50, 100, 200, 300 C). GC-MS studies showed that phenols were selectively extracted from soil at 50 C but various other compounds (e.g. polyaromatic hydrocarbons) were extracted along with the phenols at 300 degrees C. In the case of CZE, phenols extracted from the soil, at 300 C were separated with good resolution at pH 9.7, and co-extracted compounds did not interfere with the analysis. The analytical values obtained by GC-MS and CZE were generally of similar magnitude.

Prediction of the behaviour of organic pollutants using cloud point extraction

A preconcentration study based on the cloud point phenomenon was carried out for a set of triazine herbicides, three of them chloro-substituted and three of them methylthio-substituted. Concentration factors and recoveries were calculated as function of the percentage of the non-ionic surfactant Triton X-114 employed. From these values, obtained from a cloud point extraction (CPE) procedure, the distribution coefficient between the Triton X-114 micelles and water, Kc, prior to CPE was calculated for each triazine and related to the corresponding octanol-water partition coefficient, Kow. In order to confirm the results obtained with the triazine herbicides, two sets of data from chemically different organic pollutants--organophosporous and chlorophenols--obtained from the literature were assessed, concluding that they display a similar behaviour to that of the triazine herbicides. This can be used to predict the CPE behaviour of other organic pollutants from their octanol-water partition coefficients. The Kc values were compared with the analyte concentration ratio in the surfactant-rich phase and aqueous phase (Ksa) with a view to obtaining a link between the analyte behaviour prior to and after cloud point extraction procedures.

Determination of herbicides, including thermally labile phenylureas, by solid-phase microextraction and gas chromatography-mass spectrometry

A method for the determination of 10 herbicides, including thermally unstable compounds, has been developed. The method uses solid-phase microextraction (SPME) with a polyacrylate fibre. Separation, identification and quantification were accomplished with gas chromatography-mass spectrometry. The herbicides chosen belong to different chemical groups and were alachlor, atrazine, chlorotoluron, diclofop, diflufenicam, ethofumesate, isoproturon, linuron, terbutryn and trifluralin. In the present work we studied the chromatographic behaviour of three phenylureas as a function of the medium and injection mode employed. The compounds generated as a function of the solvent used in direct injection of the phenylureas (ethyl acetate, methanol and methanol-water) and those obtained when injection was accomplished using the polyacrylate fibre were determined. The results allow us to propose a method for the determination of stable and thermally unstable herbicides as long as a preconcentration step involving SPME is carried out. In the proposed method, the limits of detection varied between 0.02 microg/l for ethofumesate and 0.11 microg/l for chlorotoluron. The method was applied to the determination of these herbicides in surface and ground water samples, performing quantification by standard addition calibration. The contents of chlorotoluron and atrazine found were significantly equal to those obtained using HPLC after a preconcentration stepwith styrene-divinylbenzene sorbents.

Solid-phase extraction and sample stacking-micellar electrokinetic capillary chromatography for the determination of multiresidues of herbicides and metabolites

Micellar electrokinetic capillary chromatography (MEKC) with diode array detection was used for the separation of 13 compounds (eight herbicides widely used in agriculture: metribuzin, lenacil, ethofumesate, atrazine, terbutryn, isoproturon, chlorotoluron and linuron, and five of their principal degradation products; namely, deethylatrazine, 2-hydroxyatrazine, deethyl-2-hydroxyatrazine, deisopropylatrazine and 3-chloro-4-methylphenylurea). Peak separation for the 13 analytes was not successful when a single surfactant system was employed, neither sodium dodecyl sulfate (SDS) nor dioctyl sulfosuccinate (DOSS) sodium salt. However, a mixture of these herbicides was successfully separated using a mixed micellar system involving SDS-DOSS in less than 14 min. An application study of an on-line concentration technique for MEKC was carried out to enhance sensitivity. The optimized on-line stacking procedure consisted simply of the addition of 50 mM of sodium chloride to the injection sample, the stacking effect being more intensive as analyte polarity increased. When this stacking procedure was combined with an off-line sample preconcentration step, based on solid-phase extraction, analytes could be detected in the ppb range. The whole method was applied to ultra-high-quality and natural waters. Linear relationships between the analytical signal and the initial analyte concentration were found to be independent of the type of water, except for the more polar analytes for which small differences were observed.

Evolution over time of the agricultural pollution of waters in an area of Salamanca and Zamora (Spain)

A survey of the herbicides present in surface and groundwaters was conducted in 1999 in an area of the provinces of Salamanca and Zamora (Central-Western Spain) to assess the degree of pollution of the agricultural land and seasonal changes in the presence of herbicide residues. Ten sites were sampled and screened for 17 herbicides commonly used in the area; the compounds were ureas, triazines, amides, and others. A previously optimised method involving solid-phase extraction with polymeric cartridges, followed by HPLC with diode array detection, was used to monitor the herbicides. Of the 17 compounds examined, eight were found: chlorotoluron (41% of total detections), terbutryn (21%), atrazine (14%), linuron (7%), isoproturon and metolachlor (5.5% each), lenacil (4%) and metamitron (2%). Of the detections, 66% corresponded to river water samples (three sites). The herbicides found in groundwaters (seven sites) were: chlorotoluron, atrazine, terbutryn, linuron, and isoproturon, all of which are classified as probable or transient leachers in Europe. The temporal evolution of the herbicide content in river waters shows that the observed pollution is a function of time and is related to the application and use of the herbicides. By contrast, the pollution found in groundwaters was lower than that seen in surface waters, except that due to chlorotoluron, which, additionally, remained almost constant throughout the study period.

Determination of herbicides and metabolites by solid-phase extraction and liquid chromatography evaluation of pollution due to herbicides in surface and groundwaters

A procedure based on solid-phase extraction (SPE) has been developed for the simultaneous preconcentration of three widely used herbicides and seven of their most common degradation products. The compounds studied were atrazine and its metabolites, desethylatrazine, desethyldesisopropylatrazine (DEDIA), 2-hydroxyatrazine, desethyl-2-hydroxyatrazine and desisopropyl-2-hydroxyatrazine (DIHA), terbutryne and its metabolite 2-hydroxyterbutylazine, and chlorotoluron and its metabolite 3-chloro-4-methylphenylurea. A HPLC system with diode array detection was used for the separation, identification and quantification of all these analytes. In the SPE preconcentration step, different types of sorbent were studied: C18 on silica and polymeric sorbents (Oasis and LiChrolut EN), the best results being obtained with the styrene-divinylbenzene cartridge and when the elution was performed with methanol and ethyl acetate. The detection limits obtained were between 0.1 microg l(-1) for DIHA and DEDIA and 0.02 microg l(-1) for the other analytes. The method used permitted the determination of these herbicides in drinking water at the concentration levels demanded by current legislation. The proposed method was used to evaluate the presence and evolution with time of these herbicides and their degradation products in samples of surface and ground waters from agricultural zones of the provinces of Salamanca and Zamora (basins of the Rivers Guarefia and Almar), Spain.

Comparative study of separation and determination of triazines by micellar electrokinetic capillary chromatography and nonaqueous capillary electrophoresis: application to residue analysis in natural waters

The separation and determination of a mixture of chloro- and methylthiotriazines in water samples by both micellar electrokinetic capillary chromatography (MEKC) and nonaqueous capillary zone electrophoresis (NA-CZE) were compared. The characteristics of both methods proved to be very similar in terms of separation efficiency and analysis times, but application of these methods for the analysis of triazines in natural waters, with a prior preconcentration step, revealed significant differences. A preconcentration step by solid-phase extraction (SPE) with Oasis HLB cartridges was accomplished for the determination of triazines at sub-ppb levels in drinking and river waters; when NA-CZE was used after this SPE step, electropherograms with fewer interferences and more stable baselines were obtained than when separation was carried out using MEKC. Another aspect related to the application to real samples was the lack of precision encountered upon evaluating the electrophoretic signals generated when using SPE coupled with NA-CZE. Here, we demonstrate the importance of choosing an appropriate internal standard for analyte quantification. It is recommended that a triazine belonging to the same family as that of the triazine to be determined should be used as internal standard.

Surfactant cloud point extraction and preconcentration of organic compounds prior to chromatography and capillary electrophoresis

The use of preconcentration steps based on phase separation by the cloud point technique offers a convenient alternative to more conventional extraction systems. It has been used successfully for the preconcentration of species of widely differing character and nature, such as metal ions, proteins and other biomaterials, or organic compounds of strongly differing polarity. Here we address the most recent analytical applications of this methodology when used as an isolation and trace enrichment step prior to the analysis of organic compounds (polycyclic aromatic hydrocarbons, polychlorinated compounds, pesticides, phenolic derivatives, aromatic amines, vitamins, etc.) via liquid and gas chromatography or capillary electrophoresis.