Determination of electroactive organic acids by anion-exchange chromatography using a copper modified electrode

An ion-chromatographic method combined with electrochemical detection at a copper-based chemically modified glassy carbon electrode (Cu-GC) has been shown to provide a simple analytical approach for the determination of some common organic acids in alkaline medium. Under the optimized isocratic chromatographic conditions (i.e. 0.1 M NaOH plus 80 mM CH3COONa), organic acids such as gallic, ascorbic, gluconic, lactobionic, galacturonic and glucuronic acid could be separated in less than 20 min. Under constant potential amperometric detection (i.e. 0.55 V vs. Ag-AgCl) the Cu-GC modified electrode allowed detection limits between 2 and 5 pmol for all investigated organic acids while the linear dynamic range spanned generally over three orders of magnitude. Examples of applications included the separation and quantitation of some common organic acids in vinegar, honey and tea samples, are given.     

Simultaneous determination of active ingredients in blueberry wine by CE-AD

A method based on capillary electrophoresis coupled with amperometric detection（CE-AD） has been developed for the separation and determination of kaempferol,ferulic acid,vanillic acid,caffeic acid,gallic acid and protocatechuic acid in blueberry wine for the first time.In order to get the optimum conditions of separation,the effects of working electrode potential,pH value and concentration of running buffer,separation voltage and injection time on CE-AD were investigated.Under the optimum conditions, the analytes could be separated in Na₂B₄O₇-KH₂PO₄ buffer at pH 7.8 within 18 min.A 300μm diameter carbon disk electrode had good current responses at +0.95 V（vs.SCE） for all analytes.The responses were linear with concentrations over three orders of magnitude with detection limits（S/N = 3） at 10^-8 g/mL magnitude for the analytes and the recoveries were in the range of 95.8- 106.7%.The method could be successfully applied to the analysis of real sample with satisfactory results and therefore recommended for use by the quality control departments of fruit wine producers.     

A dynamically modified microfluidic poly(dimethylsiloxane) chip with electrochemical detection for biological analysis

Separation and direct detection of amino acids, glucose and peptide in a 3.1 cm separation channel made of poly(dimethylsiloxane) (PDMS) with end-column amperometric detection at a copper microdisk electrode was developed. This system is the integration of a normal sized working electrode with electrochemical detection on a PDMS microfabricated device. The PDMS channels dynamically modified by 2-morpholinoethanesulfonic acid (MES) show less adsorption and more enhanced efficiency than that of unmodified ones when applied to separations of these biological molecules. The migration time is less than 100 s and the reproducibility of migration time is satisfactory with relative standard deviation (RSD) of 2.8% in 19 successive injections. The limits of detection of arginine (Arg), glucose, and methionine-glycine (Met-Gly) are estimated to be 2.0, 8.5, and 64.0 microM at S/N = 3, approximately 0.5-16.0 fmol, respectively. Variances influencing the separation efficiency and amperometric response, including injection, separation voltage, detection potential, or concentration of buffer and additive, are assessed and optimized.     

Electrocatalytic Properties of a Novel Poly‐1‐Naphthylamine‐Modified Electrode Using Ascorbic Acid as Molecule Probe

A glassy carbon electrode was modified with an electropolymerized film of 1‐naphthylamine in aqueous solution. The electrocatalytic properties of this modified electrode (ME) were investigated using ascorbic acid (AA) as probe molecule. The electrochemical behavior of AA in buffer solution was examined by voltammetry and amperometry. The results showed that the ME exhibited good electrocatalytic activity towards the oxidation of AA, as a consequence, it can be used as amperometric sensor of this analyte in a flow injection system with good sensitivity. Calibration curves were linear over the concentration range 0.05–1.50 mM with a phosphate buffer solution pH 3 as the carrier, the detection limit was 1 ppm (S/N=3). The methods were applied to the determination of AA in beverages and pharmaceutical products. A good correlation with a reference method was attained.     

Analysis of polyphenols in white wine by CZE with amperometric detection using carbon nanotube-modified electrodes

A method for the simultaneous detection of five polyphenols (caffeic, chlorogenic, ferulic and gallic acids and (+)-catechin) by CZE with electrochemical detection was developed. Separation of these polyphenols was performed in a 100 mM borate buffer (pH 9.2) within 15 min. Under optimized separation conditions, the performance of glassy carbon (GC) electrodes modified with multiwalled carbon nanotube layer obtained from different dispersions was examined. GC electrode modified with a dispersion of multi-walled carbon nanotubes (CNT) in polyethylenimine has proven to be the most suitable CNT-based electrode for its application as amperometric detector for the CZE separation of the studied compounds. The excellent electrochemical properties of this electrode allowed the detection of the selected polyphenols at +200 mV and improved the efficiency and the resolution of their CZE separation. Limits of detection below 3.1 μM were obtained with linear ranges covering the 10?? to 10?? M range. The proposed method has been successfully applied for the detection (ferulic, caffeic and gallic acids and (+)-catechin) and the quantification (gallic acid and (+)-catechin) of polyphenols in two different white wines without any preconcentration step. A remarkable signal stability was observed on the electrode performance despite the presence of potential fouling substances in wine.     

Screening of seized cocaine samples using electrophoresis microchips with integrated contactless conductivity detection

This study describes the development of a new analytical method for the separation and detection of cocaine (COC) and its adulterants, or cutting agents, using microchip electrophoresis (ME) devices coupled with capacitively coupled contactless conductivity detection (C⁴D). All the experiments were carried out using a glass commercial ME device containing two pairs of integrated sensing electrodes. The running buffer composed of 20 mmol/L amino‐2‐(hydroxymethyl) propane‐1,3‐diol and 10 mmol/L 3,4‐dimethoxycinnamic acid provided the best separation conditions for COC and its adulterants with baseline resolution (R > 1.6), separation efficiencies ranging from (2.9 ± 0.1) to (3.2 ± 0.2) × 10⁵ plates/m, and estimated LOD values between 40 and 150 μmol/L. The quantification of COC was successfully performed in four samples seized by the Brazilian Federal Police Department and all predicted values agree with values estimated by the reference method. Some other interfering species were detected in the seized samples during the screening procedure on ME–C⁴D devices. While lidocaine was detected in sample 3, the presence of levamisole was observed in samples 2 and 4. However, their concentrations were estimated to be below the LOQ. ME–C⁴D devices have proved to be quite efficient for the identification and quantification of COC with errors lower than 10% when compared to the data obtained by a reference method. The approach herein reported offers great potential to be used for on‐site COC screening in seized samples.     

Analysis of phenolic xenoestrogens by pressurized CEC with amperometric detection

A new method, pressurized CEC with end‐column amperometric detection using carbon paste electrode, has been developed for the separation and determination of five phenolic xenoestrogens in chicken eggs and milk powder samples. Efficient separation of five analytes was performed by pressurized CEC using a mobile phase consisting of 60% v/v ACN and 40% v/v Tris buffer (5 mmol/L, pH 8.0), +6 kV of applied voltage and 7.0 MPa of supplementary pressure. Detection limits of 50, 5, 2, 10 and 20 ng/mL for pentachlorophenol, bisphenol‐A, 2,4‐dichlorophenol, 4‐tert‐octylphenol and 4‐nonylphenol, respectively, were achieved using carbon paste electrode as working electrode and +0.8 V as detection potential. Matrix solid phase dispersion extraction method had been employed during sample preparation procedure, and mean recoveries ranged from 79.2 to 102.6% at different concentrations of phenolic xenoestrogens for spiked egg and milk powder samples were obtained.     

Microchip capillary electrophoresis with electrochemical detector for fast measurements of aromatic amino acids

A method based on microchip capillary electrophoresis with amperometric detection was developed for the rapid separation and direct detection of oxidizable aromatic amino acids (without prior derivatization). The working electrode was a thick-film carbon strip electrode positioned opposite the outlet of the separation channel. Factors influencing the separation and detection processes were examined and optimized. The five aromatic amino acids, tyrosine, 5-hydroxytryptophan, tryptophan, p-aminobenzoic acid, and m-aminobenzoic acid, can be well separated within 5 min using a separation voltage of 2000 V and a 25 mM phosphate buffer (pH 7.0) run buffer containing 50 mM sodium dodecylsulfate. Most favorable amperometric detection was obtained at +0.95 V. Linear calibration plots are observed for micromolar concentrations of the oxidizable amino acids. The new protocol offers good stability and for reproducibility, with relative S.D. of less than 5% for both migration times and peak currents (n=8). It should be useful for the analysis of aromatic amino acids, as desired for life sciences.     

CZE separation of amitrol and triazine herbicides in environmental water samples with acid-assisted on-column preconcentration

A simple analytical scheme for the detection and quantification of amitrol and triazine herbicides (atrazine, ametryn and atraton) and degradation product (2‐hydroxyatrazine) in environmental water samples by CZE is reported. On‐column preconcentration of analytes from untreated water samples (mineral, spring, tap and river water) is accomplished by introducing an acid plug (200 mM citrate of pH 2.0) after the sample and then proceeding with the CZE separation, using 100 mM formiate buffer of pH 3.5 as running buffer and 25.0 KV as separation voltage. UV detection at 200 nm provides LODs from 50 to 300 nM in untreated samples and they were lowered tenfold by sample preconcentration by evaporation. Calculated recoveries were typically higher than 90%. Minimal detectable concentration of the electroactive amitrol could be decreased about 20‐fold when electrochemical detection was employed by monitoring the amperometric signal at +800 mV using a carbon paste electrode (LOD of 9.6 nM, 0.81 μg/L, versus 170 nM, 14.3 μg/L, using amperometric and UV detection, respectively) in untreated water samples.     

Carbon paste-based electrochemical detectors for microchip capillary electrophoresis/electrochemistry

The first reported use of a carbon paste electrochemical detector for microchip capillary electrophoresis (CE) is described. Poly(dimethylsiloxane) (PDMS)-based microchip CE devices were constructed by reversibly sealing a PDMS layer containing separation and injection channels to a separate PDMS layer that contained carbon paste working electrodes. End-channel amperometric detection with a single electrode was used to detect amino acids derivatized with naphthalene dicarboxaldehyde. Two electrodes were placed in series for dual electrode detection. This approach was demonstrated for the detection of copper(II) peptide complexes. A major advantage of carbon paste is that catalysts can be easily incorporated into the electrode. Carbon paste that was chemically modified with cobalt phthalocyanine was used for the detection of thiols following a CE separation. These devices illustrate the potential for an easily constructed microchip CE system with a carbon-based detector that exhibits adjustable selectivity.     

The use of poly(dimethylsiloxane) surface modification with gold nanoparticles for the microchip electrophoresis

Poly(dimethylsiloxane) (PDMS) microfluidic channels modified by citrate-stabilized gold nanoparticles after coating a layer of linear polyethylenimine (LPEI) were successfully used to separate dopamine and epinephrine, which were difficult to be separated from baseline in native and hybrid PDMS microchannels. In-channel amperometric detection with a single carbon fibre cylindrical electrode was employed. Experimental parameters of separation and detection processes were optimized in detail. The analytes were well separated within 100 s in a 3.7 cm long separation channel at a separation voltage of +800 V using a 30 mM phosphate buffer solution (PBS, pH 7.0). Linear responses of them were obtained both from 25 to 600 μM with detection limits of 2 μM for dopamine and 5 μM for epinephrine, respectively. The modified PDMS channels have a long-term stability and an excellent reproducibility within 2 weeks.     

Determination of aspartame by ion chromatography with electrochemical integrated amperometric detection

In this paper, the separation and determination of the sweetener aspartame by ion chromatography coupled with electrochemical amperometric detection is reported. Sodium saccharin, acesulfame-K and aspartame were separated using 27.5 mmol/l NaOH isocratic elution on a Dionex IonPac AS4A-SC separation column. Aspartame can be determined by integrated amperometric detection without interference from the other two sweeteners. The method can be applied to the determination of aspartame in powered tabletop, fruit juice and carbonated beverage samples, and the results obtained by integrated amperometry were in agreement with those obtained using a UV detection method. A method for determining analytes with an NH₂ group by ion chromatography with integrated amperometry was developed.     

Use of different buffers for detection and separation in determination of physio‐active components in oolong tea infusion by CZE with amperometric detection

With a view of simultaneous determination of physio‐active ingredients in oolong tea infusion: sugars, amino acids, epigallocatechin gallate and ascorbic acid, a novel CZE with amperometric detection method was studied. Operated in a wall‐jet configuration, 100 mmol/L NaOH was used in detecting cell to lead the electrocatalysis oxidation behaviors of the analytes on a 300 μm diameter copper‐disc electrode (working electrode), while in separating capillary, a mild alkaline running buffer consisting in a mixture of 30 mmol/L borate and 40 mmol/L phosphates charged and carried analytes to detecting end. The methodology research was performed for system stability and suitability. Under the optimal CE conditions, analytes could be separated within moderate time period. Good linearity between peak area and concentration existed over three orders of magnitude; lower RSD and LOD were achieved. The oolong tea infusion was assayed and result was satisfactory.     

毛细管区带电泳法分析绿茶中的痕量成份

采用毛细管电泳/安培检测法(CE/AD)同时分离测定了绿茶中的芦丁、没食子酸、槲皮素、绿原酸等生物活性成分的含量, 考察了运行缓冲液酸度、浓度、分离电压、氧化电位和进样时间等实验参数对分离、检测的影响。在最优化条件下, 以300 μm碳圆盘电极为检测电极, 检测电位为+ 950 mV (vs. SCE) , 60 mmol/L硼酸盐运行缓冲液(pH 8.7)中, 上述各组分在20 min内可实现基线分离。各组分浓度与峰电流在3个数量级范围内呈良好线性, 检出限(S/N=3)在1.0×10^-7到1.0×10^-4g^.mL^-1范围,四种标样7次平行进样的相对标准偏差(RSD)小于3.0 %。该方法已成功地应用于绿茶中生物活性成分的测定, 结果令人满意。     

Study on Rhizoma Chuanxiong based on capillary electrophoresis with amperometric detection

<正>A high-performance capillary electrophoresis with amperometric detection(CE-AD) method has been developed for the analysis of seven bioactive ingredients,namely ferulic acid(FA),vanillin,vanillic acid,p-hydroxybenzoic acid,caffeic acid,gallic acid and protocatechuic acid,in Rhizoma Chuanxiong.The effects of several factors such as the acidity and concentration of running buffer,the separation voltage,the applied potential to working electrode and the injection time were investigated.Under the optimum conditions,the seven analytes could be well separated within 21 min at the separation voltage of 16 kV in a 60 mmol/L borax running buffer(pH 8.7).A 300μm diameter carbon disk electrode has a good response at potential of +950 mV(vs.SCE) for all analytes.Good linear relationship was established over three orders of magnitude with detection limits(S/N = 3) ranged from 3.3×10~(-7) to 6.7×10~(-9)g/mL.This proposed method has been successfully applied for the determination and comparison of different batches of Rhizoma Chuanxiong samples based on their characteristic electrochemical profiles.     

Capillary electrophoresis-amperometric determination of antioxidant propyl gallate and butylated hydroxyanisole in foods.

Capillary electrophoretic separation coupled with end-column amperometric detection for the simultaneous quantification of butylated hydroxyanisole (BHA) and propyl gallate (PG) in food was developed. Important factors affecting separation and detection, such as the running buffer, separation voltage, and detection potential, were investigated in detail. An improved working electrode preparation method was used, where a carbon disk of 33 microm in diameter was sealed in a tip and positioned opposite the outlet of a capillary. The experiments indicated that the preparation method was simple, and the obtained electrode exhibited good flexibility and stability for the determination of phenolic antioxidants. The separation was carried out within 5 min using a 50 cm length capillary, with a solution containing 5 mM phosphate and 5 mM borax of pH 8.84 as a separation buffer, and a separation potential of 20 kV. Amperometric detection was achieved with an applied potential of 0.70 V versus Ag|AgCl| saturated KCl. There was excellent linearity between the peak current and the concentrations of the analytes in the range of 1.8 - 180.2 microg/mL for BHA and 10.6 - 212.2 microg/mL for PG, respectively. Relative standard deviations of 4.92% for BHA and 5.27% for PG were obtained, respectively. The developed method was successfully applied for the determination of antioxidants in several commercial foods.     

Electroanalytical Determination of Some Phenolic Acids by High‐Performance Liquid Chromatography at Gold Electrodes

The electrochemical and amperometric behavior of a gold electrode was investigated towards the oxidation of several common phenolic acids in neutral phosphate solutions. Au electrodes show an appreciable stability and reproducibility of the amperometric signals by using a constant applied potential of 1.0 V vs. Ag/AgCl. Separations of selected phenolic acids using a reverse phase C₁₈ analytical column with a mobile phase containing 10 mM NaH₂PO₄ plus 10 mM Na₂HPO₄ (pH 7) and methanol as organic modifier, are achieved isocratically in less than 30 min. The detection limits at the level of nmol/L and linear ranges of four‐five orders of magnitude are generally achieved. The proposed chromatographic strategy coupled with the electrochemical detection at the Au electrode was successful tested for the quantitative determination of phenolic acids in beer, red wine and brandy with good sensitivity and recovery.     

Microchip reversed-phase liquid chromatography with packed column and electrochemical flow cell using polystyrene/poly(dimethylsiloxane)

A microchip pressure-driven liquid chromatography (LC) with a packed column and an electrochemical flow cell has been developed by using polystyrene (PS) and poly(dimethylsiloxane) (PDMS). The cylindrical separation column with packed octadecyl silica particles was fabricated in the PS substrate. The three electrode system (working, reference, and counter electrode) for amperometric detection was fabricated onto the PS substrate, using the Au deposition, photolithography, and chemical etching. The detector flow cell was formed by sealing the electrode system with a PDMS chip containing a channel. In this flow cell, the effect of working electrode width (in the direction of flow) on chromatographic parameters, such as peak width and peak resolution were studied in electrode width ranging 50-5,000 microm. The effect of electrode width on sensitivity (current intensity, current density, and S/N ratio) was also examined. The sensitivity was discussed by simulating the concentration profile generated around the working electrode. The effects of the column packing size and the column size on the separation efficiency were examined. In this study, a good separation of three catechins was successfully achieved and the detection limits for (+)-catechin, epicatechin, and epigallocatechin gallate were 350, 450, and 160 nM, respectively.     

Simultaneous determination of flavonoids and phenolic acids in Chinese herbal tea by beta-cyclodextrin based capillary zone electrophoresis

Kaempferol, apigenin, rutin, quercetin, luteolin and ferulic acid are separated and detected in Chinese herbal tea using capillary zone electrophoresis coupled to amperometric detection. The phosphate running buffer also contains ß-cyclodextrin (ß-CD), which assists in separation and gives excellent separations within 20 min and detection limits as low as 10 ng mL^?1 (S/N = 3). The effects of working electrode potential, pH and concentration of running buffer, concentration of ß-CD, separation voltage and injection time were investigated. The method was applied to analyze tea samples with recoveries in the range of 90.0 to 107.0%. The method offers high separation efficiency, short analysis time, small sample consumption, and good repeatability.     

Determination of chloride, chlorate and perchlorate by PDMS microchip electrophoresis with indirect amperometric detection

In this work, chloride, chlorate and perchlorate are fast separated on PDMS microchip and detected via in-channel indirect amperometric detection mode. With PDMS/PDMS microchip treated by oxygen plasma, anions chloride (Cl-), chlorate (ClO3-), and perchlorate (ClO4-) are separated within 35s. Some parameters including buffer salt concentration, buffer pH, separation voltage and detection potential are investigated in detail. The separation conditions using 15 mM (pH 6.12) of 2-(N-morpholino)ethanesulfonic acid (MES)+L-histidine (L-His) as running buffer, -2000 V as separation voltage and 0.7 V as detection potential are optimized. Under this condition, the detection limits of Cl-, ClO3-, and ClO4- are 1.9, 3.6, and 2.8 microM, respectively.