Pesticide analysis by micellar electrokinetic capillary chromatography

On-capillary sample concentration using sample stacking for the improvement of detection limits for various pesticides separated by micellar electrokinetic capillary chromatography was examined. The dependence of the stacking on different parameters was investigated. An approximately 30-fold preconcentration was achieved by applying sample stacking. Employing a two-step enrichment process (solid-phase extraction combined with sample stacking), detection limits were improved and the sample volume for SPE was reduced. In addition, the total time for the analysis was considerably reduced. Detection limits were between 0.01 and 0.1 ng/ml under these enrichment conditions.     

Combination of solid-phase extraction and large-volume stacking with polarity switching in micellar electrokinetic capillary chromatography for the determination of traces of nonsteroidal anti-inflammatory drugs in saliva

A reliable MEKC method for the identification and quantitation of traces of the nonsteroidal anti-inflammatory drugs (NSAIDs) ketoprofen, fenbufen and indomethacin in saliva is proposed. Using CE to analyze biological samples often requires suppressing the interferences and peak broadening typically resulting from high-conductivity sample matrices. We addressed this problem by using Microcon, a centrifugal filter device, to reduce the viscosity of saliva and exclude most higher-molecular-mass substances. This initial pretreatment was followed by the combined used of off-line SPE to isolate and concentrate the analytes, and large-volume stacking with polarity switching (LVSS) in the capillary. These two preconcentration steps allow the determination of NSAIDs at concentrations above 0.1 microg/L; therefore, the proposed SPE/LVSS/MEKC method affords a 500-fold sensitivity enhancement relative to conventional CE injection. The LODs obtained afford the determination of NSAIDs in saliva, where analytes can be present at the microgram-per-liter level.     

Method development for the determination of teicoplanin in patient serum by solid phase extraction and micellar electrokinetic chromatography

In-hospital deaths caused by the infection of methicillin-resistant Staphylococcus aureus (MRSA) are on the increase worldwide. Teicoplanin is a potent glycopeptide antibiotic against MRSA. A rapid and cost-saving micellar electrokinetic chromatography (MEKC) method combined with solid phase extraction (SPE) was developed and then validated to quantify teicoplanin in patient serum in this work. The method includes the following steps: (1) pretreatment of the serum samples with 10 M urea to denature proteins, (2) application of SPE by using an OASIS HLB cartridge to clean up and concentrate the serum samples, and (3) use of MEKC for sample analysis. Under the optimized conditions, the SPE recovery of teicoplanin is higher than 90%. The six major components of teicoplanin could be baseline-separated from one another and endogenous materials in 12 min with a background electrolyte composed of 20 mM sodium tetraborate buffer pH 8.8, 40 mM sodium dodecyl sulfate, and 11% (v/v) ACN. The relative standard deviation (R.S.D.) of the peak area ratios for method repeatability (n = 6) and intermediate precision (inter-day, n = 3) were found to be lower than 4.18% and 5.30%, respectively. The calibration curves were linear between the chromatographic response and total teicoplanin concentration over the range of 5 μg/mL to 55 μg/mL. Limit of detection (LOD) for each of the six components was found to be lower than 0.06 μg/mL. Pearson’s correlation revealed that a good correlation (r = 0.98) was obtained between the SPE-MEKC method and the fluorescence polarization immunoassay (FPIA) method. The developed method can be used to quantitatively determine serum teicoplanin concentration in patients for dose monitoring and clinical research.     

Trace analysis of rapamycin in human blood by micellar electrokinetic chromatography

A capillary electrophoretic method with UV detection at 278 nm has been developed for analysis of the immunosuppressant rapamycin (sirolimus) in human blood at low microgram per liter levels. Separation has been achieved in an acidic carrier electrolyte containing sodium dodecylsulfate and 30% (v/v) acetonitrile. For sample clean-up and preconcentration, an off-line solid-phase extraction step using a silica-based reversed-phase material and an on-capillary focussing technique were employed. The latter allows the injection of increased sample volumes without excessive band broadening. Although this new method is less sensitive than existing liquid chromatographic procedures combined with mass spectrometry, it is fully suited to routine analysis of rapamycin in blood from patients treated with this drug. Last but not least the low costs make it an attractive alternative to established methods.     

Micelle to trapping solution stacking in micellar electrokinetic chromatography

An analytical strategy micelle to trapping solution stacking (MSS) was developed in acidic buffer in micellar electrokinetic chromatography (MEKC). The stacking mechanism is based on the transport, release, capturing of molecules bound to micelle carriers that are made to collapse into trapping solution (TS) to serve as the medium to contain and stacking the analytes. Tetrandrine and fangchinoline were selected as model mixture using sodium dodecyl sulfate (SDS) micelles as carrier to demonstrate this stacking method. The experiments by MSS-MEKC were carried out and further compared with those by normal MEKC. The results reveal that 113–123-fold improvements in the detection sensitivity was obtained for the analytes, and separation and determination of tetrandrine and fangchinoline in Stephaniae tetrandrae S. Moore and Fengtongan capsules were finished under optimum conditions using the sample matrix containing 8.0 mM SDS and TS containing 50 mM H₃PO₄–55% (v/v) ethanol.     

Determination of neonicotinoid insecticides in environmental samples by micellar electrokinetic chromatography using solid‐phase treatments

A sensitive and reliable method based on MEKC has been developed and validated for trace determination of neonicotinoid insecticides (thiamethoxam, acetamiprid, and imidacloprid) and the metabolite 6‐chloronicotinic acid in water and soil matrices. Optimum separation of the neonicotinoid insecticides was obtained on a 58 cm long capillary (75 μm id) using as the running electrolyte 40 mM SDS, 5 mM borate (pH 10.4), and 5% (v/v) methanol at a temperature of 25°C, a voltage of 25 kV and with hydrodynamic injection (10 s). The analysis time was less than 7 min. Prior to MEKC determination, the samples were purified and enriched by carrying out extraction‐preconcentration steps. For aqueous samples, off‐line SPE with a sorptive material such as Strata‐X (polymeric hydrophobic sorbent) and octadecylsilane (C₁₈) was carried out to clean up and preconcentrate the insecticides. However, for soil samples, matrix solid‐phase dispersion (MSPD) was applied with C₁₈ used as the dispersant. Good linearity, accuracy, and precision were obtained and the detection limits were in the range between 0.01 and 0.07 μg mL^?1 for river water and 0.17 and 0.37 μg g^?1 for soil samples. Recovery levels reached greater than 92% for all of the assayed neonicotinoids in river water samples with Strata‐X. In soil matrices, the best recoveries (63–99%) were obtained with MSPD.     

Retention indices in micellar electrokinetic chromatography

The use of retention indices in micellar electrokinetic chromatography (MEKC) is evaluated both from a theoretical and a practical point of view. Fundamental equations for the determination of retention indices in MEKC are described, showing that retention indices are independent of the surfactant concentration. Possibilities as well as limitations of different homologous series as reference standards are described. In addition, the practical application of retention indices for identification, investigation of solute-micelle interactions, characterization and classification of pseudo-stationary phases and determination of solute lipophilicity are discussed.     

Verification of statistical-overlap theory in micellar electrokinetic chromatography

The limited peak capacity of neutral compounds in micellar electrokinetic chromatography (MEKC) causes peak overlap in a simple 38-compound sample that is predicted by statistical-overlap theory (SOT). The low-concentration sample was prepared in-house from several compound classes to span the entire migration-time range and was resolved partially in a pH=7 phosphate buffer containing 50 mM sodium dodecyl sulfate. Peaks, singlets, doublets, and other multiplets were identified on the basis of known migration times and were counted at 13 voltages spanning 4 – 26 kV. These numbers agreed well with predictions of a simple SOT based on the assumption of an inhomogeneous Poisson distribution of migration times. Because the dispersion theory of MEKC is simple, the standard deviations of single-component peaks were modeled theoretically. As part of a new way to implement SOT, probability distributions of the numbers of peaks, singlets, and so on, were computed by Monte Carlo simulation. These distributions contain all theoretical information on peak multiplicity predictable by SOT and were used to evaluate the agreement between experiment and theory. The peak capacity of MEKC was calculated numerically and substituted into the simplest equations in SOT, affirming that peak overlap arises from limited peak capacity.     

在线富集-微乳液毛细管电动色谱法分析食品塑料袋中邻苯二甲酸酯类化合物

建立了微乳液毛细管电动色谱(MEEKC)模式下,采用常规样品堆积模式(normal stacking mode, NSM)和反向极性堆积模式(reversed electrode polarity stacking mode, REPSM)两种在线富集邻苯二甲酸酯类化合物(phthalate esters, PAEs)的简便、有效方法。与常规MEEKC方法相比,REPSM-MEEKC方法使4种PAEs的检测灵敏度提高了937.5～7143倍。考察了常规MEEKC的分离条件,分别对影响两种富集过程的一些因素进行了研究,同时对两种富集方法的重现性和检出限等进行了考察。NSM-MEEKC和REPSM-MEEKC对邻苯二甲酸酯类化合物的检出限(按信噪比(S/N)=3计)分别为0.021～0.33 mg/L和0.7～4 μg/L。其中,灵敏度更高的REPSM-MEEKC方法已成功应用于食品塑料袋中邻苯二甲酸酯类化合物的测定,加标回收率为89.1%～105.6%,结果令人满意。     

Comparative study for the analysis of parabens by micellar electrokinetic capillary chromatography with and without large-volume sample stacking technique

The separation and determination of four parabens (methyl, ethyl, propyl, and butyl p-hydroxybenzoate) which are commonly used as preservatives in cosmetic products, by micellar electrokinetic capillary chromatography (MEKC) with and without large-volume sample stacking (LVSS) technique were compared. As an effective on-line concentration technique, LVSS was successfully combined with MEKC to determine neutral parabens in an acidic media. The effects of some typical parameters such as sample volume, buffer pH, temperature, and concentration of surfactant were examined. The detection limits for this LVSS-MEKC method were found to be 3.0 × 10⁻⁷ M for each of the parabens based on the signal-to-noise ratio of 3, which were around 300 times lower than normal MEKC technique. The curves of peak response versus concentration were linear from 1.0 × 10⁻⁶ to 5.0 × 10⁻⁵ M with regression coefficients of 0.9987, 0.9960, 0.9925 and 0.9864, respectively. A simple and easy-manipulative sample preparation method was developed and validated by analyzing commercially available cosmetic samples. It was found that with current sample preparation process and instrumentation system, 0.5 g of sample is enough for the analysis of parabens preservatives in cosmetic product with satisfactory results.     

Analysis of carbofuran, carbosulfan, isoprocarb, 3-hydroxycarbofuran, and 3-ketocarbofuran by micellar electrokinetic chromatography

We developed an analytical method for the detection and quantitation of five pesticides and some of their metabolites - 3-hydroxycarbofuran, 3-ketocarbofuran, carbofuran, carbosulfan, and isoprocarb - using micellar electrokinetic chromatography coupled with a UV-Vis detector. The optimum separation conditions were 20 mM phosphate buffer (pH 8.0) containing 15 mM sodium dodecyl sulfate. The detection wavelength was set at 200 nm and the applied voltage was 12.5 kV. Under these conditions, baseline separation of five pesticides was achieved in 15 min, and the detection limits (S/N = 3) of 3-hydroxycarbofuran, 3-ketocarbofuran, carbofuran, carbosulfan, and isoprocarb were 0.3, 0.3, 0.3, 4.0, and 0.3 μM, respectively. The linear ranges for 3-hydroxycarbofuran, 3-ketocarbofuran, carbofuran, and isoprocarb were between 1.0 and 50.0 μM and that for carbosulfan was between 10.0 and 100.0 μM, with R(2) larger than 0.995. When applied to the analysis of a carbofuran-spiked rice sample, this approach yielded results with excellent repeatability (3.3%, n = 5), reproducibility (4.5%, n = 5), separation efficiency (>2.1 × 10(4) theoretical plates), and recovery (95.5 ± 1.4%, n = 5).     

混合胶束电动毛细管色谱法分离测定妈富隆片剂中的炔雌醇含量

胆汁酸钠（SC）－十二烷基硫酸钠（SDS）混合胶束电动毛细管色谱法分离测定妈富隆片剂中的炔雌醇，分离缓冲液为SC（55mmol/L)-SDS(15mmol/L)-Tris磷酸（50mmol/L)(pH8.05),分离电压20kV，温度20℃,75μm（i.d)*57.5酮为内标，炔雌醇质量浓度在75．15－901．8μg/mL之间呈良好的线性关系，加样回收率96．4％－104．5％，RSD为3．6％－4．9％（n=3),可用于复方制剂中炔雌醇的含量测定。     

Separation of anabolic steroids by micellar electrokinetic capillary chromatography

Summary The separation of seven analogous anabolic steroids was studied by micellar electrokinetic capillary chromatography (MECC). The retention order was found to be dependent on polarity. All of these steroids were well separated by the addition of organic modifiers to the separation buffer. Of the organic modifiers tested, 1-propanol gave the best separation, better than methanol or acetonitrile.     

Determination of piribedil in pharmaceutical formulations by micellar electrokinetic capillary chromatography

A fast and simple micellar electrokinetic capillary chromatographic method was developed for the analysis of piribedil in pharmaceutical formulations. The effects of buffer concentration, buffer pH, sodium dodecyl sulphate (SDS) concentration, organic modifier, applied voltage and injection time were investigated. Optimum results were obtained with a 50 mM borate buffer at pH 8.0 containing 50 mM SDS by using a fused silica capillary (50 m internal diameter, 72 cm effective length). The sample was injected hydrodynamically for 4 s at 50 mbar pressure and the applied voltage was +30 kV. The detection wavelength was set at 205 nm. Diflunisal was used as an internal standard. The analysis was performed at 25 °C and the total run time was 14 min. The method was suitably validated with respect to linearity range, limit of detection and quantification, precision, accuracy, specificity and robustness. The linear calibration range was 5–100 g mL^–1 and the limit of detection was determined as 1 g mL^–1. The method developed was successfully applied to the determination of piribedil in pharmaceutical formulations. The results were compared with a spectrophotometric method reported in the literature and no significant difference was found statistically.     

Analysis of positional isomers of hydroxylated aromatic cytokinins by micellar electrokinetic chromatography

A micellar electrokinetic chromatography (MEKC) method was developed for the separation of six positional isomers of hydroxylated aromatic cytokinins (topolin and topolin riboside), including ortho-topolin, meta-topolin, para-topolin, ortho-topolin riboside, meta-topolin riboside, and para-topolin riboside. Optimum resolution and analysis time (ca. 20 min) for the six aromatic cytokinin standards were achieved with a running buffer at pH 8.0 consisting of 20 mM boric acid, 50 mM sodium dodecyl sulfate (SDS), and 20% v/v methanol. The method has good reproducibility and has been successfully applied to detect the presence of a putative ortho-topolin in coconut water extract sample purified using C(18) and mixed-mode solid-phase extraction (SPE) columns. Other advantages of this MEKC method are short analysis time, low solvent consumption, and separation of positional isomers which could be achieved by a simple aqueous buffer system without the use of expensive chromatographic columns. In addition, a high-performance liquid chromatography (HPLC) method with baseline separation of the six topolin and topolin riboside standards was developed for the confirmation of the endogenous ortho-topolin in coconut water sample. Finally, the presence of ortho-topolin was further confirmed by liquid chromatography-tandem mass spectrometry (LC-MS/MS) based on its characteristic fragmentation pattern.     

Direct multicomponent analysis of beer samples constituents using micellar electrokinetic capillary chromatography

A capillary electrophoretic method was developed using micellar electrokinetic capillary chromatography (MEKC) with diode-array detection to analyze simultaneously 26 beer constituents in a single procedure, including alcohols, iso-alpha-acids, amino acids, flavonoids, isoflavonoids, a vitamin, purine and pyrimidine bases. After filtration, sample components were separated with an uncoated capillary and a 25 mM sodium borate and 110 mM SDS buffer at pH 10.5. Analyses were run at 14 kV and 8 s of hydrodynamic injection with UV detection at 210 nm and 270 nm. The proposed method was successfully applied to the direct determination of beer constituents without any sample cleanup procedures.