On-line coupling of capillary electrophoresis to hydride generation atomic fluorescence spectrometry for arsenic speciation analysis

A novel hyphenated technique, on-line coupling of capillary electrophoresis (CE) to atomic fluorescence spectrometry (AFS), was developed for speciation analysis of four environmentally significant and toxic forms of arsenic: arsenite, arsenate, monomethylarsenic acid, and dimethylarsenic acid. Baseline separation of the four arsenic species was achieved by capillary electrophoresis in a 50 cm x 100 microm i.d. fused-silica capillary at 20 kV and using a 20 mmol L(-1) phosphate buffer (pH 6.5). A hydride generation (HG) technique was employed to convert the arsenic species from the CE effluent into their respective hydrides. The CE-AFS interface was constructed on the basis of a cross design for introducing a sheath flow around the CE capillary and a Pt electrode, which provided an electrical connection for stable electrophoretic separations and allowed on-line volatile hydride formation. A laboratory-made gas-liquid separator was used to isolate the generated volatile species from the reaction mixture solution, and an argon flow was used to transport the volatile hydrides into the atomizer of AFS for on-line detection. The precision (RSD, n = 7) ranged from 2.1 to 3.1% for migration time, from 2.8 to 4.2% for peak area response, and from 2.0 to 4.1% for peak height response for the arsenic species at the 1 mg L(-1) (as As) level. The detection limits were in the range of 9-18 microg L(-1) (as As). The recoveries of the four arsenic species in locally collected water samples and urine sample ranged from 91 to 115%. The developed technique was successfully applied to the speciation of the water-methanol extractable arsenic in a sediment sample.     

测定汞的原子荧光法和冷原子吸收法比对研究

从监测方法和质量控制的角度出发,比对研究了测定水中汞的原子荧光法和冷原子吸收法,并提出了质量控制指标.研究结果表明:(1)两种方法具有可比性;(2)质量控制指标与样品浓度之间没有明显相关性;(3)对于标准样品,两种方法的室内相对标准偏差(RSD)、室间相对标准偏差(RSD′)和相对误差(RE)结论一致,即RSD≤5％,RSD′≤10.0％,RE≤±15.0％;(4)两种方法的标准样品和实际样品的加标回收率基本一致,为85％～115％,原子荧光法的空白加标回收率略低于冷原子吸收法.     

Probing mercury species-DNA interactions by capillary electrophoresis with on-line electrothermal atomic absorption spectrometric detection

The interactions of inorganic mercury Hg(II), methylmercury (MeHg(I)), ethylmercury (EtHg(I)), and phenylmercury (PhHg(I)) with DNA have been probed by capillary electrophoresis with on-line electrothermal atomic absorption spectrometric detection (CE-ETAAS) in combination with circular dichroism and Fourier transform infrared spectroscopy. The CE-ETAAS assay allows sensitive probing of the level of DNA damage by mercury species, extraction of thermodynamic and kinetic information on the interactions of mercury species with DNA, and provides direct evidence for the formation of mercury species-DNA adducts. The binding affinity of mercury species to DNA increases in order of Hg(II) < EtHg(I) approximately PhHg(I) approximately MeHg(I). The interactions of mercury species with DNA follow a first-order kinetics for mercury species and zero-order kinetics for DNA. Mercury highly covalently coordinates to endocyclic and exocyclic N sites of DNA bases. However, the interactions of DNA with mercuric species cause no transition of the DNA original conformation. The results reveal that organomercuric species exhibit stronger affinity and faster binding to DNA and show more potential damage to DNA than Hg(II) in view of the kinetic and thermodynamic evaluations. Moreover, MeHg(I) exhibits the fastest binding to DNA, suggesting that MeHg(I) enjoys superiority over the other mercuric species for rapid formation of a stable complex with DNA, whereas Hg(II) shows the slowest binding to DNA. The present study provides new evidence and understanding of the binding modality of mercuric species to DNA.     

Experimental studies of electroosmotic flow dynamics during sample stacking for capillary electrophoresis

Electroosmotic flow dynamics during a field-amplified sample stacking experiment have been studied experimentally using the periodic photobleaching of a dilute, neutral fluorophore added to the separation buffer. The effects of hydrodynamically injecting different sample plug lengths containing a mixture of arsenic compounds dissolved in 0.125 mM (120, 240, and 600 s) and 41.7 microM (27, 45, and 74 s) phosphate buffer with a separation buffer concentration of 12.5 mM phosphate buffer were examined. Changes in electroosmotic flow during sample stacking and separation were monitored at a rate of 1 Hz. The observed effects of increasing the sample plug length on electroosmotic flow and electrophoretic current agreed qualitatively with predictions by theoretical models presented in the literature. Electroosmotic flow changes on the order of 100% (1.6-3.3 mm/s) were observed. Broadening of the flow monitoring peaks has been used to examine parabolic flow due to the discontinuous buffer systems used for sample stacking.     

Hydride generation interface for speciation analysis coupling capillary electrophoresis to inductively coupled plasma mass spectrometry

A novel hydride generation (HG) interface for coupling capillary electrophoresis (CE) with inductively coupled plasma mass spectrometry (ICPMS) is presented in this work. The CE-HG-ICPMS interface was applied to the separation and quantitation of common arsenic species. Lack of a commercially available HG interface for CE-ICPMS led to a three concentric tube design allowing alleviation of back pressure commonly observed in CE-HG-ICPMS. Due to the high sensitivity and element-specific detection of ICPMS, quantitative analysis of As(III), As(V), monomethylarsonic acid, and dimethylarsinic acid was achieved. Optimization of CE separation conditions resulted in the use of 20 mmol L(-1) sodium borate with 2% osmotic flow modifier (pH 9.0) and -20 kV applied potential for baseline resolution of each arsenic species in the shortest time. Hydride generation conditions were optimized through multiple electrophoretic separation analyses with 5% HCl and 3% NaBH(4) (in 0.2% NaOH) determined to be the optimum conditions. After completion of system optimization, detection limits obtained for the arsenic species were less than 40 ng L(-1) with electromigration time precision less than 1% within a total analysis time of 9.0 min. Finally, the interface was used for speciation analysis of arsenic in river and tap water samples.     

Indirect determination of sulfide at ultratrace levels in natural waters by flow injection on-line sorption in a knotted reactor coupled with hydride generation atomic fluorescence spectrometry

A simple and sensitive nonchromatographic approach for indirect determination of sulfide at ultratrace levels in natural waters based on its selective precipitation with Hg2+ on the inner wall of a knotted reactor (KR) was developed for flow injection on-line sorption coupled with hydride generation atomic fluorescence spectrometry (HG-AFS). With the Hg2+ pH kept at 2.0, the HgS precipitation was formed in the KR after a reaction time of 120 s. A 10% (v/v) HCl was introduced to elute the remnant inorganic mercury and to merge with the KBH4 solution (0.05% m/v) for HG-AFS detection. Under the optimal experimental conditions, the sample throughputs were 20 h(-1). The detection limit was found to be 0.05 microg L(-1), and the relative standard deviation (RSD, n = 11) for determination of 2.0 microg L(-1) sulfide was 3.3%. The developed method was successfully applied to the determination of sulfide in a variety of natural water samples and wastewater samples with the gas-phase separation and sorption apparatus.     

Effects of analyte velocity modulation on the electroosmotic flow in capillary electrophoresis.

Modulation of the electroosmotic flow in capillary zone electrophoresis by modulation of the driving voltage gives rise to a flow profile that changes between laminar and flat profiles. The changing flow profile induces a radial movement of sample species to and from the capillary surface. The induced sample concentration gradient can be monitored by carefully probing the capillary surface. The resulting signal is a derivative of the normal-shaped peak. Derivative-shaped peaks can be observed with cations, but not with anions. Anions are unable to approach the double-layer region and therefore are unaffected by the modulation process.     

Nanosemiconductor-based photocatalytic vapor generation systems for subsequent selenium determination and speciation with atomic fluorescence spectrometry and inductively coupled plasma mass spectrometry

We reported novel Ag-TiO(2)- and ZrO(2)-based photocatalytic vapor generation (PCVG) systems as effective sample introduction techniques for further improving the sensitivity of the atomic spectrometric determination of selenium for the first time, in which the conduction band electron served as a "reductant" to reduce selenium species including Se(VI) and convert them directly into volatile H(2)Se, which was easily separated from the sample matrix and underwent more effectively subsequent atomization and/or ionization. These two PCVG systems helped us to overcome the problem encountered in the most conventional KBH(4)/OH(-)-H(+) system, in that Se(VI) was hardly converted into volatile selenium species without the aid of prereduction procedures. The limits of detection (LODs) (3σ) of the four most typical Se(IV), Se(VI), selenocystine ((SeCys)(2)), and selenomethionine (SeMet) species were, respectively, down to 1.2, 1.8, 7.4, and 0.9 ng mL(-1) in UV/Ag-TiO(2)-HCOOH, and 0.7, 1.0, 4.2, and 0.5 ng mL(-1) in UV/ZrO(2)-HCOOH with the relative standard deviations (RSDs) lower than 5.1% (n = 9 at 1 μg mL(-1)) when using atomic fluorescence spectrometry (AFS) under flow injection mode. They reached 10, 14, 18, and 8 pg mL(-1) in UV/Ag-TiO(2)-HCOOH, and 6, 7, 10, and 5 pg mL(-1) in UV/ZrO(2)-HCOOH with the RSDs lower than 4.4% (n = 9 at 10 ng mL(-1)) when using inductively coupled plasma mass spectrometry (ICPMS). After the two PCVG systems were validated using certified reference materials GBW(E)080395 and SELM-1, they were applied to determine the total Se in the selenium-enriched yeast sample and used as interfaces between high-performance liquid chromatography (HPLC) and AFS or ICPMS for selenium speciation in the water- and/or enzyme-extractable fractions of the selenium-enriched yeast.     

DNA sequencing by capillary electrophoresis with four-decay fluorescence detection

A scheme for multiplex detection of dye-labeled DNA fragments in DNA sequencing is described in which on-the-fly, frequency-domain fluorescence lifetime detection is used to discriminate among the dye-labeled fragments of the four terminal bases in a single-lane CE separation. Two four-dye systems were evaluated, one excited at 488 nm and the other, at 514 nm. The 488 nm system proved successful for four-decay detection. Base calling was achieved either directly from on-the-fly lifetimes or from lifetime-resolved electropherograms recovered for each base from the electropherogram of the mixture of sequencing reaction products. The latter method was found to be more accurate (99% for two bases and 98.5% for three bases) and could achieve longer read lengths, but it was unsuccessful for sequencing of all four bases. The first method gave a base-calling accuracy of 96% for four-base sequencing over the fragment length range of 41-220 bases.     

Control of electroosmotic flow and wall interactions in capillary electrophoresis capillaries by photografted zwitterionic polymer surface layers

A novel capillary with covalently bonded zwitterionic surface modification was prepared by photograft polymerization of the zwitterionic monomer N,N-dimethyl-N-methacryloxyethyl-N-(3-sulfopropyl)ammonium betaine, onto the inner surface of a UV-transparent fused-silica capillary. Although the zwitterionic moieties in the resulting polymeric "tentacles" comprise both a positive quaternary ammonium group and a negative sulfonate group, the coating has a net zero charge. The electroosmotic flow (EOF) was therefore extensively suppressed on the grafted capillary compared to the native silica capillary and to the silica capillary that had been activated for graft polymerization by reaction with 3-(methacryloyl)oxypropyltrimethoxysilane. It was also found that the EOF can be varied by adding chaotropic anions or divalent cations such as perchlorate ion and magnesium ion to the running buffer, due to the interaction between these ions and zwitterionic functional group. This provides a new way of altering the EOF and the wall interaction without changing the pH or the overall ionic strength of the separation buffer. The influence of pH and ionic strength of separation buffer on the EOF were also investigated to optimize the separation conditions. Good separations of a mixture containing eight inorganic anions were achieved within 5 min under optimal conditions by capillary zone electrophoresis. The newly prepared capillary was also well suited for the separation of peptides or proteins.     

Dynamic coating using polyelectrolyte multilayers for chemical control of electroosmotic flow in capillary electrophoresis microchips

Poly(dimethylsiloxane) (PDMS) capillary electrophoresis (CE) microchips were modified by a dynamic coating method that provided stable electroosmotic flow (EOF) with respect to pH. The separation channel was coated with a polymer bilayer consisting of a cationic layer of Polybrene (PB) and an anionic layer of dextran sulfate (DS). According to the difference in charge, PB- and PB/ DS-coated channels supported EOF in different directions; however, both methods of channel coating exhibited a pH-independent EOF in the pH range of 5-10 due to chemical control of the effective zeta-potential. The endurance of the PB-coated layer was determined to be 50 runs at pH 3.0, while PB/DS-coated chips had a stable EOF for more than 100 runs. The effect of substrate composition and chip-sealing methodology was also evaluated. All tested chips showed the same EOF on the PB/DS-coated channels, as compared to uncoated chips, which varied significantly. No significant variation for separation and electrochemical detection of dopamine and hydroquinone between coated and uncoated channels was observed.     

The capillary cold trap as a suitable instrument for mercury speciation by volatilization, cryogenic trapping, and gas chromatography coupled with atomic absorption spectrometry

An innovative accessory for speciation analysis has been developed. The system is based on the combination of cryogenic trapping and gas chromatographic separation, carried out within the same capillary. The instrument, hyphenating derivatization, gas-phase extraction, preconcentration, and analyte separation, is semiautomated, and all operational parameters are adjustable via an in-house-developed control unit, which regulates the selected parameters throughout the analysis process. Species detection was carried out by atomic absorption spectrometry. The detection limits achieved were 33, 39, and 71 ng L(-1) for dimethylmercury, methylmercury, and inorganic mercury, respectively. A complete chromatogram could be obtained within three minutes, resulting in the duration of one whole analysis cycle of about 15 min. The proposed method was applied to mercury speciation in freeze-dried tuna fish powder after microwave-assisted extraction, finding that mercury is present at 80% as methylmercury and about 20% as inorganic mercury, in this kind of biological material.     

氢化物发生-原子荧光光谱法同时测定食盐中的砷和汞

建立一种氢化物发生-原子荧光光谱法同时测定食盐中砷和汞的方法.研究样品前处理对测定结果的影响,探索同时测定砷、汞的最佳实验条件.优化条件下,砷、汞质量浓度分别在0～10.0 μμg/L和0～2.0μg/L范围内,线性关系良好.方法检出限:As为0.0043 μg/L,Hg为0.003 2 μg/L;相对标准偏差:As为0.37％,Hg为0.61％;样品加标回收率:As为97.80％ ～ 102.40％,Hg为96.78％～ 100.80％.试验表明:该方法具有操作简便、快速、准确度高等优点,可用于食盐中砷汞的同时测定.     

Theoretical description of the influence of external radial fields on the electroosmotic flow in capillary electrophoresis

The effect of applying a radial voltage on the electroosmotic flow in capillary electrophoresis has been studied from a theoretical point of view. Based on Stern's model for the electric double layer on the surface of a fused silica capillary and on the Gouy-Chapman theory for the diffuse layer, equations describing the relation between the electroosmotic mobility and the radial electric field were derived. The thickness of the stagnant solution layer on the surface of the capillary, an important parameter in the calculations, was estimated from the electroosmotic mobility found in high-pH solutions. The theory developed predicts the experimental findings that the effect of the radial field levels off at high applied voltages and that it is smaller when the electroosmotic mobility without radial field is already high. The theoretical results were compared with experimental data taken from the literature. A good quantitative agreement was found.     

Suppression of electroosmotic flow and prevention of wall adsorption in capillary zone electrophoresis using zwitterionic surfactants

Addition of zwitterionic surfactants such as dodecyldimethyl(3-sulfopropyl)ammonium hydroxide, hexadecyldimethyl(3-sulfopropyl)ammonium hydroxide, and coco (amidopropyl)hydroxyldimethylsulfobetaine (Rewoteric AM CAS U) to an electrophoretic buffer suppress the electroosmotic flow by 50-90%. Onset of suppression occurs at approximately the critical micelle concentration of the surfactant. CAS U effectively suppresses the electroosmotic flow over the pH range 3-12. Addition of 2 mM CAS U to the electrophoretic buffer prevents adsorption of cationic proteins lysozyme, α-chymotrypsinogen A, cytochrome c, and ribonuclease A. Migration time reproducibility for these proteins is ～1% RSD within 1 day and 2-5% from day to day. Efficiencies in excess of 750?000 plates/m and recoveries of >80% were observed for protein injections from distilled water. Alternatively if 2 mM CAS U is added to samples, recoveries were quantitative, although efficiencies decreased to 325?000-600?000 plates/m. The natural electroosmotic flow of the capillaries is regenerated simply by rinsing with sodium hydroxide.     

Determination of mercury in coal by isotope dilution cold-vapor generation inductively coupled plasma mass spectrometry

A method based on isotope dilution cold-vapor inductively coupled plasma mass spectrometry (ID-CV-ICPMS) has been developed for high-accuracy determinations of mercury in bituminous and sub-bituminous coals. A closed-system digestion process employing a Carius tube is used to completely oxidize the coal matrix and chemically equilibrate the mercury in the sample with a 201Hg isotopic spike. The digestates are diluted with high-purity quartz-distilled water, and the mercury is released as a vapor by reduction with tin(II) chloride. Measurements of 201Hg/202Hg isotope ratios are made using a quadrupole ICPMS system in time-resolved analysis mode. The new method has some significant advantages over existing methods. The instrument detection limit is less than 1 pg/mL. The average blank (n = 17) is 30 pg, which is roughly 1 order of magnitude lower than the equivalent microwave digestion procedure. The detection limit in coal is blank limited and is approximately 40 pg/g. Memory effects are very low. The relative reproducibility of the analytical measurements is approximately 0.5% for mercury concentrations in the range 10-150 ng/g. The method has been used to measure mercury concentrations in six coal reference materials, SRM 1632b (77.4 ng/g), SRM 1632c (94.3 ng/g), BCR 40 (433.2 ng/g), BCR 180 (125.0 ng/g), BCR 181 (135.8 ng/g), and SARM 20 (252.6 ng/g), as well as a coal fly ash, SRM 1633b (143.1 ng/g). The method is equally applicable to other types of fossil fuels including both crude and refined oils.     

Field-amplified sample injection combined with water removal by electroosmotic flow pump in acidic buffer for analysis of phenoxy acid herbicides by capillary electrophoresis

A procedure that combines two common stacking techniques, field-amplified sample injection and water removal, with an electroosmotic flow pump, is used to separate phenoxy acid herbicides by capillary zone electrophoresis. Before sample loading, a long plug of water was hydrodynamically injected into the capillary both to serve as the medium to permit a high electric field strength and to contain sample anions. Because of this long length of water, the number of ions injected into the capillary was greatly increased. Electrokinetic injection at reversed voltage was then used for introducing negatively charged ions from the diluted sample into the column. The water was removed from the capillary using the electroosmotic flow (EOF) pump when the EOF of the background electrolyte was suppressed. This method afforded a sensitivity enhancement of greater than 3,000 times. Combined with solid-phase extraction, detection limits for the phenoxy acid herbicides as low as 0.01 ng/mL could be achieved.     

Determination of methylmercury in environmental matrixes by on-line flow injection and atomic fluorescence spectrometry

The precision and bias of monomethylmercury (MMHg) determinations in environmental samples can be improved by directly coupling and automating the numerous steps involved with analysis of this toxic Hg species. We developed a simple and robust mercury speciation analyzer (MSA) for measurement of MMHg in environmental matrixes. This on-line hyphenated system couples the main analytical steps, including sample introduction, aqueous-phase ethylation, Tenax preconcentration, and gas chromatographic separation, to cold vapor atomic fluorescence detection and data acquisition. Here we describe the MMHg-MSA, present results of laboratory optimization and performance tests, and compare the reproducibility between dual analytical channels. With alternating sample concentration and analysis, a dual-channel system permits six high-accuracy MMHg determinations per hour. Additional advantages compared to the traditional manual method include ease of operation and high precision (<5% relative standard deviation). The MSA is applicable to the determination of MMHg in various environmental matrixes, and it can be fully automated. This method was validated by analysis of MMHg in certified reference materials of sediment and biological tissue. Estimated detection limits for MMHg with the MSA are approximately 0.01 ng g(-1) for a 0.1-g sample of dry sediment or fish and approximately 0.01 ng L(-1) for 0.15 L of water.