On-column liquid-liquid-liquid microextraction coupled with base stacking as a dual preconcentration method for capillary zone electrophoresis.

A simple and efficient dual preconcentration method of on-column liquid-liquid-liquid microextraction (LLLME) coupled with base stacking was developed for capillary zone electrophoresis (CZE) in this paper. Four N-methyl carbamates were used as target compounds to evaluate the enrichment means. The carbamates in sample solutions (donor phase) were extracted into a dodecanol phase immobilized on a porous hollow fiber, hydrolyzed and back extracted into 0.20 microL running buffer (acceptor phase) of 30 mmol/L methylamine hydrochloride (pH 11.6) containing 0.5 mmol/L tetradecyltrimethylammonium bromide inside the hollow fiber, stacked further with 0.5 mol/L NaOH injected at -10 kV for 60s, and separated by CZE. Analytical parameters affecting the LLLME, base stacking and CZE were investigated, including sample solution volume, pH and temperature, extraction time, stirring rate, buffer component, buffer pH, NaOH concentration, stacking time, etc. The enrichment factors of the carbamates were higher than 1100. The relative standard deviation (RSD) of peak height and limits of detection (LODs) were 4.5-5.5% (n=6) and 2-4 ng/mL (S/N=3) for standard solutions, respectively. The proposed method was applied to the analysis of vegetable and fruit samples with the RSD less than 6.0% (n=3) and LODs of 6-10 ng/g (S/N=3). The calibration solutions were prepared by diluting the stock solutions with blank sample solutions, and the calibration concentrations ranged from 0.012 to 1.0 microg/mL (r>0.9951). The analytical results demonstrated that the LLLME coupled with base stacking was a simple, convenient and reliable on-column sample pretreatment method for the analysis of anionic analytes in CZE.

Determination of kanamycin A, amikacin and tobramycin residues in milk by capillary zone electrophoresis with post-column derivatization and laser-induced fluorescence detection.

An analytical method for kanamycin A, amikacin and tobramycin of aminoglycoside (AG) antibiotics in milk samples was proposed using capillary zone electrophoresis (CZE) with post-column derivatization and laser-induced fluorescence detection. A simple and convenient homemade coaxial-gap reactor was adopted in the post-column derivatization of the AGs with 1.0 mmol/L naphthalene-2,3-dicarboxaldehyde and 8.0 mmol/L 2-mercaptoethanol in 35 mmol/L sodium tetraborate buffer (pH 10.0) of 30% (v/v) methanol. 50 mmol/L sodium acetate (pH 5.0) containing 0.5 mmol/L cetyltrimethyl ammonium bromide was used as the separation buffer. The linear calibration concentrations and detection limits were from 2.1 x 10(-5) to 5.0 x 10(-2)g/L and in the range of 7 x 10(-6) to 2 x 10(-5)g/L, respectively. The recoveries of the AGs in milk samples were from 81.6 to 93.1% (n=3).

Determination of benzoic acid and sorbic acid in food products using electrokinetic flow analysis-ion pair solid phase extraction-capillary zone electrophoresis.

An electrokinetic flow analysis system (EFA), consisting of one electroosmotic pump, five solenoid valves and one on-line homemade solid phase extraction (SPE) unit, combined with capillary zone electrophoresis (CZE) was proposed to determine benzoic acid and sorbic acid in food products. Tetrabutylammonium bromide (TBAB) was adopted as an ion pair reagent to improve the retention of the preservatives on C(8)-bonded silica sorbent, which was also used to remove sample matrices. By using the SPE unit, the EFA-SPE-CZE system was able to perform the SPE operation and CZE separation simultaneously. With a modified interface of EFA and CZE, the buffer consumption was reduced to 130 microL for each running. The preservatives were separated and determined under optimized conditions with p-hydroxybenzoic acid as an internal standard. The relative standard deviation (R.S.D.) of peak area for each analyte was less than 3.1% (n=5) and the limits of detection (LODs) ranged from 10 to 20 ngmL(-1) (K=3, n=11).

Post-column reactor of coaxial-gap mode for laser-induced fluorescence detection in capillary electrophoresis.

A post-column reactor with coaxial-gap mode is developed for laser-induced fluorescence detection (LIF) in capillary electrophoresis (CE). The reactor can be assembled simply and conveniently, in which a thin polyimide sleeve of 10-mm length obtained from the capillary coating is used to align separation and reaction capillary with a 20 microm gap. Naphthalene-2,3-dicarboxaldehyde and 2-mercaptoethanol are used as derivatization reagents and delivered into the reaction capillary through the annulus between the separation capillary and polyimide sleeve and the gap of two capillaries by gravity. A reaction distance from the gap to detection point is 5mm. For the post-column reactor of CE-LIF, several configuration parameters are optimized, including liquid level difference between the derivatization solution and outlet buffer, annular dimension between the outer diameter of etched separation capillary and the inner diameter of polyimide sleeve, and reaction distance, etc. The detection limits in the range from 8.0x10(-8) to 1.0x10(-6) mol/L and linear calibration range more than two orders of magnitude are obtained for amino acids. The separation efficiency ranges from 1.35x10(5) to 1.67x10(5) theoretical plates.

Determination of amino acids in tobacco samples by capillary electrophoresis/indirect absorbance detection with isolation of the electrolysis compartment and p-Aminobenzoic acid as a background electrolyte.

A fast, convenient and sensitive method of capillary zone electrophoresis (CZE) and indirect UV detection was proposed for the determination of 16 amino acids. p-Aminobenzoic acid (PAB) was selected as a background electrolyte (BGE). An isolated cell included a BGE buffer part and an electrode buffer one, which were jointed with a glass frit. The isolated cell can prevent PAB from the electrode reaction and improve the stability of the detection baseline. The separation conditions of amino acids were investigated, such as different BGEs, BGE concentration, buffer pH and electroosmotic flow (EOF) modifiers. Under the selected separation conditions, 14 amino acid peaks could be separated in 12 min. The detection limits of the amino acids were in the range of 1.7 - 4.5 micromol/L. The isolated cell is suitable for reagents reacting on the electrodes in capillary electrophoresis. The proposed method has been successfully applied to the determination of the amino acids in tobacco samples.

Study on pressurizing electroosmosis pump for chromatographic separation.

A porous core electroosmosis pump was studied and improved in accordance with the electroosmosis theory. Hexamethylene tetraamine (HMTA) was used as the additive of pump carrier solution to improve the flow rate stability and delivery efficiency. The influences of the electric field strength, porous core dimension and acetonitrile concentration of carrier solution on the pump flow rate and output pressure were investigated in detail. The improved electroosmosis pump can provide not only steady flow rate and large flow range, but also moderate output pressure. With the pump carrier solution of 0.5mmol/L HMTA and the working voltage of 4950V, the pump output pressure, flow rate and delivery efficiency achieved 1.1MPa, 1.3mL/min and 3.2mL/(minmA), respectively. The pump can be employed for mobile phase delivery in the reversed-phase chromatographic separation of monolithic silica columns.