On-line concentration of s-triazine herbicides in micellar electrokinetic chromatography using a cationic surfactant

Optimization and validation of the kinetic spectrophotometric method for quantitative determination of the pesticide atrazine and its application in infant formulae and cereal-based baby food

BACKGROUND

Pesticides are potentially toxic to humans and can produce both acute and chronic health effects, depending on the quantity and the ways in which a person is exposed. Exposure to pesticides can cause serious health problems. Infants and young children are particularly sensitive to these contaminants because their brains and organ systems are not fully developed. For this reason, it is important to determine the quantities of pesticides in baby food.

RESULTS

The aim of this study was to develop a kinetic-spectrophotometric method for atrazine determination and to apply it to determine pesticide in baby-food samples, using solid-phase extraction (SPE) followed by the kinetic-spectrophotometric method and the high-performance liquid chromatography (HPLC) method. This method is based on the inhibition effect of atrazine (the oxidation of sulfanilic acid (SA) by hydrogen peroxide in the alkaline medium in the presence of the Co²⁺ ion). Under the experimental conditions used, atrazine showed a linear dynamic range of 0.5 to 5.0 μg mL^-1 , and from 5.0 to 70.00 μg mL^-1 with relative standard deviations (RSD) from 1.91% to 9.41%. The limit of detection and the limit of quantification were 0.074 and 0.225 μg mL^-1 , respectively. The kinetic method was successfully applied to determine the atrazine concentration in spiked samples after SPE of samples. High-performance liquid chromatography was used to verify the results.

CONCLUSION

The proposed method is highly sensitive, simple, easy, requires cheap reagents, and leads to good recovery levels. It is linear, precise, and accurate. It can be used successfully for the routine analysis of atrazine in infant formulae and cereal-based food samples. © 2019 Society of Chemical Industry.

On-line concentration of environmental pollutant samples by using capillary electrophoresis

This chapter reviews the theory and methodological developments of on-line concentration techniques for the determination of environmental pollutant samples, such as organic and inorganic compounds in capillary zone electrophoresis (CZE) and also in micellar electrokinetic chromatography (MEKC). Topics covered include a variety of on-line preconcentration strategies, which are now generally referred to as sample stacking and sweeping techniques. For each technique, surveyed methods are tabulated in order to assist in method selection. Innovative applications of sample stacking and sweeping to advanced environmental research are also emphasized. In addition, other comparative on-line concentration methods for environmental samples, namely, isotachophoretic stacking and anion and cation selective exhaustive injection-sweeping are briefly discussed.

Flow manipulation for sweeping with a cationic surfactant in microchip capillary electrophoresis

Flow manipulation in sweeping microchip capillary electrophoresis (CE) is complicated by the free liquid communication between channels at the intersection, especially when the electroosmotic flows are mismatched in the main channel. Sweeping in traditional CE with cationic micelles is an effective way to concentrate anionic analytes. However, it is a challenge to transfer this method onto microchip CE because the dynamic coating process on capillary walls by cationic surfactants is interrupted when the sample solution free of surfactants is introduced into the microchip channels. This situation presents a difficulty in the sample loading, injection and dispensing processes. By adding surfactant at a concentration around the critical micelle concentration and by properly designing the voltage configuration, the flows in a microchip were effectively manipulated and this sweeping method was successfully moved to microchip CE using tetradecyltrimethylammonium bromide (TTAB). The sweeping effect of cationic surfactant in the sample solution was discussed theoretically and studied experimentally in traditional CE. The flows in a microchip were monitored with fluorescence imaging, and the injection and sweeping processes were studied by locating the detection point along the separation channel. A detection enhancement of up to 500-fold was achieved for 5-carboxyfluorescein.

Unlimited-volume electrokinetic stacking injection in sweeping capillary electrophoresis using a cationic surfactant

Sweeping is an effective and convenient way for online sample preconcentration in micellar electrokinetic chromatography. The usual procedure includes a hydrodynamic injection step carried out by applying pressure to the sample vial followed by the subsequent sweeping and separation processes. The injected sample volume is limited by the dimensions of the capillary because a part of the capillary has to be left free of sample solution for the subsequent sweeping and separation steps. In addition, when a short capillary, such as 4-10 cm, is used for sweeping, the injected sample volume is small even if the entire capillary is filled with sample solution. To solve this problem, an electrokinetic stacking injection (EKSI) scheme was developed by using a cationic surfactant, dodecyltrimethylammonium bromide, for sweeping in capillary electrophoresis. An experimental model was proposed, and the entire process was theoretically analyzed. According to the theoretical discussion, the optimal conditions for two model analytes, 5-carboxyfluorescein (5-FAM) and sodium fluorescein (FL), were experimentally determined. The injected sample plug lengths for 5-FAM and FL under 20.1 kV for 60 min were experimentally estimated as 836 and 729 cm, corresponding to 28- and 24-fold the effective capillary length, respectively. The EKSI scheme resulted in increased detection factors for 5-FAM and FL of 4.5 x 10(3) and 4.0 x 10(3) using 60-min injection relative to a traditional pressure injection.

On-line sample preconcentration by sweeping with dodecyltrimethylammonium bromide in capillary zone electrophoresis

On-line sample preconcentration of oligonucleotides with a new sweeping carrier was developed by using dodecyltrimethylammonium bromide (DTAB) below the critical micelle concentration (CMC). The sweeping results with DTAB below and above the CMC were compared. The use of DTAB below the CMC benefits the preconcentration of the oligonucleotides, while the use of DTAB above the CMC is good for hydrophobic small molecules. The factors affecting the sweeping results were optimized and this method was evaluated by constructing calibration curves for thrombin aptamers. The sweeping scheme produced a 112-fold sensitivity enhancement for the oligonucleotides relative to that run in a running buffer without DTAB. The sweeping method developed here can be a good reinforcement of the preconcentration scheme by sweeping when less-hydrophobic analytes or large negatively-charged molecules need to be preconcentrated.

Dynamic pH junction technique for on-line preconcentration of peptides in capillary electrophoresis

A method based on the presence of a dynamic pH junction within the capillary to induce band narrowing for enhanced detection sensitivity for some peptides is presented. This technique is predicated on a sharp reduction in an analyte's migration velocity following a reversal of its electrophoretic direction from the acidic sample zone to the basic BGS zone. Larger-than-usual injection volumes of samples in relatively high-conductivity matrices were enabled, without degrading peak shape, resolution and efficiency. The size of the original sample plug was reduced by as much as 38-fold, and improvement in detector response in terms of peak height by as much as 124-fold was obtained. The effects of pH and concentration of the sample matrix, and the length of sample injection on the efficiency of the technique are discussed.

Analysis of pesticides in soy milk combining solid-phase extraction and capillary electrophoresis-mass spectrometry

In this work, the determination of a group of triazolopyrimidine sulfoanilide herbicides (cloransulam-methyl, metosulam, flumetsulam, florasulam, and diclosulam) in soy milk by capillary electrophoresis-mass spectrometry (CE-MS) is presented. The main electrospray interface (ESI) parameters (nebulizer pressure, dry gas flow rate, dry gas temperature, and composition of the sheath liquid) are optimized using a central composite design. To increase the sensitivity of the CE-MS method, an off-line sample preconcentration procedure based on solid-phase extraction (SPE) is combined with an on-line stacking procedure (i.e. normal stacking mode, NSM). Samples could be injected for up to 100 s, providing limits of detection (LODs) down to 74 microg/L, i.e., at the low ppb level, with relative standard deviation values (RSD,%) between 3.8% and 6.4% for peak areas on the same day, and between 6.5% and 8.1% on three different days. The usefulness of the optimized SPE-NSM-CE-MS procedure is demonstrated through the sensitive quantification of the selected pesticides in soy milk samples.

On-line sample concentration techniques in capillary electrophoresis: velocity gradient techniques and sample concentration techniques for biomolecules

Methods with a high sensitivity and high separation efficiency are goals in analytical separation techniques. On-line sample concentration techniques in capillary electrophoresis (CE) separations have rapidly grown in popularity over the past few years because they achieve this goal. This review describes the methodology and theory associated with a number of different techniques, including electrokinetic and chromatographic methods. For small molecules, several on-line concentration methods based on velocity gradient techniques are described, in which the electrophoretic velocities of the analyte molecules are manipulated by field amplification, sweeping, and isotachophoretic migration, resulting in the on-line concentration of the analyte zones. In addition, the on-line concentration methods for macromolecules are described, since the techniques used for macromolecules (DNAs and proteins), are different from those for small molecules, with respect to either mechanism or methodology. Recent studies relating to this topic are also discussed, including electrophoretic and chromatographic techniques on capillary or microchip.

Pesticide analysis by capillary electrophoresis

In this work, a critical and updated revision of the current situation of the analysis of pesticides by Capillary Electrophoresis (CE) is presented. The review has been written in two main sections. The first one presents a thorough revision of the various offline and on-line sample preconcentration procedures that have been used in conjunction with CE to analyze these compounds. The second part reviews the various detection strategies (i.e., UV, LIF, MS, and electrochemical) and CE modes that have been applied to the analysis of pesticides. Future trends that can be expected from this hot research area are also discussed.

Polymer-brush stationary phases for open-tubular capillary electrochromatography

Synthesis of poly(2-hydroxyethyl methacrylate) (PHEMA) brushes from the inside of silica capillaries by surface-initiated atom transfer radical polymerization (ATRP) yields unique stationary phases for open-tubular capillary electrochromatography (OT-CEC). Although PHEMA brushes have only a small effect on the separation of a set of phenols and anilines, derivatization of PHEMA with ethylenediamine (en) allows baseline resolution of several anilines that co-elute from bare silica capillaries. Derivatization of PHEMA with octanoyl chloride (C8-PHEMA films) affords even better resolution in the separation of a series of phenols and anilines. Increasing the thickness of C8-PHEMA coatings by a factor of 2 enhances resolution for several solute pairs, presumably because of an increase in the effective stationary phase to mobile phase volume ratio. Thus, this work demonstrates that thick polymer brushes provide a tunable stationary phase with a much larger phase ratio than is available from monolayer wall coatings. Through appropriate choice of derivatizing reagents, these polymer brushes should allow separation of a wide range of neutral molecules as well as compounds with similar electrophoretic mobilities.

Investigation of preconcentration strategies for the trace analysis of multi-residue pesticides in real samples by capillary electrophoresis

In this work, on-line preconcentration strategies were investigated for the multi-residue analysis of pesticides in drinking water and vegetables using micellar electrokinetic chromatography. Among the on-line strategies, sweeping and stacking with reverse migration of micelles (SRMM), with and without the insertion of a plug of water before sample injection, were contrasted. A new version of SRMM was also introduced. The modification consisted of momentarily applying a positive voltage at the inlet vial right after sample has been injected, increasing the efficiency by which the analytes are captured. Nine pesticides from different classes, carbendazim (benzimidazole), simazine, atrazine, propazine and ametryn (triazine), diuron and linuron (urea), carbaryl and propoxur (carbamate), were baseline separated in less than 6 min with a electrolyte composed of 20 mmol l(-1) phosphate buffer at pH 2.5, containing 25 mmol l(-1) sodium dodecyl sulfate and 10% methanol. Limits of detection (LODs) in the order of 2-46 microg l(-1) for the pesticides under investigation were obtained solely using the on-line strategies. Enrichment factors of 3-18-fold were obtained. These factors were computed as the improvement of the concentration LODs with respect to the reference condition (injection of 10 s at 2.5 kPa pressure). The proposed methodologies were applied to the analysis of pesticides in complex matrices such as carrot extracts where the detection of 2.5 microg l(-1) was illustrated. By combining off-line solid-phase extraction and the proposed on-line strategies, the detection of pesticides in drinking water at the 0.1 microg l(-1) level was conceived.

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.

On-line sample preconcentration techniques in micellar electrokinetic chromatography

This review provides an overview as well as a practical understanding of on-line sample concentration techniques in micellar electrokinetic chromatography (MEKC). MEKC as well as other capillary electrophoretic modes suffer from low concentration sensitivity due to minute sample volume and limited optical pathlength for on-capillary photometric detection. Two on-line sample preconcentration techniques, sample stacking and sweeping are known to be effective techniques for enhancement of the concentration sensitivity in MEKC. Sample stacking occurs as ions cross a boundary that separates regions of the high electric field sample zone and the low electric field background solution zone. The difference in migration velocity of pseudostationary phases within the two zones is the key to achieving the focusing effect. Sweeping is defined as the picking and accumulating of analytes by the pseudostationary phase that penetrates the sample zone devoid of pseudostationary phase. In this review, several examples of the sample stacking and sweeping under different experimental conditions are given, besides many references to applications.

On-line sample preconcentration in micellar electrokinetic chromatography using ion-pair reagents

To improve the detection sensitivity of some aromatic carboxylic acids and naphthalenesulfonic acids, the use of ion-pair reagents was examined in sweeping micellar electrokinetic chromatography (MEKC) with an anionic sodium dodecyl sulfate (SDS) micelle. Tetraalkylammonium (TAA) salts were used as ion-pair reagents to improve the sweeping efficiency. The effects of the alkyl chain length of the TAA groups and the TAA salt concentration on sweeping were examined. Under optimized conditions, about 400-fold enhancement in detection sensitivity was obtained in terms of peak heights by addition of ion-pair reagents in sweeping MEKC. This value was about 10 times greater than that obtainable by the SDS micelle used alone.

Electrophoretic analysis of amines using reversed-phase, reversed-polarity, head-column field-amplified sample stacking and laser-induced fluorescence detection

This paper describes the use of reversed-phase, reversed-polarity head-column field-amplified sample stacking (HCFASS) for on-line sample concentration in conventional capillary electrophoresis. The effective stacking efficiency was determined as a function of sodium hydroxide concentration in the sample matrix. Results concur with theoretical predictions where stacking efficiency depends on the conductivity (electric field strength) and electrophoretic mobility in the sample matrix solution. Fluorescein isothiocyanate-derivatized aniline and 2,4-dimethylaniline were dissolved in sodium hydroxide (800 microM), separated in a phosphate running buffer (0.05 M, pH 9.0) and detected utilising laser-induced fluorescence. The use of reversed-phase, reversed-polarity HCFASS with laser-induced fluorescence detection yielded sensitivity improvements with respect to normal injection schemes in excess of three orders of magnitude, and a limit of detection as low as 10(-13) M.

Extraction of phyllanthusols A and B from Phyllanthus acidus and analysis by capillary electrophoresis

Extracts of roots Phyllanthus acidus were examined by free zone capillary electrophoresis, micellar electrokinetic chromatography (MEKC), and MEKC using the sweeping technique which involves application of a negative potential to the inlet end of the capillary and very much longer than conventional injection times. The latter technique, using a buffer of 50 mM sodium dihydrogen phosphate (pH 2) containing 80 mM sodium dodecylsulphate and 30% methanol was found to allow complete resolution of the active constituents of P. acidus, phyllanthusols A and B, from each other and from other extracted components in under 30 min. Several other components could be detected when hydrodynamic injection times of 500 s were used. The separation, combined with an appropriate extraction procedure and using an internal standard of proguanil, permitted quantification of both phyllanthusols. Calibrations were linear over the range 2-8 micrograms/mL for phyllanthusol A, and 1-4 micrograms/mL for phyllanthusol B. Within-day and day-to-day repeatability RSDs were below 10%, and the precision of extraction RSD was around 14%. The limits of quantification and detection were 0.55 and 0.24 microgram/mL, respectively.

Sweeping: concentration mechanism and applications to high-sensitivity analysis in capillary electrophoresis

Sweeping in capillary electrophoresis (CE) involves the interaction of a pseudostationary phase (PS) in the separation solution and a sample in the matrix that is free of the PS used. The PS includes not only the PSs employed in electrokinetic chromatography, but also complexation reagents such as borate. The sample matrix could have a lower, similar, or higher conductance than the separation solution. Thus, the basic condition for sweeping is a sample matrix free of the additive. The accumulation of analyte molecules during the interaction makes this interesting phenomenon very useful as an on-line preconcentration method for CE. Preconcentration occurs due to chromatographic partitioning, complexation, or any interaction between analytes and PS. Contact between analyte and PS is facilitated by the action of electrophoresis and is independent of electroosmosis. The analyte, PS, or both should have electrophoretic velocities when an electric field is applied. The extent of preconcentration is dictated by the strength of the interaction involved. From tens to several thousand-fold improvements in detector response for many neutral and charged analytes have been achieved with this technique, suggesting sweeping as a general approach to on-line preconcentration in CE. The mechanism and applications of the sweeping phenomenon under different experimental conditions are discussed in this review, with particular emphasis on a better understanding of the sweeping mechanism under reduced electric field (high conductivity) in the sample zone.

Separation and on-line preconcentration by sweeping of charged analytes in electrokinetic chromatography with nonionic micelles

The separation and on-line preconcentration by sweeping of charged analytes in electrokinetic chromatography using a neutral pseudostationary phase is described. Under neutral or basic conditions, the electrophoretic migration of anionic analytes towards the anode is overcome by a high, cathode-directed electroosmotic flow; hence, they experience net migration towards the cathode, and the system is run at positive polarity mode. The separation and the retention factor, k, are dependent on both the analyte's electrophoretic mobility and its interaction with the pseudostationary phase. The versatility of the sweeping mechanism is then shown in this system. The charged analyte, prepared in a matrix free of the pseudostationary phase, penetrates the pseudostationary phase zone upon application of voltage. Analyte molecules are consequently accumulated and concentrated. As a demonstration, the separation and preconcentration of phenol derivatives using nonionic surfactants of the alkyl polyoxyethylene ether type (Brij 35 and Brij 58) yielded peak height enhancements up to 100-fold. The efficiency of sample stacking was also found to be improved with the use of a high viscosity background solution.