Ultra-low-temperature non-aqueous capillary electrophoretic separation--77 K fluorescence spectroscopic detection for the on-line identification of photo-converted analytes of trans-resveratrol

We demonstrate here, for the first time, that non-aqueous capillary electrophoresis (NACE) can be interfaced with any ultra-low-temperature (ULT) separation method and 77 K fluorescence spectroscopy (FS). This novel ULT-NACE-FS system consists of a modular CE system, a dry ice bath, and instrumentation for 77 K fluorescence detection. The ULT-NACE method serves to separate structurally similar molecules by a combination of a low electrophoresis current and a high voltage at approximately -70 degrees C. When the ULT-NACE-separated analytes move into the quartz Dewar flask and traverse into the capillary detection window, liquid nitrogen was added, thus freezing the separating analyte zones, allowing the collection of 77 K fluorescence spectra for on-line spectral fingerprint identification. The first application of the ULT-NACE-FS system is described for the analysis of photo-converted analytes of trans-resveratrol; prospects and future applications of ULT-NACE-FS are also briefly addressed.     

On-line identification of trans-resveratrol in red wine using a sweeping technique combined with capillary electrophoresis/77 K fluorescence spectroscopy

The feasibility of combining the techniques of on-line concentration and capillary electrophoresis/low-temperature fluorescence spectroscopy (CE/LTFS) for the detection and identification of trans-resveratrol in red wine at 77 K is demonstrated for the first time. This technique, involving sweeping-micellar electrokinetic chromatography (sweeping-MEKC), was used for the initial on-line concentration and separation, after which a cryogenic molecular fluorescence experiment was performed at 77 K. In comparison with normal-MEKC mode, a approximately 1500-fold improvement in detection sensitivity could be obtained when the sweeping-MEKC was applied. The proposed method permits not only the separation and detection of trans-resveratrol from red wine extracts but also ensures that the on-line spectrum is readily distinguishable and can be unambiguously assigned at 77 K.     

Determination of lysergic acid diethylamide (LSD) by application of online 77 K fluorescence spectroscopy and a sweeping technique in micellar electrokinetic chromatography

The principal advantage of the use of Shopl'skii effect (low temperature spectrum) is that spectral sharpening occurs both in absorption and emission. However, thus far using the technique of capillary electrophoresis/low temperature fluorescence spectroscopy (CE/LTFS) either at 77 or 4.2 K remains difficult to obtain an on-line spectrum, if the analyte is present at low concentration. This paper examines the feasibility of combining the techniques of online concentration and CE/LTFS to identify LSD and related compounds in urine at 77 K. To improve sensitivity, sweeping-micellar electrokinetic chromatography (sweeping-MEKC) and cation-selective exhaustive injection-sweep-micellar electrokinetic chromatography (CSEI-sweep-MEKC) were used for on-line concentration which resulted in detection limits of approximately 20 approximately 60 ppt, respectively.     

A chip-based capillary electrophoresis-contactless conductivity microsystem for fast measurements of low-explosive ionic components

A miniaturized analytical system for separating and detecting inorganic explosive residues, based on the coupling of a micromachined capillary electrophoresis (CE) chip with a contactless conductivity detector is described. The low electroosmotic flow (EOF) of the poly(methylmethacrylate) (PMMA) chip material facilitates the rapid switching between analyses of cations and anions using the same microchannel and run buffer (and without an EOF modifier), and hence offers rapid (< 1 min) measurement of seven explosive-related cations and anions. Experimental parameters relevant to the separation and detection processes have been optimized. Addition of a 18-crown-6 ether modifier has been used for separating the peaks of co-migrating potassium and ammonium ions. The ionic-explosive microchip system combines the distinct advantages of contactless conductivity detection with the attractive features of plastic CE microchips. The new microsystem offers great promise for monitoring explosive-related ions at the sample source, with significant advantages of speed/warning, efficiency, cost, or sample size.     

Carbon nanotube/polystyrene composite electrode for microchip electrophoretic determination of rutin and quercetin in Flos Sophorae Immaturus

In this report, carbon nanotube/polystyrene (CNT/PS) composite electrodes have been fabricated as sensitive amperometric detectors of microchip capillary electrophoresis (CE) for the determination of rutin and quercetin in Flos Sophorae Immaturus. The composite electrode was fabricated on the basis of the in situ polymerization of a mixture of CNT and styrene in the microchannel of a piece of fused silica capillary under heat. The surface morphologies of the composite in the electrodes were observed by using a scanning electron microscope. The performance of this unique system has been demonstrated by separating and detecting rutin and quercetin. The new CNT-based CE detector offered significantly lower detection potentials, yielded substantially enhanced signal-to-noise characteristics, and exhibited resistance to surface fouling and hence enhanced stability. It demonstrated long-term stability and reproducibility with a relative standard deviation of less than 5% for the peak current (n = 20) and should also find a wide range of applications in conventional CE, flowing injection analysis, and other microfluidic analysis systems.     

Carbon nanotube/poly(methyl methacrylate) composite electrode for capillary electrophoretic measurement of honokiol and magnolol in Cortex Magnoliae Officinalis

This paper describes the development and the application of a novel carbon nanotube/poly(methyl methacrylate) (CNT/PMMA) composite electrode as a sensitive amperometric detector of CE. The composite electrode was fabricated on the basis of the in situ polymerization of a mixture of CNT and prepolymerized methylmethacrylate in the microchannel of a piece of fused-silica capillary under heat. The performance of this unique system has been demonstrated by separating and detecting honokiol and magnolol in traditional Chinese medicine, Cortex Magnoliae Officinalis. Factors influencing their separation and detection processes were examined and optimized. Honokiol and magnolol were well separated within 7 min in a 40 cm long capillary at a separation voltage of 15 kV using a 50 mM borate buffer (pH 9.2). The new CNT-based CE detector offered significantly lower operating potentials, yielded substantially enhanced S/N characteristics, and exhibited resistance to surface fouling and hence enhanced stability. It demonstrated long-term stability and reproducibility with RSDs of less than 5% for the peak current (n = 9) and should also find a wide range of applications in microchip CE, flowing injection analysis, and other microfluidic analysis systems.     

Application of a Square‐Wave Potential Program for Time‐Dependent Amperometric Detection in Capillary Electrophoresis

Use of a square‐wave potential program for time‐dependent amperometric detection of analyte zones in capillary electrophoresis (CE) is described. Electrochemical detection for CE requires that the separation field be isolated from that of the electrochemical detection. This is generally done by physically separating the CE separation field from that of the detection. By applying a time variant potential program to the detection electrode, the detector current has a time dependence that can be used to help isolate the electrochemical detection current from that of the separation. When using a 20 μm inner‐diameter capillary, we find that a square‐wave potential program decreases the RMS baseline current from 4.5×10^?10 A, found with a constant potential amperometric detection, to 1.1×10^?10 A when using a square‐wave potential program. With a 75 μm inner‐diameter capillary, the improvement is even more dramatic, from 2.3×10^?9 A with amperometric detection to 2.06×10^?10 A when using a 1 Hz square‐wave potential program. When not using the time‐dependent detection with the 75 μm capillary, the analyte zones were beneath the S/N for the system and not detected. With the square‐wave potential program and time‐dependent detection, however, the analyte zones for an electrokinetic injection of 200 μM solution of 2,3‐dihydroxybenzoic acid were observed with the 75 μm inner‐diameter capillary. The improvement in the ability to discriminate the analytical signal from the background found experimentally is consistent with modeling studies.     

Isotachophoresis as an on-line concentration pretreatment technique in capillary electrophoresis

One of the major disadvantages of capillary electrophoresis (CE) is its limited loadability. Therefore, the on-line coupling of isotachophoresis (ITP) and CE was studied with regard to its potential for the improvement of the minimum concentration that can be measured by CE. Based on the concentrating and separating power of ITP, detection limits could be lowered by at least two orders of magnitude. Especially for biological samples containing proteins, it appeared that in non-treated capillaries the electromigration characteristics are hardly influenced when isotachophoretic pretreatment is applied. The potential of ITP-CE coupling is illustrated by the analysis of o-phthaldialdehyde and fluorescein isothiocyanate derivatives of a number of amino acids.     

A three-dimensionally adjustable amperometric detector for microchip electrophoretic measurement of nitroaromatic pollutants

A microchip capillary electrophoresis (CE)–amperometric detection (AD) system has been fabricated by integrating a two-dimensionally adjustable CE microchip and an amperometric detection cell containing a one-dimensionally adjustable disc detection electrode in a Plexiglas holder. It facilitates the precise three-dimensional alignment between the channel outlet and the detection electrode without a complicated three-dimensional manipulator. The performance of this unique system was demonstrated by separating four nitroaromatic pollutants (nitrobenzene, 2,4-dinitrotoluene, 2,4,6-trinitrotoluene, and p-nitrobenzene). Factors influencing their separation and detection processes were examined and optimised. The four analytes have been well-separated within 120 s in a 75 cm long separation channel at a separation voltage of +2000 V using an electrophoretic separation medium containing 15 mM borax and 15 mM sodium dodecyl sulfate (pH 9.2). Highly linear response is obtained for the four analytes over the range of 0–5 ppm with the detection limits ranging from 12 to 52 ppb. The present system demonstrated long-term stability and reproducibility with relative standard deviations of less than 5% for the peak current (n = 9). The new approach for the microchannel–electrode alignment should find a wide range of applications in other microfluidic analysis systems.     

Monitoring environmental pollutants by microchip capillary electrophoresis with electrochemical detection

During the past decade, significant progress in the development of miniaturized microfluidic systems has occurred due to the numerous advantages of microchip analysis. This review focuses on recent advances and the key strategies in microchip capillary electrophoresis (CE) with electrochemical detection (ECD) for separating and detecting a variety of environmental pollutants. The subjects covered include the fabrication of microfluidic chips, ECD, typical applications of microchip CE with ECD in environmental analysis, and future prospects. It is expected that microchip CE-ECD will become a powerful tool in the environmental field and will lead to the creation of truly portable devices.     

毛细管电泳前沿分析法研究钾离子与18-冠醚-6体系相互作用

采用毛细管电泳前沿分析(CE／FA)法，利用间接紫外检测方法，在pH为5．10，运行电压为30kV，缓冲溶液组成为咪唑和醋酸的条件下，测定了钾离子与18-冠醚-6体系的相互作用参数，结合常数对数值lgK=3．50；在同样条件下用峰漂移法作对照实验，求得体系的结合常数对数值lgK=3．40，结果与前沿分析法基本一致。验证了两种方法测定结果的正确性。     

Simplified universal method for determining electrolyte temperatures in a capillary electrophoresis instrument with forced-air cooling

Temperature increase due to resistive electrical heating is an inherent limitation of capillary electrophoresis (CE). Active cooling systems are used to decrease the temperature of the capillary, but their capacity is limited; and in addition, they leave "hot spots" at the detection interface and at the capillary ends. Until recently, the matter was complicated by the lack of a fast and generic method for temperature determination in efficiently and inefficiently cooled regions of the capillary. Our group recently introduced such a method, termed "Universal Method for determining Electrolyte Temperatures" (UMET). UMET is a probe-less approach that requires only measuring current versus voltage for different voltages and processing the data using an iterative algorithm. Here, we apply UMET to develop a Simplified Universal Method of Temperature Determination (SUMET) for a CE instrument with a forced-air cooling system using an Agilent 7100 CE instrument (Agilent Technologies, Saint Laurent, Quebec, Canada) as an example. We collected a wide set of empirical voltage-current data for a variety of buffers and capillary diameters. We further constructed empirical equations for temperature calculation in efficiently and inefficiently cooled parts of the capillary that require only the data from a single 1-min voltage-current measurement. The equations are specific for the Agilent 7100 CE instrument (Agilent Technologies) but can be applied to all kinds of capillaries and buffers. Similar SUMET approaches can be developed for other CE instruments with forced-air cooling using our approach.     

Online identification of the fluorescent whitening agent 4,4-bis(2-sulfostyrol)biphenyl using a sweeping technique combined with capillary electrophoresis/77 K fluorescence spectroscopy

The feasibility of combining the techniques of online concentration and CE/low-temperature fluorescence spectroscopy in the detection and identification of E,E-4,4'-bis(2-sulfostyryl)biphenyl (DSBP) in synthetic detergents at 77 K is demonstrated. The technique involves the use of sweeping-MEKC, and was used for the initial online concentration and separation, after which a cryogenic molecular fluorescence experiment was performed at 77 K. The proposed method not only permits the separation and detection of E,E-DSBP in a synthetic detergent sample, but also ensures that the online spectrum is readily distinguishable and can be unambiguously assigned at 77 K. The photoconversion and isomer separation of DSBP are also described.     

An economic approach for construction of a multiplexed capillary electrophoresis system

We describe in this report an economic approach to construct a multiplexed capillary electrophoresis (CE) system with optic-fiber UV detection. To demonstrate the feasibility of this approach, a seven-lane CE setup is built. The system has a background noise of ∼10⁻⁵ absorbance unit, a limit of detection of 3-4 μM for uracil at 254 nm, and a linear dynamic range of 2.5 orders of magnitude. The apparatus consists of three major components: a UV light source, a photodiode array (PDA) and optical module assembly, and a computer. Compared to the recently reported 96-lane capillary electrophoresis system with UV-vis absorbance detection, the cost of goods is reduced by more than 10-fold, and the noise level is improved by 2 to 3 folds. Parallel CE separations of a protein mixture have been performed on this apparatus, and reproducible and high resolution separation results have been obtained. Detailed construction and characterization of the system have also been discussed.     

Demonstration of an integrated capillary electrophoresis-laser-induced fluorescence fiber-optic sensor

A unique integrated separation-based fiber-optic sensor for remote analysis, that incorporates capillary electrophoresis (CE) directly at the fiber sensing terminus is described for the first time. Based on laser-induced fluorescence detection, the sensor offers the potential for high sensitivity. Although the broad-band nature of fluorescence spectra limits selectivity, the high separation power of CE provides a unique dimension of selectivity, while permitting a design of diminutive size. Previously reported fluorescence-based sensors that utilize a chemical reagent phase to impart selectivity tend to be inflexible (not readily adaptable to the detection of different species) and "one-measurement-only" sensors. Conversely, the CE-based fiber-optic sensor described here is both versatile and reusable. The analysis speed and the potential for remote control are further attributes which make the system amenable to remote sensing. A "single-fiber" optical detection arrangement and a "single-reservoir" CE system with the fiber-optic probing the outlet of the separation capillary are employed. A preliminary evaluation of the separation characteristics of this CE-based sensor is presented. Highlights include an observed separation efficiency of up to 3000 theoretical plates (8 cm separation capillary) and migration time reproducibility of less than 10% for frontal mode CE separations. The potential utility of the sensor for remote analysis is demonstrated with separations involving the CE analysis of charged fluorescent dyes, CE analysis of metal complexes based on in situ complexation and micellar electrokinetic capillary chromatographic analysis of neutral fluorescent compounds.     

21世纪毛细管电泳技术及应用发展趋势

在21世纪，毛细管电泳技术面临着新的挑战和机遇，在其检测手段，仪器的小型化和集成化，以及分离模式上都存在着极大的发展空间，文中针对这三方面的发展趋势和毛细管电泳的应用进行了讨论。     

Improving signal to background ratio for on-the-fly fluorescence lifetime detection in capillary electrophoresis

On-the-fly fluorescence lifetime detection (OFLD) in capillary electrophoresis (CE) was previously demonstrated using a commercial multiharmonic Fourier transform (MHF) spectrofluorometer interfaced to a commercial CE system. This paper discusses optimization of the interface design for minimization of background fluorescence and scattered light, thereby maximizing the signal-to-background ratio (S/B) of the dynamic measurement. Strategies included using various combinations of optical filters including a holographic filter and longpass or bandpass filters, tilting the capillary relative to the incident laser beam, employing a confocal design and adding an iris to remove out-of-focus light, using a microscope objective in the emission beam to increase the collection efficiency, and using square instead of ciruclar capillary columns. Significant improvements in S/B for on-column, on-the-fly detection of fluorescein in CE were achieved with most modifications.     

In-column fiber-optic laser-induced fluorescence detection for CE

A highly sensitive in-column fiber-optic LIF detector for CE has been constructed and evaluated. In this detection system, a 457-nm diode-pumped solid-state blue laser was used as the excitation light source and an optical fiber (40 mum od) was used to transmit the excitation light. One end of the optical fiber was inserted into the separation capillary and was in situ positioned at the detection window. The other end of the fiber was protruded from the capillary to capture the excitation light beam from the blue laser. Fluorescence emission was collected by a 40 x microscope objective, focused on a spatial filter, and passed through a yellow color filter before reaching the photomultiplier tube. The present CE-fluorescence detection is a simple and compact optical system. It reduces the laser scattering effect from the capillary and fiber as compared to the conventional LIF detection for CE. Its utility was successfully demonstrated by the separation and determination of D-penicillamine labeled with naphthalene-2,3-dicarboxaldehyde. The detection limit was 0.8 nM (S/N = 3). The present detection scheme has been proven to be attractive for sensitive fluorescence detection for CE.     

Application of a capillary‐assembled microfluidic system for separation of cephalosporins

This paper demonstrates a simple and easy setting up of a fused‐silica capillary‐assembled microfluidic system (μCE). This system incorporates a split‐flow pressure injection of the sample into a microfluidic system made from PDMS and a short (～20 cm) length of fused‐silica capillary as a separation unit. The on‐capillary detection was carried out by fiber optic spectrometry. A mixture of six cephalosporin antibiotics was separated in the μCE system and the obtained results were compared to those achievable by conventional CE. The six components could be separated within 8.5 min with the number of theoretical plates around 10 000.     

Sensitive determination of sulfonamides in environmental water by capillary electrophoresis coupled with both silvering detection window and in‐capillary optical fiber light‐emitting diode‐induced fluorescence detector

A new detector, silvering detection window and in‐capillary optical fiber light‐emitting diode‐induced fluorescence detector (SDW‐ICOF‐LED‐IFD), is introduced for capillary electrophoresis (CE). The strategy of the work was that half surface of the detection window was coated with silver mirror, which could reflect the undetected fluorescence to the photomultiplier tube to be detected, consequently enhancing the detection sensitivity. Sulfonamides (SAs) are important antibiotics that achieved great applications in many fields. However, they pose a serious threat on the environment and human health when they enter into the environment. The SDW‐ICOF‐LED‐IFD‐CE system was used to determine fluorescein isothiocyanate (FITC)‐labeled sulfadoxine (SDM), sulfaguanidine (SGD) and sulfamonomethoxine sodium (SMM‐Na) in environmental water. The detection results obtained by the SDW‐ICOF‐LED‐IFD‐CE system were compared to those acquired by the CE with in‐capillary optical fiber light‐emitting diode‐induced fluorescence detection (ICOF‐LED‐IFD‐CE). The limits of detection (LODs) of SDW‐ICOF‐LED‐IFD‐CE and ICOF‐LED‐IFD‐CE were 1.0–2.0 nM and 2.5–7.7 nM (S/N = 3), respectively. The intraday (n = 6) and interday (n = 6) precision of migration time and corresponding peak area for both types of CE were all less than 0.86% and 3.68%, respectively. The accuracy of the proposed method was judged by employing standard addition method, and recoveries obtained were in the range of 92.5–102.9%. The results indicated that the sensitivity of the SDW‐ICOF‐LED‐IFD‐CE system was improved, and that its reproducibility and accuracy were satisfactory. It was successfully applied to analyze SAs in environmental water.