Improving sensitivity by large-volume sample stacking combined with sweeping without polarity switching by capillary electrophoresis coupled to photodiode array ultraviolet detection

An easy, simple, and highly efficient on-line preconcentration method for polyphenolic compounds in CE was developed. It combined two on-line concentration techniques, large-volume sample stacking (LVSS) and sweeping. The analytes preconcentration technique was carried out by pressure injection of large-volume sample followed by the EOF as a pump pushing the bulk of low-conductivity sample matrix out of the outlet of the capillary without the electrode polarity switching technique using five polyphenols as the model analytes. Identification and quantification of the analytes were performed by photodiode array UV (PDA) detection. The optimal BGE used for separation and preconcentration was a solution composed of 10 mM borate-90 mM sodium cholate (SC)-40% v/v ethylene glycol, without pH adjustment, the applied voltage was 27.5 kV. Under optimal preconcentration conditions (sample injection 99 s at 0.5 psi), the enhancement in the detection sensitivities of the peak height and peak area of the analytes using the on-line concentration technique was in the range of 18-26- and 23-44-fold comparing with the conventional injection mode (3 s). The detection limits for (-)-epigallocatechin (EGC), (-)-epicatechin (EC), (+)-catechin (C), (-)-epigallocatechin gallate (EGCG), and (-)-epicatechin gallate (ECG) were 4.3, 2.4, 2.2, 2.0, and 1.6 ng/mL, respectively. The five analytes were baseline-separated under the optimum conditions and the experimental results showed that preconcentration was well achieved.     

Comparative study of aqueous and non-aqueous capillary electrophoresis in the separation of halogenated phenolic and bisphenolic compounds in water samples

Capillary zone electrophoresis methods, based on either aqueous and non-aqueous solutions as running buffers and UV spectrophotometric detection, have been developed and optimized for the separation of several halogenated phenolic and bisphenolic compounds, suspected or proved to exhibit hormonal disrupting effects. Both aqueous capillary electrophoresis (CE) and non-aqueous capillary electrophoresis (NACE) methods were suitable for the analysis of compounds under study. The separation of the analytes from other 25 potentially interfering phenolic derivatives was achieved with NACE method. Large-volume sample stacking using the electroosmotic flow pump (LVSEP) was assayed as on-column preconcentration technique for sensitivity enhancement. LVSEP-CE and LVSEP-NACE improved peak heights by 5-26 and 16-330 folds, respectively. To evaluate their applicability, the capillary electrophoresis methods developed were applied to the analysis of water samples, using solid-phase extraction as sample pre-treatment process.     

Determination of components in propolis by capillary electrophoresis and photodiode array detection

The capillary electrophoretic separation of components in propolis, a commonly used natural medicine, was investigated. Optimum conditions for the separation were established. Photodiode-array detection permitted the rapid identification of the components in the samples analysed. The determination of these components, including caffeic acid, dimethylcaffeic acid, isoferulic acid and quercetin, was performed on a commercial propolis sample.     

Ultrasound-assisted extraction of phenolic compounds from strawberries prior to liquid chromatographic separation and photodiode array ultraviolet detection

Ultrasound-assisted extraction was used for the determination of phenolic compounds present in strawberries. The optimization study of the extraction was carried out using spiked samples (100 mg/kg). The sample immersed in an aqueous solution containing hydrochloric acid (0.4 M) was sonicated for 2 min (duty cycle 0.2 s, output amplitude 20% of the nominal amplitude of the converter, applied power 100 W with the probe placed 1cm from the bottom of the water bath and 5 cm from the walls of the precipitate glass). Subsequent separation was carried out by liquid chromatography (LC) with photodiode array UV detection. Calibration curves using the standard addition in green strawberries typically gave linear dynamic ranges of 2-300 mg/l for all analytes; R(2) values exceeded 0.996 in all cases. The method was applied to two types of strawberries to demonstrate the applicability of the proposed method, which is much faster and produces less analyte degradation than methods as solid-liquid, subcritical water and microwave-assisted extraction.     

Analysis of phenolic acids by non-aqueous capillary electrophoresis after electrokinetic supercharging

Electrokinetic supercharging (EKS), a new and powerful on-line preconcentration method for capillary electrophoresis, was utilized in non-aqueous capillary electrophoresis (NACE) to enhance the sensitivity of phenolic acids. The buffer acidity and concentration, leader and terminator length and electrokinetic injection time were optimised, with the optimum conditions being: a background electrolyte of 40 mM Tris-acetic acid (pH 7.9), hydrodynamic injection of 50 mM ammonium chloride (22 s, 0.5 psi) as leader, electrokinetic injection of the sample (180 s, -10 kV), hydrodynamic injection of 20 mM CHES (32 s, 0.5 psi) as terminator, before application of the separation voltage (-25 kV). Under these conditions the sensitivity was enhanced between 1333 and 3440 times when compared to a normal hydrodynamic injection with the sample volume <3% of the capillary volume. Detection limits for the seven phenolic acids were in the range of 0.22-0.51 ng/mL and EKS was found to be 3.6-7.9 times more sensitive than large-volume sample stacking and anion selective exhaustive injection for the same seven phenolic acids.     

毛细管电泳结合压力辅助电动进样测定水样中4种酚类雌激素

在毛细管电泳的胶束电动色谱（MEKC）模式下,采用压力辅助电动进样（PAEKI）的进样方式在线富集4种酚类雌激素（PEs）。对影响PAEKI的进样电压、进样时间等进行考察,并与传统的压力进样比较。结果表明,在最优的PAEKI条件下（-9 kV,0.3 psi（约2.1 kPa）,0.4 min）,4种PEs在7 min内基线分离,线性关系良好,相关系数（r）大于0.9936,己烷雌酚和双烯雌酚的线性范围为0.05~5 mg/L、双酚A和己烯雌酚的线性范围为0.1~10 mg/L;检出限（S/N=3）为0.0071~0.017 mg/L,富集倍数为11~15。使用该MEKC-PAEKI法对自来水和湖水水样进行测定,得到定量限（S/N=10）分别为0.029~0.064 mg/L和0.033~0.079 mg/L;加标回收率为75.6%~110.1%,相对标准偏差（n=5）为4.6%~11.8%。PAEKI不需要使用其他试剂,只需对电泳仪的参数进行适当调整即可实现对分析物的在线富集,简单、快速、自动化程度高。     

Separation and determination of phenolic compounds by capillary electrophoresis with chemiluminescence detection

A methodology for multi-class pesticide determination at trace level in lanolin is presented. Gel permeation chromatography on a Bio-Beads SX-3 column followed by a dual GC chromatographic determination has been developed. The effluent of the analytical column (50% diphenyl–methyl- or 14% cyanopropyl–phenylpolysiloxane) was split into an electron-capture and a nitrogen–phosphorus detection system. The chromatographic system was optimised for 28 pesticides commonly used to control sheep pests and corresponding to organochlorine, organophosphorus and pyretroid classes. Identification has been carried out by gas chromatography coupled to negative chemical ionization mass spectrometry. Recoveries ranged from 72 to 94% and the detection limits from 20 to 97 ng/g depending on the pesticide class, the RSDs were below 10%. Finally, the developed analytical methodology has been successfully applied to the determination of pesticides in several lanolin samples.     

Extraction and determination of phenolic derivatives in water samples by using polyoxyethylene surfactants and liquid chromatography with photodiode array detection

Methodology based on the cloud-point phenomenon was applied to the comparative study of 3 different polyoxyethylene nonionic surfactants in order to extract and preconcentrate a group of phenolic derivatives in water samples; these phenolic compounds, which were determined by liquid chromatography with UV detection, included 11 pollutants given priority by the U.S. Environmental Protection Agency. The optimum conditions for the extraction and preconcentration of phenolic compounds were established for each surfactant. The surfactant that gave the best extraction and preconcentration of the analytes under study was polyoxyethylene 6 lauryl ether (C12E6) with detection limits of <3.5 microg/L for all the phenolic compounds tested. The method was applied to seawater and depurated wastewater samples.     

Analysis of drugs, natural and bioactive compounds containing phenolic groups by capillary electrophoresis coupled to mass spectrometry

This review summarizes the use of capillary electrophoresis (CE) coupled to mass spectrometry (MS) for the analysis of phenolic compounds and its latest developments. Special attention is paid to the different interfaces. The instrumental setups are discussed and demonstrated in a high number of real applications.     

Photochemical reactivity of sulfamethoxazole and other sulfa compounds with photodiode array detection

The photochemical activities of six sulfa compounds [sulfacetamide (CET), sulfadiazine (DIA), sulfaguanidine (GUA), sulfamerazine (MER), sulfamethoxazole (SMX), and sulfamethizole (MET)] under different experimental conditions such as photolysis time, solvent and buffer pH are investigated by photodiode array (PDA) spectrophotometry. With no photolysis, the sulfa drugs CET and DIA show no absorbance at 332 nm and the other compounds only modest absorbance. Upon photolysis for 4 min, absorbance enhancements at 332 nm of three to four times for GUA and MET and 12-15 times for SMX and MER are observed. For CET and DIA after photolysis, the (absorbance) l/mg is now approximately 0.01-0.02 units. Although two pH optima of approximately 3-4 and 7 are noted, the optimum solvent for photolysis is ethanol without pH adjustment. For flow injection (FI) with on-line photolysis and PDA detection, a mobile phase of 100% ethanol with a step flow rate from 0.1 to 1 ml/min is used providing a 4-min reaction time. The FI detection limit for SMX with photolysis at 330 nm is 1 mg/l. The relative standard deviation data (n=4) of seven individual points in a calibration curve from 5 to 150 mg/l are 0-4%. The recovery of SMX from pharmaceutical tablets is 99.7% indicating no interference from trimethoprim which is not photochemically active.     

Optimization of phenolic compounds analysis by capillary electrophoresis

Operational parameters like migration time, temperature, voltage, composition of background electrolyte and content of organic modifier were optimized in CZE for the determination of lignin-like phenolic compounds.The applied background electrolyte buffer consisted of a Na₂B₄O₇, KH₂PO₄ aqueous solution, pH 9.15 using acetonitrile as organic modifier with UV-detection. Compounds, such as acetosyringone, acetovanillone, syringealdehyde, p-hydroxyacetophenone, vanillin, syringic acid, ferulic acid, p-hydroxybenzaldehyde, p-coumaric acid, vanillic acid and p-hydroxybenzoic acid were applied as reference compounds.The quality and quantity of different phenolic compounds obtained upon alkaline CuO oxidation of a commercial humic acid were determined with CZE using ethylvanillin as internal standard.The optimized CZE revealed has being an appropriate method since it is quick, sensitive and quantitative and does not require a time-consuming sample preparation.     

Indirect fluorescence detection of phenolic compounds by capillary electrophoresis on a glass device

A micromachined capillary electrophoresis system has been fabricated on a glass device for the separation and indirect fluorescence detection of phenols. Using this device two phenols viz., 2,4-dichlorophenol and pentachlorophenol, were separated within 12 s compared to under 19 min on a conventional capillary electrophoresis system using direct ultraviolet detection. The precision of the glass device ranged from 12.7%–16.7% compared to 0.42%–4.9% for the conventional system. Both systems showed good linearity in the concentration range of 0.8– 6.38 mM for the glass device and 5–130 μM for the conventional system. The relationship between temperature and high voltage with baseline drift was also investigated. These results provide a foundation for the development of a miniaturised chemical analysis system for the on-line analysis of phenols in water. Received: 24 January 2000 / Revised: 27 March 2000 / Accepted: 29 March 2000     

Indirect fluorescence detection of phenolic compounds by capillary electrophoresis on a glass device

A micromachined capillary electrophoresis system has been fabricated on a glass device for the separation and indirect fluorescence detection of phenols. Using this device two phenols viz., 2,4-dichlorophenol and pentachlorophenol, were separated within 12 s compared to under 19 min on a conventional capillary electrophoresis system using direct ultraviolet detection. The precision of the glass device ranged from 12.7%-16.7% compared to 0.42%-4.9% for the conventional system. Both systems showed good linearity in the concentration range of 0.8-6.38 mM for the glass device and 5-130 microM for the conventional system. The relationship between temperature and high voltage with baseline drift was also investigated. These results provide a foundation for the development of a miniaturised chemical analysis system for the on-line analysis of phenols in water.     

Analysis of methylglyoxal in water and biological matrices by capillary zone electrophoresis with diode array detection

We describe a new method for the determination of methylglyoxal in water and biological matrices, using o-phenylenediamine as derivatizing agent and solid-phase extraction followed by capillary zone electrophoresis with diode array detection. 25 mM sodium phosphate running buffers at pH 2.2, 30 kV, and 25 degrees C allowed the best instrumental conditions for the optimum separation of methylglyoxal in a suitable analytical time (< 10 min), using an uncoated fused-silica capillary of 75 microm inner diameter and an effective length of 45.1 cm with an extended light path and the wavelength set to 200 nm. Under optimized instrumental conditions, good reproducibility of the migration time (< 1.1%), precision (< 5%), an excellent linear dynamic range from 0.1 to 3.6 mg/L (r(2) = 0.9997), and low limits of detection (7.2 microg/L) were obtained for methylglyoxal measurements, using the internal standard methodology. Assays on laboratory-spiked tap and ground water samples allowed a remarkable accuracy, presenting yields of 95.0 +/- 4.3 and 94.0 +/- 1.1%, respectively, and good performance to determine methylglyoxal in beer and yeast cells suspensions matrices was also obtained at trace level. The present methodology is a cost-effective alternative for routine quality control analysis, showing to be reliable, sensitive, and with a low sample volume requirement to monitor methylglyoxal in water and biological matrices.     

Solid-phase microextraction coupled to high-performance liquid chromatography to determine phenolic compounds in water samples

Solid-phase microextraction (SPME) coupled to high-performance liquid chromatography (HPLC) with ultraviolet (UV) and electrochemical detection (ED) has been applied to determine 11 phenolic compounds considered priority pollutants by the US Environmental Protection Agency. 85 microm polyacrylate fibers were used to extract the analytes from the aqueous samples. Two different designs of the liquid chromatograph were compared in combination with SPME. Dynamic and static modes of desorption in both HPLC designs were compared and the variables affecting both absorption and desorption processes in SPME-HPLC were optimized. Static desorption in both HPLC systems showed better recoveries for the phenolic compounds. The performance of the SPME-HPLC-UV-ED method was evaluated with river water and wastewater samples. The method enabled the determination of phenolic compounds at low levels in these water samples.     

Determination of methylmercury and phenylmercury in water samples by liquid-liquid-liquid microextraction coupled with capillary electrophoresis

A novel method for determination of methylmercury (MeHg) and phenylmercury (PhHg) by liquid-liquid-liquid microextraction (LLLME) coupled with capillary electrophoresis (CE) with ultraviolet (UV) technique was developed. The method based on MeHg and PhHg was complexed with 1-(2-pyridylazo)-2-naphthol (PAN) to form hydrophobic complexes. When the sample solution was stirred, analytes were extracted into the organic layer (200 microL toluene) and back-extracted simultaneously into the 4.0 microL 0.1% (w/v) l-cysteine microdrop. The factors affecting on the LLLME of two mercury species, including sample pH, complex reagent concentration, extraction time, volume of organic solvent, stirring rate and phase volume ratio, were investigated. Under the optimized conditions, the detection limits (S/N=3) of MeHg and PhHg were 0.94 and 0.43 ngmL(-1) (as Hg), respectively. The precisions (RSDs, c=10 ngmL(-1), n=7) were in the range of 3.3-3.4% for migration time, 6.1-7.2% for peak area response, and 6.7-7.5% for peak height response for the two mercury species. The enrichment factors of 324 for MeHg and 210 for PhHg were obtained with 40 min LLLME. The developed method was successfully applied to the determination of trace amounts of MeHg and PhHg in water samples.     

Ionic liquid-based dispersive liquid-liquid microextraction with back-extraction coupled with capillary electrophoresis to determine phenolic compounds

Ionic liquid (IL) based dispersive liquid-liquid microextraction (DLLME) with back-extraction coupled with capillary electrophoresis ultraviolet detection was developed to determine four phenolic compounds (bisphenol-A, β-naphthol, α-naphthol, 2, 4-dichlorophenol) in aqueous cosmetics. The developed method was used to preconcentrate and clean up the four phenolic compounds including two steps. The analytes were transferred into room temperature ionic liquid (1-octyl-3-methylimidazolium hexafluorophosphate, [C(8) MIM][PF(6) ]) rich-phase in the first step. In the second step, the analytes were back-extracted into the alkaline aqueous phase. The effects of extraction parameters, such as type and volume of extraction solvent, type and volume of disperser, extraction and centrifugal time, sample pH, salt addition, and concentration and volume of NaOH in back-extraction were investigated. Under the optimal experimental conditions, the preconcentration factors were 60.1 for bisphenol-A, 52.7 for β-naphthol, 49.2 for α-naphthol, and 18.0 for 2, 4-dichlorophenol. The limits of detection for bisphenol-A, β-naphthol, α-naphthol and 2, 4-dichlorophenol were 5, 5, 8, and 100 ng mL(-1), respectively. Four kinds of aqueous cosmetics including toner, soften lotion, make-up remover, and perfume were analyzed and yielded recoveries ranging from 81.6% to 119.4%. The main advantages of the proposed method are quick, easy, cheap, and effective.     

Electrochemical detection of phenolic compounds using cylindrical carbon-ink electrodes and microchip capillary electrophoresis

A simple method to fabricate cylindrical carbon electrodes for use in capillary electrophoresis (CE) microchips is described. The electrodes were fabricated using a metallic wire coated with carbon ink. Several experimental variables were studied in order to establish the best conditions to fabricate the electrode. Finally, the electrodes were integrated in a poly(dimethylsiloxane) microchip and used for the analysis of phenolic compounds. Using the optimum conditions, the analysis of a mixture of dopamine, epinephrine, catechol, and 4-aminophenol was achieved in less than 240 s, showing good linear responses (R² = 0.999) in the 0.1-190 μM range, and limits of detection (without the use of stacking or a decoupler) of 140 and 105 nM for dopamine and epinephrine, respectively.     

非水介质毛细管电泳电导法检测盐酸胺碘酮

采用非水介质毛细管电泳电导检测法对盐酸胺碘酮进行检测。探讨了缓冲溶液的种类、pH和浓度、分离电压、进样时间、进样高度等因素对检测效果的影响,建立了测定盐酸胺碘酮的新方法。用乙醇作为非水介质,在30mmol L三羟甲基氨基甲烷 15mmol L柠檬酸(pH6.90)运行缓冲溶液中,盐酸胺碘酮在5～200mg L范围内的线性回归方程为y=74.94x-7.83,r=0 999。检出限(S N=3)为0.5mg L,样品回收率为98.9%。适用于含盐酸胺碘酮的药剂的分析。