Acupuncture injection for field amplified sample stacking and glass microchip‐based capillary gel electrophoresis

Acupuncture sample injection is a simple method to deliver well‐defined nanoliter‐scale sample plugs in PDMS microfluidic channels. This acupuncture injection method in microchip CE has several advantages, including minimization of sample consumption, the capability of serial injections of different sample solutions into the same microchannel, and the capability of injecting sample plugs into any desired position of a microchannel. Herein, we demonstrate that the simple and cost‐effective acupuncture sample injection method can be used for PDMS microchip‐based field amplified sample stacking in the most simplified straight channel by applying a single potential. We achieved the increase in electropherogram signals for the case of sample stacking. Furthermore, we present that microchip CGE of ΦX174 DNA‐HaeⅢ digest can be performed with the acupuncture injection method on a glass microchip while minimizing sample loss and voltage control hardware.     

pH-independent large-volume sample stacking of positive or negative analytes in capillary electrophoresis

In capillary electrophoresis, the short optical path length associated with on-column UV detection imposes an inherent detection problem. Detection limits can be improved using sample stacking. Recently, large-volume sample stacking (LVSS) without polarity switching was demonstrated to improve detection limits of charged analytes by more than 100-fold. However, this technique requires suppression of the electroosmotic flow (EOF) during the run. This necessitates working at a low pH, which limits using pH to optimize selectivity. We demonstrate that LVSS can be performed at any buffer pH (4.0-10.0) if the zwitterionic surfactant Rewoteric AM CAS U is used to suppress the EOF. Sensitivity enhancements of up to 85-fold are achieved with migration time, corrected area, and peak height reproducibility of 0.8-1.6%, 1.3-3.7%, and 0.8-4.9%, respectively. Further, it is possible to stack either positively or negatively charged analytes using zwitterionic surfactants to suppress the EOF.     

Simultaneous stacking of cationic and anionic compounds in single run capillary zone electrophoresis by two-end field amplified sample injection

In order to extend the application of field amplified sample injection (FASI) in high throughput analysis, a convenient and simple procedure, namely two-end field amplified sample injection (TE-FASI), was developed for the simultaneous stacking of cationic and anionic compounds in a single run capillary zone electrophoresis (CZE). Following the capillary-filling with a buffer of high conductivity, water plug was loaded into each end of the capillary; and two high-field strength zones were generated at both heads of the column when high voltage was applied. Therefore, under suppressed EOF cations and anions can be selectively FASI stacked at anode and cathode head, respectively. After separation, the stacked anions and cations are detected by a common detector placed in the center of the capillary. Under the optimized conditions, the limits of detection for the model cationic (matrine and oxymatrine) and anionic (5-sulfosalicylic acid) compounds were determined as 0.2, 0.2 and 0.06 ng/mL, respectively. Compared with non-stacking conditions, the sensitivities of these compounds were enhanced 1003-, 1330- and 1380-fold, respectively. The results of reproducibility, linearity and real sample analysis show that the proposed procedure is promising to be applied for the simultaneous quantification detection of trace cationic and anionic analytes.     

Separation of alkylbenzyl quaternary ammonium compounds by capillary zone electrophoresis with sample stacking injection

Summary A systematic investigation of operational buffer systems, sample preparation and instrument parameters for achieving the best possible performance for determinating an homologous series of N-benzyl-N-alkyl-N,N-dimethylammonium chloride compounds by capillary zone electrophoresis with direct UV detection. The most effective separation was achieved within 3.5 min with the addition of acetonitrile (40%) in a phosphate buffer (20 mM pH 5.2) using a 40 cm fused-silica capillary operating at 25 KV and 20°C. Degassing of all electrolyte solutions and samples was very important. The linearity and repeatability for each compounds were satisfactory. To improve detection limits, on-column sample preconcentration, sample stacking, was investigated achieving a tenfold enrichment factor and quantitation limits about 10⁻⁷M.     

Capillary electrophoresis of methylmercury with injection by sample stacking

A procedure for separation and quantitation of methylmercury by capillary electrophoresis using sample stacking as the injection technique is presented. The CE conditions have been optimized in order to separate the methylmercury from the excess cysteine peak and to concentrate large volumes of sample obtaining a low detection limit. Under the proposed operational conditions, the detection limit (S/N = 3) was 12 ng g⁻ and the limit of quantitation (S/N = 10) was 20 ng g⁻¹ with a linear range of 20–100 ng g⁻¹ (as methylmercury in samples). The method was tested using different reference materials with a certified methylmercury content.     

毛细管电泳-场强放大样品堆积法检测染发剂中的7种苯胺类物质

建立了毛细管电泳-场强放大样品堆积测定染发剂中4,4′-二氨基二苯甲烷、苯胺、邻甲氧基苯胺、对氨基苯甲醚、3,4-二甲基苯胺、间氨基苯酚、1-萘胺7种苯胺类物质的分析方法。在优化的缓冲溶液体系(0.15 mol/L NaH2PO4,0.015 mol/L 三乙醇胺, pH 2.3)下7种分析物在6.5 min内实现基线分离。考察了样品中添加的磷酸浓度和乙腈浓度、水柱长度、电动进样时间与电压对场强放大富集效率及重现性的影响。最佳的富集条件为: 水柱注入3.45 kPa(0.5 psi)×6 s,样品中添加40%(v/v)乙腈和0.6×10~3mol/L磷酸,进样电压与进样时间为10 kV×10 s。线性范围为3～1000 μg/L(R2＞0.996),检出限为0.26～2.75 μg/L,将已有方法的检测灵敏度提高了1～3个数量级。在2种市售黑色染发剂中均检测到间氨基苯酚,含量分别为7.32 mg/g和1.34 mg/g。平均加标回收率为74%～108%。该方法灵敏度高、快速、重现性好、成本低,可供多种样品基质中痕量苯胺类污染物及其他阳离子物质的测定借鉴使用。     

柱头场放大样品富集技术测定镍

利用毛细管电泳在线化学发光检测技术,研究了镍离子场放大样品富集过程。结果表明,利用该技术可使灵敏度显著改善,浓缩因子达5 8×104,镍离子检出限为1 2×10-10mol/L。预进一段水柱可使检出限达到7 0×10-11mol L。还考察了进样方式、样品基体浓度、进样时间、进样电压对检测灵敏度的影响。     

Field amplified injection in the presence of salts for capillary electrophoresis.

Salts in the sample are detrimental to the stacking by the field-amplified injection. However, physiological samples often contain salts at levels of about 1% which can diminish the peak height or cause band spreading instead of stacking. Using different analytes which contain salts, we demonstrate that the presence of acetonitrile at 66% in the sample reverses the deleterious effect of salts and favors the stacking by the electrokinetic injection. The advantage of this type of stacking is that it favors certain analytes over others and it can give, in some instances, better theoretical plate numbers.     

Microsequential injection: anion separations using 'lab-on-valve' coupled with capillary electrophoresis

Microsequential injection (microSI) has been successfully coupled with capillary electrophoresis (CE). Presented is the microSI-CE system, interfaced with an integrated Lab-on-Valve (LOV) manifold that provides an efficient sample delivery conduit and a versatile means of sample pretreatment along with total automation of the separation process. Programmable microSI protocols control all critical system peripherals to perform various types of CE sample injections automatically such as electrokinetic (EK) injection, hydrodynamic (HD) injection, and head column field amplification (HCFA) sample stacking injection. Novel features of the microSI-CE technique are demonstrated on assays of samples containing 10 anions that had been used previously as a model system. Calibration studies by EK sample injection yielded linear concentration ranges of 0.5-3.0 mM with linear regression responses of r2 = 0.9999 for both chloride and sulfate using conductivity corrected peak area (CCPA) as concentration responses. Calibration using an internal standard was studied at the same concentration range giving r2 = 0.9992 for both chloride and sulfate and r2 = 0.9997 for both when CCPA correction was deployed. With HCFA sample stacking injection, a linear concentration dynamic range of 0.034-3.419 mM for chloride and 0.014-1.408 mM for sulfate were produced with linear regression responses of r2 = 0.9999 for chloride and r2 = 0.9998 for sulfate.     

Analysis of food colorants by capillary electrophoresis with large-volume sample stacking

A technique combining an on-capillary concentration method known as large-volume sample stacking and high-efficiency CE separation has been developed to analyze and detect colorants in several food samples, such as soft drinks, jellies and milk beverages. Following optimization, this technique significantly reduced the limits of detection of eight food colorants commonly used in food products by up to two orders of magnitude when compared with the conventional capillary electrophoresis method. The developed technique was able to successfully determine colorants in food samples that had concentrations as low as 0.1-0.5 microg/ml.     

Analysis of naphthalenesulfonate compounds by cyclodextrin-mediated capillary electrophoresis with sample stacking

This study systematically investigates the optimal conditions for analyzing the positional isomers of multi-charged naphthalenesulfonate compounds by cyclodextrin-mediated capillary electrophoresis (CE). Specifically, this work employs large-volume sample injection with the electrode polarity switching technique. The most effective separation and sample stacking conditions were 15 mM borate buffer with a mixture of beta- and gamma-cyclodextrin (concentration ratio 3:7 mM) at pH 9.2, and the sample hydrodynamic injection of up to 60 s at 3 p.s.i. (around 1.8 microl, and 1 p.s.i. = 6.9 kPa). Significantly selective and sensitive improvements were observed and a more than 100-fold enrichment was achieved (based on peak area). The reproducibility of migration time and quantitative results of stacking CE can be improved by using an internal standard. The quantitation limits of these naphthalenesulfonate isomers, based on a signal-to-noise ratio above 10, can be about 4 microg/l with UV detection. This method was successfully applied to determine the trace amount of naphthalenesulfonate isomers in a spiked drinking water sample.     

Sensitive profiling of biogenic amines in human urine by capillary electrophoresis with field amplified sample injection

In order to monitor biogenic amines in human urine, a method based on field‐amplified sample injection combined with capillary electrophoresis and direct UV absorption detection was developed. Dopamine, tyramine, tryptamine, serotonin and epinephrine were effectively separated and identified in human urine samples, and detection limits were 0.072, 0.010, 0.027, 0.010 and 0.120 µmol/L, respectively. Detection limits comparable to laser‐induced fluorescence detection or solid phase extraction combined with capillary electrophoresis were achieved. Parameters affecting electrophoretic system detection sensitivity were investigated. Optimal separation conditions were obtained using as background electrolyte a pH 6.5 mixture of 2‐(morpholino)ethanesulfonic acid 20 mmol/L and 30 mmol/L phosphate buffer, containing 0.05% hydroxypropylcellulose and 10% v/v methanol. Injections of the sample solution were performed by applying a voltage of 12 kV for 50 s. Recovery and accuracy ranged between 89.4 and 94.9%, and 89 and 112%, respectively. The method was successfully applied on actual urine samples (from a healthy volunteer): target bioamine content was consistent with endogenous levels reported in the literature. The proposed method is simple, fast and inexpensive and can be conveniently employed in work‐related stress studies. The affordability and noninvasive sampling of the method allow epidemiological studies on large number of exposed persons to be performed. Copyright © 2013 John Wiley & Sons, Ltd.     

Quality evaluation of six bioactive constituents in goji berry based on capillary electrophoresis field amplified sample stacking

Goji berry, fruits of the plant Lycium barbarum L., has long been used as traditional medicine and functional food in China. In this work, a simple and easy‐operation on‐line concentration capillary electrophoresis (CE) for detection flavonoids in goji berry was developed by coupling of field amplified sample stacking (FASS) with an electroosmotic (EOF) pump driving water removal process. Due to the EOF pump and electrokinetic injection showing different influence on the concentration, the analytes injection condition should be systemically studied. Thereafter, the verification of the analytes injection conditions was achieved using response surface experimental design. Under the optimum conditions, 86–271 folds sensitivity enhancement upon normal capillary zone electrophoresis (CZE, 50 mbar × 5 s) were achieved for six flavonoids, and the detection limits ranged from 0.35 to 1.82 ng/mL; the LOQ ranged from 1.20 to 6.01 ng/mL. Eventually, the proposed method was applied to detect flavonoids in 30 goji berry samples from different habitats of China; and the results indicated that the flavonoids were rich in the eluent of 30–60% methanol, which provided a reference for extraction of goji berry flavonoids.     

Organic solvent high-field amplified stacking for basic compounds in capillary electrophoresis

Many water-miscible organic solvents, especially acetonitrile and acetone, bring along significant degrees (approximately 30 times) of stacking by electroinjection through high-field amplified injection for the basic compounds compared to that for aqueous buffers or water. The relative stacking of different compounds in acetonitrile or acetone is different compared to that for water. Stacking by electroinjection in organic solvents is less stringent and easier to accomplish in practice. Acids and salts, in aqueous solutions, can ruin the stacking for both organic and aqueous solvents; however, this effect can be better tolerated by diluting the sample in acetonitrile. Thus, this stacking is termed "organic solvent high-field amplified injection". This stacking by electroinjection is enhanced by increasing the electrophoresis buffer concentration and can be better than that by pressure injection. From the practical aspects, some cationic drugs present in serum such as amiodarone can be detected at the therapeutic levels by electroinjection on the capillary after protein precipitation by acetonitrile.     

An automated electrokinetic continuous sample introduction system for microfluidic chip-based capillary electrophoresis

An automated and continuous sample introduction system for microfluidic chip-based capillary electrophoresis (CE) was developed in this work. An efficient world-to-chip interface for chip-based CE separation was produced by horizontally connecting a Z-shaped fused silica capillary sampling probe to the sample loading channel of a crossed-channel chip. The sample presentation system was composed of an array of bottom-slotted sample vials filled alternately with samples and working electrolyte, horizontally positioned on a programmable linearly moving platform. On moving the array from one vial to the next, and scanning the probe, which was fixed with a platinum electrode on its tip, through the slots of the vials, a series of samples, each followed by a flow of working electrolyte was continuously introduced electrokinetically from the off-chip vials into the sample loading channel of the chip. The performance of the system was demonstrated in the separation and determination of FITC-labeled arginine and phenylalanine with LIF detection, by continuously introducing a train of different samples. Employing 4.5 kV sampling voltage (1000 V cm(-1) field strength) for 30 s and 1.8 kV separation voltage (400 V cm(-1) field strength) for 70 s, throughputs of 36 h(-1) were achieved with <1.0% carryover and 4.6, 3.2 and 4.0% RSD for arginine, FITC and phenylalanine, respectively (n = 11). Net sample consumption was only 240 nL for each sample.     

Flow/sequential injection sample treatment coupled to capillary electrophoresis. A review

Various flow sample handling approaches coupled to capillary electrophoresis (CE) are reviewed, covering the research in this field in the 12 years since the milestone year of 1997, when practical interfaces to on-line couple flow injection (FI) and capillary electrophoresis were first developed independently by two research groups. Some previous attempts are also presented. Since 1997 a plethora of ingenious coupled systems have been developed. Although several reviews are available on various aspects of the topic, we have opted for a comprehensive overview of all FI-CE systems, as well as related and similar systems. This coupling has thus also led to the development of systems based on hybrids between the classical and microchip approaches. Truly microchip FI-CE systems are also included in this review.The developed systems have been used for various sample treatments, including on-line membrane-assisted sample treatment, column-based preconcentration, on-line derivatization and monitoring, to name just a few. The utility of coupling flow sampling to CE has been demonstrated in various practical applications that are discussed in detail. The current state-of-the-art and foreseeable future developments are also discussed.     

Analysis of glutathione by capillary electrophoresis based on sample stacking

Glutathione is a small peptide, which participates in cellular oxidation-reduction and detoxification. It is present in most biological tissues at different concentrations. The oxidized and reduced forms of the peptide were measured in erythrocytes and myocardial tissue by capillary electrophoresis based on stacking. After tissue homogenization or hemolysis of the red blood cells, the samples were deproteinized with acetonitrile and injected filling about 13% of the capillary volume. The electrophoresis was performed at 10 kV using a separation buffer of 250 mM borate, 50 mM Tris, pH 8.0. Sample stacking increased the sensitivity of detection by 10-20-fold.     

Determination of tetracyclines in human urine samples by capillary electrophoresis in combination with field amplified sample injection

A sensitive method using CZE‐UV detection has been developed for the determination of five tetracycline antibiotics in human urine samples. To improve the sensitivity of the method, an on‐line preconcentration strategy, named field‐amplified sample injection, has been developed, based on the electrokinetic injection of the sample, which requires only a 1:100 dilution with sample solvent before injection. Under optimum conditions, sensitivity enhancement factors ranged from 450 to 800 for the studied compounds. The applicability of the proposed method was demonstrated by the determination of these antibiotics in spiked urine samples. The limits of quantification were lower than 0.8 mg/L and the precision (intra‐ and inter‐day), expressed as %RSD was below 14%. Recoveries ranged from 92.1 to 96.7%. Thus, the proposed procedure is a simple, fast and efficient strategy which could be used as therapeutic drug monitoring in human urine samples.     

Analysis of linear alkylbenzenesulfonates by capillary zone electrophoresis with large-volume sample stacking

A systematic investigation of optimal conditions for determining the homologues of linear alkylbenzenesulfonates (LAS) by capillary zone electrophoresis (CZE) using the large-volume sample stacking technique was presented. The most effective sample stacking and separation conditions was 20 mM borate buffer with 30% acetonitrile at pH 9.0, and the sample hydrodynamic injection of up to 90 s at 4 p.s.i. (1 p.s.i. = 6,892.86 Pa) (around 711 nl). Under such conditions, approximately a 100-fold enrichment factor was achieved based on peak heights. The reproducibility of migration time and quantitative results of stacking CZE can be improved by using internal standards. Quantitation limits of the homologues of LAS were 0.002-0.01 mg/l under these enrichment conditions. The analysis of real samples of laundry and dishwashing detergents was performed. The established high-performance liquid chromatography method was applied to evaluate the stacking CZE method, and compatible results were obtained.     

Rapid and sensitive determination of strychnine and brucine in human urine by capillary electrophoresis with field‐amplified sample stacking

A simple, rapid, sensitive and low‐cost method using capillary electrophoresis (CE) coupled with field‐amplified sample stacking (FASS) has been developed and validated for the simultaneous determination of strychnine and brucine residues in human urine. Before sample loading, a water plug (3.5 kPa, 3 s) was injected to contain sample cations and to permit FASS. Electrokinetic injection at a voltage (20 kV, 25 s) was then used to introduce cations. Separation was performed using 20 mM acetate buffer (pH 3.8) with an applied voltage of 20 kV. The calibration curves were linear over a range of 8.00–2.56 ∞ 10² ng/mL (r = 0.9995) for strychnine and 10.0–3.20 × 10² ng/mL (r = 0.9999) for brucine. Extraction recoveries in urine were greater than 79.6 and 82.8% for strychnine and brucine, respectively, with an RSD of less than 4.9%. The detection limits (signal‐to‐noise ratio 3) for strychnine and brucine were 2.00 and 2.50 ng/mL, respectively. A urine sample from one healthy female volunteer (26 years old, 50 kg) was pretreated and analyzed. Strychnine and brucine levels in urine could be detected 24 h after administration. On these grounds, this method was feasible for application to preliminary screening of trace levels of abused drugs for both doping control and forensic analysis. Copyright © 2009 John Wiley & Sons, Ltd.