Metal complex separation with on-line sample preconcentration in micellar electrokinetic chromatography.

Micellar electrokinetic chromatography (MEKC) using a cationic surfactant as a pseudostationary phase was examined to separate anionic metal cyclohexane-1,2-diaminetetraacetic acid (CDTA) complexes. Cetyltrimethylammonium chloride (CTAC) was employed as the cationic surfactant micelle, its addition leading to EOF reversal. Cu(II), Co(II), Zn(II), Mn(II) and Pb(II) were used as test analytes, and the complete separation was obtained by MEKC. On-line sample preconcentration by sweeping was also examined to improve the detection sensitivity. From 15- to 42-fold increases in the detection sensitivity in terms of the peak heights were obtained by sweeping with a cationic micelle in the presence of high EOF. The limits of detection were in the range (0.6 - 1.8) x 10(-6) M with UV detection without any off-line preconcentration step.     

Analysis of ecdysteroids by micellar electrokinetic chromatography with on-line preconcentration

In order to enhance the UV detection sensitivity, an application study of an on-line preconcentration technique for micellar electrokinetic chromatographic (MEKC) was carried out. The simultaneous determination of four test ecdysteroids, 20-hydroxyecdysone, ajugasterone C, polypodine B and ponasterone A has been investigated by using the normal stacking mode in MEKC with UV detection. The effects of anionic surfactant composition and concentration, the applied voltage, the pH buffer, the kind and the amount of organic solvent and the injection time on the analyte resolution were evaluated. The optimised conditions for the separation involved the use of a 50 mM borate as the running buffer containing 50 mM of a mixture of sodium dodecyl sulphate (SDS) and sodium cholate (SC) in the ratio of 1:1 together with a concentration of 10% (v/v) of 2-PrOH at pH 9.0. Hydrodynamic injection of 12 s at 50 mbar and separation voltage of 20 kV at temperature of 20 degrees C were employed. These conditions allowed a repeatability separation within 21 min. Concentration detection limit for the neutral analytes studied improve about an order of magnitude. The method was also applied to the determination of ecdysteroids in a real sample.     

Determination of endo- and exogenous corticosteroids by cyclodextrin-modified micellar electrokinetic chromatography with the use of on-line preconcentration

A method was proposed for the simultaneous determination of steroids of the endo- and exogeneous origin with the use of micellar electrokinetic chromatography, which provides the determination of residual medicines in biological fluids and the control of medication efficiency at different steroidogenesis abnormalities. The introduction of a macrocyclic addition of β-cyclodextrin (from 1 to 6 mM) into a buffer electrolyte solution (25 mM phosphoric acid, 20 mM sodium dodecyl sulfate, 4.5 mM urea, pH 2.5) or into a sample solution decreases the detection limit by a factor of 20–200.     

Strategies for the on-line preconcentration and separation of hypolipidaemic drugs using micellar electrokinetic chromatography

Three strategies were investigated for the simultaneous separation and on-line preconcentration of charged and neutral hypolipidaemic drugs in micellar electrokinetic chromatography (MEKC). A background electrolyte (BGE) consisting of 20 mM ammonium bicarbonate buffer (pH 8.50) and 50 mM sodium dodecyl sulfate (SDS) was used for the separation and on-line preconcentration of the drugs. The efficiencies of sweeping, analyte focusing by micelle collapse (AFMC), and simultaneous field-amplified sample stacking (FASS) and sweeping, were compared for the preconcentration of eight hypolipidaemic drugs in different conductivity sample matrices. When compared with a hydrodynamic injection (5 s at 50 mbar, 0.51% of capillary volume to detection window) of drug mixture prepared in the separation BGE, improvements of detection sensitivity of 60-, 83-, and 80-fold were obtained with sweeping, AFMC and simultaneous FASS and sweeping, respectively, giving limits of detection (LODs) of 50, 36, and 38 μg/L, respectively. The studied techniques showed suitability for focusing different types of analytes having different values of retention factor (k). This is the first report for the separation of different types of hypolipidaemic drugs by capillary electrophoresis (CE). The three methods were validated then applied for the analysis of target analytes in wastewater samples from Hobart city.     

Determination of doxycycline in pharmaceuticals and human urine by micellar electrokinetic capillary chromatography

Micellar electrokinetic capillary chromatography (MEKC) was performed at 25 °C and 30 kV (under a pressure of 15 mbar), using 30 mM borate buffer containing 60 mM sodium dodecysulfate (SDS) and 5% (v/v) methanol as background electrolyte (pH 9.0) to determine doxycycline. UV detection was at 350 nm. The method was shown to be specific, accurate (recovery was 100.3 ± 1.0%), linear over the tested range (correlation coefficient 0.9995) and precise (RSD <1.9%). The method was used to determine doxycycline in tablets, capsules and human urine after oral application.     

Determination of ibuprofen and tetrazepam in human urine by micellar electrokinetic capillary chromatography

A new micellar electrokinetic capillary chromatography method (MEKC) is proposed for the determination of ibuprofen and tetrazepam in human urine samples over a concentration range of therapeutic interest. A fused silica capillary (60 cm × 75 μm) is used. Ibuprofen and tetrazepam are detected via UV detection at 220 and 228 nm, respectively. Separation is performed at 25 °C and at a separation voltage of 30 kV, with 15 mM borate buffer (pH 10.2) containing 40 mM sodium dodecylsulfate as the electrolyte solution. Under these conditions the analytes were separated in <11 min. Sulfamethazine is used as an internal standard. Prior to determination, the samples are purified and enriched by means of an extraction–preconcentration step with a preconditioned C₁₈ cartridge and by eluting the compounds with methanol. Good linearity, accuracy, precision, robustness and solution stability were achieved for the technique. Detection limits of 200 μg L⁻¹ for ibuprofen and 300 μg L⁻¹ for tetrazepam were obtained. These analytes were then determined in real urine using the technique.     

Selective extraction of alkaloids in human urine by on-line single drop microextraction coupled with sweeping micellar electrokinetic chromatography

A novel method of on-line single drop microextraction (SDME) coupled with sweeping micellar electrokinetic chromatography (MEKC) for the selective extraction and dual preconcentration of alkaloids was developed. In this technique, analytes of three alkaloids were firstly extracted from 4.0 mL basic aqueous sample solution (donor phase, 500 mM NaOH) into a layer of n-octanol at temperature 30 °C with the stirring rate of 1150 rpm, then back-extracted into the acidified aqueous acceptor (acceptor phase, 50 mM H₃PO₄) suspended at the tip of a capillary at 650 rpm. Then, the aqueous acceptor was introduced into capillary by hydrodynamic injection with a height difference of 15 cm between the inlet and outlet of capillary for 300 s, and analyzed directly by on-line sweeping MEKC. With the selective SDME, we were able to extract three alkaloids without any interfering components in human urine samples. Under the optimum conditions, the proposed method achieved limits of detections (LOD) of between 0.2 ng mL⁻¹ and 1.5 ng mL⁻¹ with 1583–3556-fold increases in detection sensitivity for three analytes, which indicated that it was a promising method for analysis of alkaloids in human urine.     

胶束在线扫集毛细管电泳法测定三聚氰胺

研究胶束在线扫集毛细管电泳法测定三聚氰胺的可行性,结果表明,与区带毛细管电泳相比,胶束在线扫集毛细管电泳法富集倍数提高约60倍。缓冲体系为140 mmol/L SDS+20 mmol/L NaH2PO4(pH 2.20)+10%(体积分数)甲醇,分离电压-18 kV,进样时间30 s,测量波长214 nm。考察了SDS浓度、pH、进样时间、运行电压等因素对分离测定的影响情况。在优化条件下,三聚氰胺在9 min时出峰,峰面积RSD≤3.7%。方法检出限、线性范围、相关系数分别为:0.13μg/mL、0.50~32.0μg/mL、0.9997。方法可用于奶粉中三聚氰胺的分离测定。     

Determination of ephedrine and codeine in human urine by cation-selective exhaustive injection and sweeping micellar electrokinetic chromatography

A sensitive method for the determination of ephedrine and codeine in human urine by capillary electrophoresis(CE)was described.In order to improve the sensitivity,two online concentration techniques including cation-selective exhaustive injection (CSEI)and sweeping micellar electrokinetic chromatography(sweeping-MEKC)were used.Under the optimum conditions,the detection limits(S/N=3)were 0.10μg/L for ephedrine and 0.80μg/L for codeine.This method was successfully applied to real urine sample analysis.     

Specific determination of cysteine in human urine by capillary micellar electrokinetic chromatography

This paper describes a method for the analysis of cysteine in human urine using capillary micellar electrokinetic chromatography and on‐column reaction with 2,2′‐dipyridyl disulfide. In this reaction cysteine is quantitatively transformed into a mixed disulfide concomitantly with formation of an equimolar amount of 2‐thiopyridone that is further separated by capillary micellar electrokinetic chromatography and determined spectrophotometrically at 343 nm. The concentration of cysteine is thus estimated indirectly from the result of 2‐thiopyridone determination. The linear detection range for concentration versus peak area for the assay is from 0.05 to 5 mM (correlation coefficient 0.989) with a detection limit of 2.5 μM and a limit of quantitation of 8.5 μM. The inter‐day reproducibility of the peak area was 2.18% and the inter‐day reproducibility of the migration time 0.51%. The method is relatively rapid, simple, and can be easily automated. Moreover, its detection limit covers the concentration range at which cysteine is present in biological samples such as human urine.     

On-line sample preconcentration in micellar electrokinetic chromatography by sweeping with anionic-zwitterionic mixed micelles

On-line preconcentration by sweeping in micellar electrokinetic chromatography using mixed micelles of sodium dodecyl sulfate (SDS)-SB-12 is presented. Because of their large micelle radius, they permit increased partitioning of hydrophobic analytes into the core. In addition, they also possess lower negative surface charge relative to pure SDS micelles so anionic analytes can be retained better due to decreased electrostatic repulsion. As the efficiency of sweeping is predicated on the magnitude of retention factors, these advantages translated to better focusing. As much as a 370-fold improvement in detector response, in terms of peak height, was obtained for some neutral steroids, while about a 360-fold improvement was obtained for some phenol derivatives, which were previously not amenable to sweeping by pure SDS micelles.     

The two-phase amphiphilic preconcentration based on surfactants to enrich phenolic compounds from diluted plant extracts and rat urine by micellar electrokinetic chromatography

A novel technology by two-phase amphiphilic preconcentration based on surfactants was established for enriching phenolic compounds by micellar electrokinetic chromatography (MEKC). The cationic surfactant cetyltrimethylammonium chloride (CTAC) was combined with the anionic analytes that existed in the sample solution before injection. The boundary was formed between CTAC and sodium dodecyl sulfate (SDS) in the background solution when the sample solution was injected into the capillary, where the analytes bound inside micelles were released due to the stronger electrostatic force between SDS and CTAC. This procedure accelerated the separation of analytes from CTAC and greatly improved the enrichment efficiency. The optimal conditions were obtained after a series of optimizations, and the sensitivity enrichment factors of the four analytes were in the range of 39–93 compared to typical injections in capillary zone electrophoresis. Good linearity for matrix-matched calibrations was established for all analytes with R² values of 0.9993–0.9997. The limits of detection (S/N = 3) for kaempferol, quercetin, salvianolic acid C, and salvianolic acid B were 0.0166, 0.0292, 0.0215, and 0.0195 µg/ml, respectively. The intracapillary RSDs of the analytes ranged from 0.8% to 1.3% for migration time and from 0.4% to 1.8% for peak areas. The developed method was successfully applied to the determination of phenolic compounds, the main compounds of Salvia miltiorrhiza Bge., and had been validated for the determination of spiked recoveries in rat urine.     

On-line preconcentration and chiral separation of propiconazole by cyclodextrin-modified micellar electrokinetic chromatography

A method for the chiral separation of propiconazole using cyclodextrin-modified micellar electrokinetic chromatography (CD-MEKC) with hydroxypropyl-gamma-cyclodextrin (HP-gamma-CD) as chiral selector is reported. The use of a mixture of 30 mM HP-gamma-CD, 50mM SDS, methanol-acetonitrile 10%:5% (v/v) in 25 mM phosphate buffer solution was able to separate two enantiomeric pairs of propiconazole. Stacking- and sweeping-CD-MEKC under neutral pH (pH 7) and under acidic condition (pH 3.0) were used as two on-line preconcentration methods to increase detection sensitivity of propiconazole. Good repeatabilities in the migration time, peak area and peak height were obtained in terms of relative standard deviation (RSD). A sensitivity enhancement factor of 100-fold was achieved using sweeping-CD-MEKC at acidic pH. This is the first report on the separation of two pairs of propiconazole enantiomers and all the enantiomers of fenbuconazole and tebuconazole using sweeping-CD-MEKC. The limit of detection (S/N=3) for the three triazole fungicides ranged from 0.09 to 0.1 microg/mL, which is well below the maximum residue limits (MRL) set by Codex Alimentarius Commission (CAC). Combination of solid-phase extraction (SPE) pretreatment and sweeping-CD-MEKC procedure was applied to the determination of selected triazole fungicides in grapes samples spiked at concentration 10-40 times lower than the MRL established by the CAC. The average recoveries of the selected fungicides in spiked grapes samples were good, ranging from 73% to 109% with RSD of 9-12% (n=3).     

A comparative study of microemulsion electrokinetic capillary chromatography and micellar electrokinetic capillary chromatography for the analysis of UV-absorbing compounds in human urine

Separations of human urine by microemulsion electrokinetic chromatography (MEEKC) and micellar electrokinetic capillary chromatography (MEKC) with respect to resolution, migration times and efficiencies were optimized and compared. The optimised MEEKC and MEKC methods were simple and fast, both of which are excellent characteristics for the complex separations required in clinical and biomedical studies. However, resolution in MEKC was significantly greater than in MEEKC although migration times were 30% faster for the optimised MEEKC method. In addition, a faster analysis method (short-end injection) specifically for routine screening purposes was also investigated. With both MEEKC and MEKC modes, this provided short separations (less than 4 min for urine) with no major compromise in resolution. In conclusion, we found that MEEKC offered no real advantage over MEKC for urine analysis.     

Characterisation of retention in micellar high-performance liquid chromatography, in micellar electrokinetic chromatography and in micellar electrokinetic chromatography with reduced flow

The retention (migration) behaviour of various barbiturates, phenylurea and triazine herbicides in micellar electrokinetic chromatography (MEKC) with uncoated fused-silica capillaries was compared with the behaviour in micellar electrokinetic chromatography with reduced electroosmotic flow (RF-MEKC) using capillaries modified with linear polyacrylamide. The error in the values of the retention factors caused by the neglection of the contribution of the electroosmotic flow in RF-MEKC was investigated and a method for correcting this error was suggested. The retention was characterised using the lipophilic and polar indices to characterise and to predict the retention as a function of the concentration of the surfactant (sodium dodecylsulphate) in the running buffer in MEKC and in RF-MEKC. Homologous series of n-alkylbenzenes and of n-alkan-2-ones were compared as the standard sets for the calibration of the retention (migration) index scale. The values of the lipophilic indices of a given solute measured in reversed-phase HPLC, MEKC and RF-MEKC are close to each other. Under ideal MEKC conditions, the values of the polarity indices are close to one for various sample solutes. However, for partially ionised compounds such as weakly acidic barbiturates, where the contribution of the electrophoretic migration is significant, the values of the polarity indices are significantly lower than one. Optimum conditions for separations of mixtures of triazine and phenylurea herbicides and of barbiturates using various techniques tested were compared.     

Determination of S-adenosylmethionine,S-adenosylhomocysteine,and methylthioadenosine in urine using solvent-modified micellar electrokinetic chromatography

A new approach for direct determination of S-adenosylmethionine (SAM), S-adenosylhomocysteine (SAH), and methylthioadenosine (MTA) in urine was developed based on MEKC by using SDS modified with isobutanol in the presence of PEG-300. Analytes were first extracted with grafted phenylborononic acid. Using a 50 µm internal diameter silica capillary of 32 cm total length filled with 0.05 M SDS, 0.05 M H₃PO₄, 5% (v/v) isobutanol, and 10% (v/v) PEG-300, LOQ of 0.15 µM for SAM and SAH, and 0.2 µM for MTA was reached. Accuracy was 92% for MTA, 109% for SAH, and 105% for SAM, intra- and interday imprecision were <2.5 and ≤3%, respectively. The total time of analysis for one sample was 10 min. Analysis of 30 urine samples from healthy volunteers showed that the median SAM and SAH levels were 12.1 and 0.73 µM, respectively. MTA levels, which were determined in urine for the first time (according to our data), were 0.43 µM, and these values correlated well with the SAM level (r = 0.748, p < 0.01).     

Separation of biogenic amines by micellar electrokinetic chromatography with on-line chemiluminescence detection

A method of on-line chemiluminescence detection with capillary electrophoresis for biogenic amines (diaminopropane, putrescine, cadaverine and diaminohexane) labeled with N-(4-aminobutyl)-N-ethylisoluminol is reported for the first time. Two separation modes, capillary zone electrophoresis and micellar electrokinetic chromatography (MEKC), were studied. The results show that excellent resolution was achieved in MEKC. Parameters affecting separation process and chemiluminescence detection have been examined in detail. Under the optimum conditions, the baseline separation of four amines was obtained within 7.5 min. The detection limits (S/N=3) of diaminopropane, putrescine, cadaverine and diaminohexane are 3.5 x 10(-8), 3.5 x 10(-8), 3.9 x 10(-8) and 1.2 x 10(-7) M, respectively. The method was applied to the analysis of biogenic amines in lake water.     

Determination of triazine herbicide residues in water samples by on-line sweeping concentration in micellar electrokinetic chromatography

A new method for the determination of atrazine, simazine and prometryn in water samples by on-line sweeping concentration technique in micellar electrokinetic chromatography （MEKC） was developed. Various parameters affecting sample enrichment and separation efficiency were systematically studied. Compared with the conventional MEKC method, up to 60-200-fold improvement in concentration sensitivity was achieved in terms of peak height by using this sweeping injection technique. The compound strychnine was used as the internal standard for the improvement of the experimental reproducibility. The limits of detection （S/ N = 3：1） for atrazine, simazine and prometryn were 9, 10 and 0.5 ng mL-1, respectively. This method has been successfully applied to the analysis of atrazine, simazine and prometryn in lake, steam and ground water.