Determination of preservatives by integrative coupling method of headspace liquid-phase microextraction and capillary zone electrophoresis.

An integrative coupling method of headspace liquid-phase microextraction (HS-LPME) and capillary zone electrophoresis (CZE) was proposed in this paper. In the method, a separation capillary was used to create a microextraction droplet of the running buffer solution of CZE, hold the droplet at the capillary inlet, extract analytes of sample solutions in the headspace of a sample vial, inject concentrated analytes into the capillary and separate the analytes by CZE. The proposed method was applied to determine the preservatives of benzoic acid and sorbic acid in soy sauce and soft drink samples, in which the running buffer solution of 50 mmol/L tetraborate (pH 9.2) was directly used to form the acceptor droplet at the capillary inlet by pressure, and the preservatives in a 6-mL sample solution containing 0.25 g/mL NaCl were extracted at 90°C for 30 min in the headspace of a 14-mL sample vial. Then the concentrated preservatives were injected into the capillary at 10 cm height difference for 20 s and separated by CZE. The enrichment factors of benzoic acid and sorbic acid achieved 266 and 404, and the limits of detection (LODs) were 0.03 and 0.01 µg/mL (S/N=3), respectively. The recoveries were in the range of 88.7-105%. The integrative coupling method of HS-LPME and CZE was simple, convenient, reliable and suitable for concentrating volatile and semi-volatile organic acids and eliminating matrix interferences of real samples.

[On-line coupling of microcolumn electrophoresis and UV-Vis spectrophotometry].

In the present paper, a laboratory-made high-performance electrophoresis microcolumn unit was prepared for UV-Vis spectrophotometer. X-ray diffraction was used in the preparation of electrophoretic microcolumns. And an analytical technique of microcolumn electrophoresis coupled with UV-Vis spectrophotometry was introduced. Uniform quartz microncrystals were prepared by hydrothermal synthesis. Their crystalline phase and morphology were identified by X-ray diffraction and scanning electron microscope, respectively. The quartz microncrystals were packed into a 2-mm i. d. fused-silica tube to prepare the electrophoretic microcolumn. With 1.5 mmol x L(-1) disodium phosphate buffer solution (pH 11.5) containing 25% (phi) methanol and 10% (phi) acetonitrile, tryptophan, phenylalanine and tyrosine were on-line separated on line and detected by microcolumn electrophoresis coupled with UV-Vis spectrophotometry without derivatization. The limits of detection were 0.037, 0.20 and 0.20 micromol x L(-1), respectively. The separation efficiency of tryptophan was 4.5 x 10(4) plates/m. The sample capacity of the electrophoretic microcolumn achieved 35 microL. It was found that the electrophoretic microcolumn packed with quartz microncrystals was able to limit Joule heat, increase sample capacity and enhance detection sensitivity. The laboratory-made electrophoretic microcolumn could be a high-performance separation unit for conventional UV-Vis spectrophotometer. The on-line coupling of microcolumn electrophoresis and UV-Vis spectrophotometry could separate and determine samples with complicated matrices, reduce zone broadening and enhance separation efficiency, so expand the analytical function of spectrophotometer in the trace analysis of mixed components with overlapped spectra.

In-line coupling headspace liquid-phase microextraction with capillary electrophoresis.

An analytical technique of in-line coupling headspace liquid-phase microextraction (HS-LPME) with capillary electrophoresis (CE) was proposed to determine volatile analytes. A special cover unit of the sample vial was adopted in the coupling method. To evaluate the proposed method, phenols were used as model analytes. The parameters affecting the extraction efficiency were investigated, including the configuration of acceptor phase, kind and concentration of acceptor solution, extraction temperature and time, salt-out effect, sample volume, etc. The optimal enrichment factors of HS-LPME were obtained with the sample volume of about half of sample vials, which were confirmed by both the theoretical prediction and experimental results. The enrichment factors were obtained from 520 to 1270. The limits of detection (LODs, S/N=3) were in the range from 0.5 to 1 ng/mL each phenol. The recoveries were from 87.2% to 92.7% and the relative standard deviations (RSDs) were lower than 5.7% (n=6). The proposed method was successfully applied to the quantitative analysis of the phenols in tap water, and proved to be a simple, convenient and reliable sample preconcentration and determination method for volatile analytes in water samples.

Design and application of a novel integrated electrochemical hydride generation cell for the determination of arsenic in seaweeds by atomic fluorescence spectrometry.

An integrated electrochemical hydride generation cell, mainly composed of three components (a gas liquid separator, a graphite tube cathode and a reticulate Pt wire anode), was laboratory constructed and employed for the detection of arsenic by coupling to atomic fluorescence spectrometry. This integrated cell was free of ion-exchange membrane and individual anolyte, with the virtues of low-cost, easy assembly and environmental-friendly. Using flow injection mode, the sample throughput could come to 120 h(-1) attributed to the small dimension of the cathode chamber. The operating conditions for the electrochemical hydride generation of arsenic were investigated in detail and the potential interferences from oxygen or various ions were also evaluated. Under the optimized conditions, no obvious oxygen quenching effects were observed. The limit of detection of As (III) for the sample blank solution was 0.2 ng mL(-1) (3sigma) and the relative standard deviation was 3.1% for nine consecutive measurements of 5 ng mL(-1) As (III) standard solution. The calibration curve was linear up to 100 ng mL(-1). The accuracy of the method was verified by the determination of arsenic in the reference materials GBW08517 (Laminaria Japonica Aresch) and GBW10023 (Porphyra crispata) and the developed method was successfully applied to determine trace amounts of arsenic in edible seaweeds.

Electrophoretic separation with 2-mm inner diameter fused-silica microcolumn packed with quartz microncrystals and its application.

The feasibility of a microcolumn electrophoresis technique was investigated with a 100mm length, 2mm I.D. fused-silica microcolumn packed with uniform quartz microncrystals prepared by hydrothermal synthesis. To evaluate the separation technique, tryptophan, phenylalanine and tyrosine were primarily separated by the microcolumn electrophoresis and detected at 216 nm without derivatization by an ordinary spectrophotometer. The separation conditions of the amino acids were optimized. With 1.5 mmol/L disodium phosphate buffer solution (pH 11.5) containing 25% (v/v) methanol and 10% (v/v) acetonitrile, the three amino acids were separated and the separation efficiency of tryptophan was 4.5x10(4)plates/m. The limits of detection were 0.035, 0.22 and 0.20 micromol/L, respectively. The sample capacity of the electrophoretic microcolumn achieved 35 microL. The proposed method was used to determine these amino acids in compound amino acid injection samples without derivatization. For the simplicity and portability of the microcolumn electrophoresis, it is studied as one of the high-performance separation techniques for an in situ and real-time electrokinetic flow analysis system. For its high detection sensitivity and large sample capacity, it can be developed for preparative electrophoresis.

On-line wall-free cell for laser-induced fluorescence detection in capillary electrophoresis.

A wall-free detection method based on liquid junction in a capillary gap was proposed for laser-induced fluorescence (LIF) of capillary electrophoresis (CE). The capillary gap of the wall-free cell was fabricated by etching a 10-mm x 50-microm I.D. fused-silica capillary to obtain a polyimide coating sleeve, decoating about 6mm at one end of both 50 microm I.D. separation and liquid junction capillary, inserting the treated capillary ends into the coating sleeve oppositely, fixing the capillaries with a gap distance of 140 microm by epoxy glue and removing the coating sleeve by burning. The theoretical model, experimental results and wall-free cell images indicated that the gap distance and applied voltage were main influence factors on the wall-free detection. Since the wall-free cell increased the absorption light path and avoided the stray light from the capillary wall, it improved the ratio of signal to noise and limit of detection (LOD) of CE-LIF. Three flavin compounds of riboflavin (RF), flavin mononucleotide sodium (FMN) and flavin adenine dinucleotide disodium (FAD) were used to evaluate the wall-free detection method. Compared with on-column cell, the LODs of the wall-free cell were improved 15-, 6- and 9-fold for RF, FMN and FAD, respectively. The linear calibration concentrations of the flavins ranged from 0.005 to 5.0 micromol/L. The column efficiency was in the range from 1.0 x 10(5) to 2.5 x 10(5) plates. The wall-free detection of CE-LIF was applied to the analysis of the flavins in spinach and lettuce leaves.

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).

On-line pretreatment and determination of parabens in cosmetic products by combination of flow injection analysis, solid-phase extraction and micellar electrokinetic chromatography.

A convenient and automated method for on-line pretreatment and determination of three parabens (i.e. methyl, ethyl and propyl p-hydroxybenzoate) in cosmetic products is proposed by using flow injection analysis (FIA), solid-phase extraction (SPE) and micellar electrokinetic chromatography (MEKC). An improved split-flow interface is used to couple SPE on C(8)-bonded silica with MEKC separation, which can avoid running buffer contamination and reduce buffer consumption, especially, containing expensive reagents. The analytes are loaded onto a C(8) column at 0.6 mL/min for 60s and eluted with a mixed eluent of 40% (v/v) 10 mmol/L sodium tetraborate buffer (pH 9.3) and 60% (v/v) ethanol at 0.75 mL/min. The MEKC separation is accomplished with a running buffer of 20 mmol/L sodium tetraborate (pH 9.3) containing 100 mmol/L sodium dodecyl sulfate (SDS) at 15 kV. For butyl p-hydroxybenzoate did not be detected in the cosmetic products, it was used as an internal standard (IS) added into the real samples. This FIA-SPE-MEKC method using IS allows the sample separation within 12 min and the sample throughput of five samples per hour with the relative standard deviation (R.S.D.) less than 2.3% (n=5). The limits of detection (LOD) are in the range from 0.07 to 0.1 microg/mL (S/N=3 and n=11). The proposed method has been used to determine three parabens in real cosmetic products satisfactorily.

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 polyphenol components in herbal medicines by micellar electrokinetic capillary chromatography with Tween 20.

A simple and convenient method of micellar electrokinetic capillary chromatography (MEKC) using polyoxyethylene sorbitan monolaurate (Tween 20) to form single micelle and methanol as a buffer additive was introduced for the simultaneous determination of five polyphenols, including scopoletin, rutin, esculetin, chlorogenic acid and caffeic acid. A running buffer solution of pH 9.3, 20 mmol/L sodium tetraborate containing 64 mmol/L Tween 20 and 9% (v/v) methanol was adopted in the separation. Because rutin and esculetin were difficult to be separated by capillary zone electrophoresis (CZE) and SDS-based MEKC, Tween 20-based MEKC was adopted and the polyphenols were separated satisfactorily. The proposed method was used to determine the polyphenol components in the herbal medicine of Cortex fraxini. The separation mechanism of Tween 20-based MEKC for the polyphenols was discussed preliminarily.

Funnelform single-drop microextraction for gas chromatography-electron-capture detection.

A funnelform single-drop microextraction was developed for gas chromatography-electron-capture detection. A solvent microdrop of 4 microL was formed at the tip of a microsyringe needle assembled with a small brass funnel in the microextraction. In the funnel, the restricted microdrop was shaken gently by circular motion in an aqueous sample solution. Eleven organochlorine and two pyrethroid pesticides were used as the model compounds for evaluating the microextraction. The parameters affecting the enrichment factor of the microextraction were investigated, including the funnel inner angle, solvent component, and microdrop volume, etc. With the optimized microextracting conditions, the enrichment factors of organochlorines and pyrethroids were 272-875 and 147-183, respectively. The detection limit was in the range of 1-12 ng/L (S/N = 3). The relative standard deviation (RSD) of peak height was less than 10.2% (n = 8). This proposed technique is simple, convenient, and efficient.

Determination of aflatoxins in high-pigment content samples by matrix solid-phase dispersion and high-performance liquid chromatography.

A fast, efficient, and cost-effective method was developed for the analysis of aflatoxins in farm commodities with high-pigment content, such as chili powder, green bean, and black sesame. The proposed method involved matrix solid-phase dispersion (MSPD) and high-performance liquid chromatography (HPLC)-fluorescence detection (FLD) with postcolumn electrochemical derivatization in a Kobra cell. The MSPD procedure combined the extraction with neutral alumina and pigment cleanup with graphitic carbon black (GCB) in a single step. The recoveries of aflatoxins ranged from 88% to 95% with the relative standard deviations (RSD) less than 6% (n = 6). The limits of detection (LODs) were 0.25 ng/g aflatoxin B1, G1, and 0.10 ng/g aflatoxin B2, G2, respectively. The analytical results obtained by MSPD were compared to those of the immunoaffinity column (IAC) cleanup method. No significant differences were found between the two methods by t-test at the 95% confidence level.

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.

On-line sequential preconcentration of inorganic anions by anion-selective exhaustive injection and base-stacking in capillary zone electrophoresis.

A sequential electrostacking method based on anion-selective exhaustive injection (ASEI) and base-stacking (BS) is presented for the preconcentration and determination of inorganic anions by capillary zone electrophoresis (CZE) in this paper. Tetradecyltrimethylammonium bromide as an electroosmotic flow (EOF) modifier was added into the buffer to suppress EOF of the capillary. Firstly, a water plug was hydrodynamically injected into the capillary. During ASEI under negative high voltage, the sample anions migrated quickly towards the boundary between the water plug and buffer in the capillary. Then an alkaline zone was injected electrokinetically to concentrate the anions further. With the sequential electrostacking method, the preconcentration factor of (0.8-1.3) x 10(5) was obtained compared with the conventionally electrokinetic injection and the relative standard deviation of peak area was 3.3-5.3% (n = 5). The detection limits of ASEI-BS-CZE for six inorganic anions were 6-14 ng/L. The proposed method has been adopted to analyze six anions in cigarette samples successfully.

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.

Response surface optimization for determination of pesticide multiresidues by matrix solid-phase dispersion and gas chromatography.

An optimized multiresidue analysis method based on matrix solid-phase dispersion (MSPD) and gas chromatography (GC) is proposed for the determination of organochlorines and pyrethroids in the tea samples. Response surface methodology (RSM) was used to optimize the extraction conditions of MSPD, such as the sorbent type, eluent composition, dichloromethane concentration and eluting volume. Desirability function approach was employed to optimize the pesticide recoveries and matrix cleanup. Compromising the recoveries and cleanup degree, MSPD was carried out with Florisil as the sorbent and n-hexane-dichloromethane (1:1, v/v) as the eluent. The pesticide recoveries in tea samples were better than 80% spiked in the concentration range of 0.01-0.05mg/kg and the relative standard deviations were lower than 7%. The quantification limits of the pesticides were in the range of 0.002-0.06mg/kg, which were lower than the maximum residue limits of the pesticides in tea samples established by the European Union.

Reversed-phase electrochromatography with a monolithic microcolumn prepared in a 2.2-mm-inner diameter fused-silica tube.

Capillary electrochromatography possesses advantages of high separation efficiency and velocity, but it also has its disadvantages due to its small inner diameter, such as poor detection sensitivity, low sample capacity, and some trouble in its column preparation. To overcome these shortcomings, a monolithic microcolumn with a surface area larger than 200 m2/g was prepared by sol-gel polycondensation of tetraethoxysilane-hydrochloric acid-poly(ethylene glycol) and filling with fine quartz sand in a 2.2-mm-i.d. fused-silica tube. The prepared microcolumn was used in the separation of aromatic compounds by reversed-phase electrochromatography. Some factors that affected electroosmotic flow were explored, such as electric field strength, buffer concentration, and buffer pH. Acetonitrile concentration in the mobile phase was investigated for phenol, benzene, and naphthalene separation. The separation results were satisfying with the electrochromatographic microcolumns. The detection limits of phenol, benzene, and naphthalene were 0.07, 0.26, and 0.04 mg/L, respectively.

On-line cation-exchange preconcentration and capillary electrophoresis coupled by tee joint interface.

An on-line preconcentration method based on ion exchange solid phase extraction was developed for the determination of cationic analytes in capillary electrophoresis (CE). The preconcentration-separation system consisted of a preconcentration capillary bonded with carboxyl cation-exchange stationary phase, a separation capillary for zone electrophoresis and a tee joint interface of the capillaries. Two capillaries were connected closely inside a 0.3 mm i.d. polytetrafluoroethylene tube with a side opening and fixed together by the interface. The preparations of the preconcentration capillaries and interface were described in detail in this paper. The on-line preconcentration and separation procedure of the analysis system included washing and conditioning the capillaries, loading analytes, filling with buffer solution, eluting analytes and separating by capillary zone electrophoresis (CZE). Several analysis parameters, including sample loading flow rate and time, eluting solution and volume, inner diameter and length of preconcentration capillary etc., were investigated. The proposed method enhanced the detection sensitivity of CE-UV about 5000 times for propranolol and metoprolol compared with normally electrokinetic injection. The detection limits of propranolol and metoprolol were 0.02 and 0.1 microg/L with the proposed method respectively, whereas those were 0.1 and 0.5 mg/L with conventional electrokinetic injection. The experiment results demonstrate that the proposed technique can increase the preconcentration factor evidently.