Field-enhanced Sample Injection Research Articles

In-capillary screening and quantifying the antioxidants of Yangxinshi Tablet using field enhanced sample injection-micellar electrokinetic chromatography.

An in-capillary 2,2'-azinobis-(3-ethylbenzthiazoline-6-sulphonate) as oxyradicals combining field enhanced sample injection with micellar electrokinetic chromatography was developed for screening and determination of the major antioxidants in Yangxinshi Tablet. To obtain simultaneous separation and detection of radicals and analytes, relevant factors were optimized separately. Under the optimum conditions, four compounds including salvianolic acid B, hyperoside, puerarin and caffeic acid were identified as the major antioxidants. All validation results covering recovery, precision, and stability demonstrated a good applicability of the method. On this basis, the total antioxidant activity was successfully evaluated in terms of the decreased peak area of radicals. There was a correlation coefficient of 0.8974 between the total contents of major antioxidants and total antioxidative activity of sample. Therefore, these four compounds were selected as combinatorial markers for quality evaluation of Yangxinshi Tablet. It was concluded that the established method presented a powerful potential to screen and quantify active ingredients in complex preparation of Chinese medicine. This article is protected by copyright. All rights reserved.

Two-step micelle-to-solvent stacking of arsenic species from foods in permanently coated tubing for capillary electrophoresis

An effective and sensitive two-step stacking of arsenic species by electrokinetic injection and micelle-to-solvent stacking (MSS) using zwitterionic surfactant 3-(N,N-dimethylpalmitylammonio) propane sulfonate was successfully developed in a co-electro-osmotic flow (co-EOF) capillary zone electrophoresis (CZE). The fused silica capillaries were coated with 1% hexadimethrine bromide solution to satisfy the condition of the same EOF direction between the test anions and capillary. The background solution was 100 mM borax buffer (pH 9.2). The proposed method was performed by a CZE system equipped with a diode array detector, and the detection wavelength was monitored at 192 nm. The separation voltage was −20 kV. The group of micelles and target analytes was stacked by field-enhanced sample injection at −10 kV for 180 s. The second stacking step used 60% MeOH to serve as an organic solvent where the analytes were brought by the micelles to the MSS boundary. The enrichment factors of As(Ⅴ), As(Ⅲ), disodium methyl arsonate hexahydrate (MMA), and dimethylarsinic acid (DMA) were 1230, 840, 3820, and 1450, respectively, compared to typical injections in CZE. The limits of detection (S/N = 3) ranged from 0.382 to 0.911 ng mL−1. The intracapillary repeatabilities (%RSD, n = 3) were 0.5–1.0% for migration time and 0.3–0.7% for peak areas. The technology of two-step stacking by MSS in co-EOF CZE together with a simple extraction procedure for As(Ⅴ), As(Ⅲ), MMA and DMA were demonstrated in kelp and rice samples.

Determination of quetiapine and its metabolites in plasma by field-enhanced sample stacking.

Quetiapine is an atypical antipsychotic drug that can be used to treat mental disorders, including schizophrenia, bipolar disorder and Alzheimer's disease. Quetiapine is mainly converted into the active metabolites of norquetiapine and 7-hydroxyquetiapine by the liver enzymes CYP3A4 and CYP2D6. In this study, liquid-liquid extraction (LLE) was used as a sample pretreatment method to eliminate interferences in plasma. tert-Butyl methyl ether was chosen as the extraction solvent. Field-enhanced sample injection (FESS), an online preconcentration technique, was used to analyze quetiapine and its metabolites norquetiapine and 7-hydroxyquetiapine in plasma. The optimal separation condition was 120 mM phosphate (pH 4.0) containing 0.005% (w/v) polyvinyl pyrrolidone and 40% (v/v) methanol. The methanol plug was 0.3 psi for 6 s, the sample was electrokinetic injection by 10 kV for 60 s at positive polarity, and the separation voltage was set at 26 kV. In this experiment, quetiapine, norquetiapine and 7-hydroxyquetiapine were successfully extracted from plasma by the LLE method and stacking and separated by FESS within 15 min. The limits of detection (S/N = 3) of quetiapine, norquetiapine and 7-hydroxyquetiapine were 0.25 ng/mL, 0.50 ng/mL and 1.00 ng/mL, respectively. The linear ranges of quetiapine, norquetiapine and 7-hydroxyquetiapine were 3-120 ng/mL and the correlation coefficients were 0.999. Compared with that of the traditional capillary zone electrophoresis method, the sensitivity enrichment of analytes was 463-835-fold. The optimal experimental conditions were successfully applied to the analysis of plasma samples from patients taking quetiapine for the treatment of schizophrenia.

Capillary Electrophoresis-Mass Spectrometry with Multisegment Injection and In-Capillary Preconcentration for High-Throughput and Sensitive Determination of Therapeutic Decapeptide Triptorelin in Pharmaceutical and Biological Matrices.

A capillary electrophoresis–tandem mass spectrometry method with a multisegment injection and an in-capillary field-enhanced sample stacking for determination of therapeutic peptide triptorelin in pharmaceutical and biological matrices was developed. The CE separation conditions were optimized in order to obtain maximal separation efficiency, analytical signal intensity and stability, and minimal adsorption of the analyzed peptide onto the capillary wall (1 M formic acid—HFo, pH 1.88). The implementation of the field-enhanced sample injection into CE improved the value of limit of detection 50 times while the multisegment injection increased the sample throughput three times in comparison to a conventional CE approach. The proposed method was characterized by favorable performance parameters, such as linearity (r2 ≥ 0.99), limit of detection (5 ng mL−1 in water matrix, 25 ng mL−1 in plasma matrix), precision (relative standard deviation, 1.5–9.4% for intraday and 2.3–11.9% for interday reproducibility), or accuracy (relative errors in the range of 80–109%). The FDA-validated method was successfully applied to the analysis of triptorelin in the commercial drug Diphereline® 0.1 mg (powder for injection) and in spiked human plasma samples. Favorable performance parameters along with proven application potentialities indicate the usefulness of the proposed method for its routine use in drug quality control laboratories and for clinical analysis, such as determination of triptorelin levels in plasma (for pharmacokinetic study).

A dual preconcentration method by combining micro matrix solid-phase dispersion extraction with field-enhanced sample injection and micelle to cyclodextrin stacking for sensitive analysis of neutral coumarins.

A rapid, sensitive, environmental friendly dual preconcentration method by combining micro matrix solid-phase dispersion extraction with field-enhanced sample injection and micelle to cyclodextrin stacking has been developed for the determination of furocoumarins. Molecular sieve, KIT-6, was used as an adsorbent in micro matrix solid-phase dispersion process. The important parameters affecting off-line and online CE preconcentration efficiency were optimized. Under the optimal experimental conditions, all analytes showed good linearity (R2 >0.999). The LODs of notopterol, isoimperatorin, and imperatorin were 0.1μg/mL, 1.2mg/kg, and 1.0mg/kg, respectively. Compared with the normal CE method, the enrichment times were up to 300. Moreover, Angelicae Dahuricae Radix was used as the mode of complex solid sample matrix to demonstrate the prospect of application of this methodology. The results showed the proposed strategy is promising for determining trace furocoumarins in complex matrix samples, which might be applied as a powerful and economic tool in monitoring illegal cosmetic adding.

Analysis of malachite green, gentian violet and their leuco metabolites in catfish and carp by micellar electrokinetic capillary chromatography

A simple, reproducible and sensitive micellar electrokinetic chromatography (MEKC) method was developed for the separation and determination of malachite green (MG), leucomalachite green (LMG), gentian violet (GV) and leucogentian violet (LGV) from catfish and carp. Field-enhanced sample injection with reverse migrating micelles (FESI-RMM) was used for on-line concentration of the analytes. The buffer contained 50 mM H3PO4-NaOH, 140 mM sodium dodecyl sulfate (SDS), 15% acetonitrile and 15% 2-propanol, at the pH of 2.0. The sample solution was diluted with water containing 5 mM SDS and injected for 15 s with -8 kV after injection of 2 s water plug. The effects of concentration of SDS and organic modifier, the sample matrix, the injection time of water plug, the injection voltage and injection time of sample on the separation and stacking efficiency were investigated. Under the optimum conditions, the analytes were well separated and by optimizing the stacking conditions, about 121, 146, 150 and 87 fold improvement in the detection sensitivities were obtained for GV, MG, LMG and LGV, respectively. The instrument detection limits (S/N = 3) of GV, MG, LMG and LGV were 40, 70, 50 and 50 ng/mL, respectively. The recoveries of GV, MG, LMG and LGV in the extracts of catfish were 87.8-96.7%, 87.9-100.2%, 96.7-101.4% and 89.8-94.2%, respectively. The GV, MG, LMG and LGV in spiked (1 μg/mL) catfish and carp extracts were successfully determined with satisfactory repeatability and recovery.

A sensitive two-step stacking by coupling field-enhanced sample injection and micelle to cyclodextrin stacking for the determination of neutral analytes

To promote the detection sensitivity of neutral analytes, a sensitive two-step stacking strategy by coupling field-enhanced sample injection and micelle to cyclodextrin stacking in reverse migrating micelles MEKC has been established by selecting jujuboside A and B as models. The stacking mechanism, affecting factors as well as analytical performances of the proposed method were investigated. Compared with typical injection, the sensitivity enhancement factors of this method were up to 140- and 152-fold for jujuboside A and B, respectively. The enhancements were 8–10 times better than only single stacking strategy by MCDS. The LODs were 0.2–0.3 μg mL−1, interday and intraday%RSD were both lower than 4.3%, and a good linearity (r>0.999) was obtained. Feasibility in real samples analysis was evaluated by using Semen Ziziphi Spinosae dispensing granule and rat's urine. The results suggested that the strategy was simple, reliable, sensitive and promising for the analysis of neutral analytes in complex sample matrix.

Solid phase extraction and capillary electrophoretic separation of racemic catecholamines by using magnetic particles coated with a copolymer prepared from poly(3,4-dihydroxyphenylalanine) and polyethyleneimine.

A method is presented for chiral separation of catecholamines present in bovine and mice blood. It combines magnetic solid phase extraction (MSPE) and chiral capillary electrophoresis (ch-CE). A copolymer consisting of poly(3,4-dihydroxyphenylalanine) and polyethyleneimine was coated onto magnetic particles (MPs) by co-deposition using the CuSO4/H2O2 system as a polymerization initiator. The coated MPs are spherical and the average diameter is about 168 ± 4nm. The thickness of the coating is approximately 19nm. The functional MPs are used as sorbents in MSPE to simultaneously extract the catecholamines epinephrine, norepinephrine and isoprenaline. Under the optimum conditions, the extraction efficiencies for those catecholamines are in the range from 92.3 to 98.3%, with relative standard deviations (RSDs) of <5.3%. The extraction can be performed within 4min. The extracts were then submitted to ch-CE. A method for field-enhanced sample injection (FESI) was used to enhance the detection sensitivities of the enantiomers. The limits of detection for catecholamine enantiomers range from 400 to 600pgmL-1. In comparison with the FESI-ch-CE method, the sensitivity enhancement factors of the MSPE/ch-CE method for catecholamines are about 10-fold. The method was applied to the determination of trace levels of catecholamine enantiomers in (spiked) bovine and mice blood. The recoveries ranged from 88.2 to 93.8%, with RSDs of <5.5%. The whole detection procedure takes less than 30min. Graphical abstract Schematic representation of the separation and detection of catecholamine enantiomers in blood by combination of polyDOPA/PEI-magnetic particles-based solid phase extraction and chiral-capillary electrophoresis.

On-line preconcentration of sodium dodecyl sulfate-protein complexes using electrokinetic supercharging method with a prefilled water plug in capillary sieving electrophoresis.

An electrokinetic supercharging (EKS) method with a prefilled water plug at the head column of capillary was developed for on-line preconcentration of sodium dodecyl sulfate (SDS)-protein complexes in capillary sieving electrophoresis (CSE). Conventional EKS is a combination of electrokinetic injection with transient isotachophoresis (tr-ITP). The capillary is first filled with background electrolyte, then an appropriate amount of a leading electrolyte is filled and electro-injection is carried out for certain duration. After that, terminating electrolyte is filled, and tr-ITP is subsequently initiated, followed by capillary electrophoresis (CE) separation. In this work, the performance of EKS was evaluated by integrating multiple sub-methods step by step, and a water plug containing polymer was introduced before electrokinetic injection in order to further improve the concentration effect. The positive effects of the sub-methods were verified, including molecular sieving effect of polymer, field enhanced sample injection (FESI) with and without a water plug, and transient isotachophoretic electrophoresis-based FESI. It was observed that analyte discrimination usually encountered in conventional electrokinetic injection was eliminated due to the similar charge to mass ratios of SDS-protein complexes. Based on these results, a hybrid on-line preconcentration method, EKS with injecting a water plug containing polymer before sample electrokinetic injection, was proposed and used to indiscriminately preconcentrate SDS-protein complexes, which provided a sensitivity enhancement factor of more than 1000. It was very suitable for the analysis of low-abundance proteins, providing the information of their molecular mass.

Enantioseparation of chiral β-blockers using polynorepinephrine-coated nanoparticles and chiral capillary electrophoresis.

A method of combining magnetic solid-phase separation (MSPE) and chiral capillary electrophoresis (CE) is developed for enantioseparation of trace amounts of β-blockers. Polynorepinephrine-functionalized magnetic nanoparticles (polyNE-MNPs) are synthesized and applied to simultaneously extract three β-blockers (carteolol, metoprolol, and betaxolol). The prepared polyNE-MNPs are spherical with a diameter of 198 ± 17nm and the thickness of the polyNE coating is about 14nm. PolyNE possesses abundant catechol hydroxyl and secondary amine groups, endowing the MNPs with excellent hydrophilicity. Under the optimum conditions, the extraction efficiencies of polyNE-MNPs for β-blockers are in the range of 89.6 to 100%, with relative standard deviations (RSDs) below 3.5%. The extraction process can be finished in 4min. Field-enhanced sample injection (FESI) in chiral CE is constructed to further enhance the sensitivities of β-blocker enantiomers. The limits of detection for β-blocker enantiomers by the FESI-CE with polyNE-MNPs are in the range of 0.401 to 1.59ngmL-1. The practicability of this method in real samples is evaluated by analysis of human urine samples. The recoveries for each enantiomer of β-blockers in the real samples range from 89.5 to 92.8%, with RSDs ranging from 0.37 to 5.9%. The whole detection process can be finished in less than 0.5h. The method demonstrates its great potential in the pharmacokinetic and pharmacodynamic studies of chiral drugs in humans. Graphical abstract ᅟ.

Rapid quantification of quinine by multi-stacking in a portable microchip electrophoresis system.

A new multi-stacking pre-concentration procedure based on field-enhanced sample injection (FESI), field-amplified sample stacking, and transient isotachophoresis was developed and implemented in a compact microchip electrophoresis (MCE) with a double T-junction glass chip, coupled with an on-chip capacitively coupled contactless conductivity detection (C4 D) system. A mixture of the cationic target analyte and the terminating electrolyte (TE) from the two sample reservoirs was injected under FESI conditions within the two sample-loading channels. At the double T-junction, the stacked analyte zones were further concentrated under field-amplified stacking conditions and then subsequently focused by transient-isotachophoresis and separated along the separation channels. The proposed multi-stacking strategy was verified under a Universal Serial Bus (USB) fluorescence microscope employing Rhodamine 6G as the model analyte. This developed approach was subsequently used to monitor the target quinine present in human plasma samples. The total analysis time for quinine was approximately 200s with a sensitivity enhancement factor of approximately 61 when compared to the typical gated injection. The detection and quantification limits of the developed approach for quinine were 3.0μg/mL and 10μg/mL, respectively, with intraday and interday repeatability (%RSDs, n = 5) of 3.6 and 4.4%. Recoveries in spiked human plasma were 98.1-99.8%.

Enantioseparation and sensitive analysis of ofloxacin by poly(3,4-dihydroxyphenylalanine) functionalized magnetic nanoparticles-based solid phase extraction in combination with on-line concentration capillary electrophoresis

Chiral separation of low concentrations of pharmaceuticals in biofluids is a challenge task. In this study, a method is developed to enantioseparate trace amount of racemic ofloxacin in urine and bovine blood by combining magnetic solid phase extraction (MSPE) and chiral capillary electrophoresis (CE). Poly(3,4-dihydroxyphenylalanine) modified magnetic nanoparticles (polyDOPA-MNPs) are prepared and used as the adsorbents in MSPE to extract ofloxacin. The polyDOPA-MNPs are spherical, about 130 nm in diameter and the polyDOPA shell is about 3 nm. The extraction process of polyDOPA-MNPs for ofloxacin includes 2 min adsorption and 2 min desorption with the assistance of sonication. Under the optimized MSPE conditions (3 mg polyDOPA-MNPs as adsorbents, sample pH 7.0, 75% (v/v) AcOH in methanol as eluent, adsorption time 2 min, desorption time 2 min), the extraction efficiency of ofloxacin is 95%. In chiral CE, pressure-assisted field-enhanced sample injection (PA-FESI) is developed to improve the detection sensitivity of ofloxacin enantiomers. The MSPE/PA-FESI chiral CE method demonstrates a good linearity in the concentration range of 0.01–0.06 μg/mL, with a detection limit as low as 0.29 ng/mL. The feasibility of the method is verified by the successful determination of trace amounts of ofloxacin enantiomers in spiked urine and bovine blood samples.

Over 1000-fold improvement in an online preconcentration of trace anionic compounds by capillary electrophoresis with ionic liquid micelle-based three-step stacking

A sensitive, simple, and environmentally friendly online three-step stacking strategy combining field-enhanced sample injection, sweeping and micelle to solvent stacking has been developed to determine trace amounts of organic anionic analytes in complex biological samples. A green ionic liquid, 1-dodecyl-3-methylimidazolium hydrogen sulfate, was first introduced as a micellar solution and used for electroosmotic flow reversal in this stacking strategy. The mechanism of stacking has been discussed, and parameters affecting three-step stacking preconcentration efficiency have been optimized. The capillary coated with an ionic liquid was easy to prepare, regenerable and repeatable (RSD<2.16%) and yielded a high efficiency. Under optimal conditions, the sensitivity improvement increased up to 2424-fold when compared with the normal capillary zone electrophoresis mode. The peak shape and separation efficiency of the model basic organic acids showed a significant enhancement when compared to traditional capillary zone electrophoresis. The optimized method showed great potential for quantitative analysis of trace concentration analytes in complex matrices.

Simultaneous electrokinetic and hydrodynamic injection and sequential stacking featuring sweeping for signal amplification following MEKC during the analysis of rapamycin (sirolimus) in serum samples.

Simultaneous electrokinetic and hydrodynamic injection of rapamycin (sirolimus) with off-line and online sample preconcentration techniques and using MEKC has been studied. Compared to conventional hydrodynamic injection, a 168-fold improvement in the signal was obtained with a combination of simultaneous electrokinetic and hydrodynamic injectionand field enhanced sample injection in conjunction with a sweeping technique called sequential stacking featuring sweeping. However, the coupling of the developed electrophoretic method and solid-phase microextraction allowed the signal intensity to increase more than 231 times. In this approach, the injection of the sample at negative polarity (anode at the detector end) into the capillary and the MEKC separation was achieved within 5min using an electrolyte (composed of 10mM sodium tetraborate and 40mM SDS) when ultraviolet (UV) detection was performed at 280nm. Thus, by combining the application of the sequential stacking featuring sweeping supported by the solid-phase microextraction clean-up procedure, the detection limit (LOD) for rapamycin in a serum sample was significantly decreased, and was set at 25ng/mL. The proposed combined simultaneous electrokinetic and hydrodynamic injection with field enhanced sample injection -sweeping technique following MEKC separation of sirolimus in human serum could be an effective tool in biomedical and clinical applications.

The increase of detection sensitivity of micellar electrokinetic capillary chromatography method of stamp pad inks components by applying a sample stacking mode for the purpose of questioned document examination

The study focused on comparison of on-line sample concentration techniques, i.e. sample stacking modes, applied in capillary electrophoresis in order to improve the detection of components of stamp pad inks so that destruction of analysed document could be minimized. In particular, such techniques were taken into consideration as: normal stacking mode/field amplified sample stacking (NSM/FASS), high-salt concentration, sweeping, electrokinetic injection + FASS, and field enhanced sample injection (FESI). At first, two mixtures of 4 red dyes and 4 violet/blue dyes were examined. As a result, in case of red dyes two environments: ACN/1 mg mL−1 aqueous NaCl (95:5, v/v) and 10-fold water dilution of BGE preceded by water plug (6 s, 0.7 psi) injection, were chosen as the most promising for further examination. Regarding the blue/violet dyes, the 100-fold water dilution of BGE was selected as giving the best increase of height of peaks. Ultimately, nine stamp pad inks in three colours (red, blue and violet) produced by different manufacturers were examined under the optimal conditions. Only 5 dots cut-out from inked paper were sampled. Almost all examined samples were possible to be differentiated basing on the obtained electrophoretic profiles. In conclusion, the developed sample stacking mode of MECC–based method was found as a useful tool in the differentiation of stamp pad inks for the purposes of forensic questioned document analysis.

Quality monitoring methods of initial and terminal manufacture of LiFePO4 based lithium ion batteries by capillary electrophoresis

Magnetic impurities of lithium ion battery degrade both the capacity and cycling rates, even jeopardize the safety of the battery. During the material manufacture of LiFePO4, two opposite and extreme cases (trace impurity Fe(II) with high content of Fe(III) background in FePO4 of initial end and trace Fe(III) with high content of Fe(II) background in LiFePO4 of terminal end) can result in the generation of magnetic impurities. Accurate determination of impurities and precise evaluation of raw material or product are necessary to ensure reliability, efficiency and economy in lithium ion battery manufacture. Herein, two kinds of rapid, simple, and sensitive capillary electrophoresis (CE) methods are proposed for quality monitoring of initial and terminal manufacture of LiFePO4 based lithium ion batteries. The key to success includes the smart use of three common agents 1,10-phenanthroline (phen), EDTA and cetyltrimethyl ammonium bromide (CTAB) in sample solution or background electrolyte (BGE), as well as sample stacking technique of CE feature. Owing to the combination of field-enhanced sample injection (FESI) technique with high stacking efficiency, detection limits of 2.5nM for Fe(II) and 0.1μM for Fe(III) were obtained corresponding to high content of Fe(III) and Fe(II), respectively. The good recoveries and reliability demonstrate that the developed methods are accurate approaches for quality monitoring of LiFePO4 manufacture.

Monitoring of vancomycin in human plasma via portable microchip electrophoresis with contactless conductivity detector and multi-stacking strategy

A portable microchip electrophoresis (MCE) coupled with on-chip contactless conductivity detection (C4D) system was evaluated for the determination of vancomycin in human plasma. In order to enhance the detection sensitivity, a new online multi-stacking preconcentration technique based on field-enhanced sample injection (FESI) and micelle-to-solvent stacking (MSS) was developed and implemented in MCE-C4D system equipped with a commercially available double T-junction glass chip. The cationic analytes from the two sample reservoirs were injected under FESI conditions and subsequently focused by MSS within the sample-loading channel. The proposed multi-stacking strategy was verified under a fluorescence microscope using Rhodamine 6G as the model analyte and a sensitivity enhancement factor (SEF) of up to 217 was achieved. The developed approach was subsequently implemented in the aqueous-based MCE, coupled to C4D in order to monitor the targeted antibiotic (in this case, vancomycin) present in human plasma samples. The multi-stacking and analysis time for vancomycin were 50s and 250s respectively, with SEF of approximately 83 when compared to typical gated injection. The detection limit of the method for vancomycin was 1.2μg/mL, with intraday and interday repeatability RSDs of 2.6% and 4.3%, respectively. Recoveries in spiked human plasma were 99.0%–99.2%.

Integration of the free liquid membrane into electrokinetic supercharging – capillary electrophoresis for the determination of cationic herbicides in environmental water samples

A new approach based on the integration of the free liquid membrane (FLM) into electrokinetic supercharging (EKS) was demonstrated to be a new powerful tool used in order to enhance online preconcentration efficiency in capillary electrophoresis (CE). A small plug of water immiscible organic solvent was used as a membrane interface during the electrokinetic sample injection step in EKS in order to significantly enhance the analyte stacking efficiency. The new online preconcentration strategy was evaluated for the determination of paraquat and diquat present in the environmental water samples. The optimised FLM-EKS conditions employed were as follows: hydrodynamic injection (HI) of 20mM potassium chloride as leading electrolyte at 50mbar for 75s (3% of the total capillary volume) followed by the HI of tris(2-ethylhexyl) phosphate (TEHP) as FLM at a 1mm length (0.1% of the capillary volume). The sample was injected at 10kV for 360s, followed by the HI of 20mM cetyl trimethylammonium bromide (CTAB) as terminating electrolyte at 50mbar for 50s (2% of the total capillary volume). The separation was performed in 12mM ammonium acetate and 30mM NaCl containing 20% MeOH at +25kV with UV detection at 205nm. Under optimised conditions, the sensitivity was enhanced between 1500- and 1866-fold when compared with the typical HI at 50mbar for 50s. The detection limit of the method for paraquat and diquat was 0.15–0.20ng/mL, with RSDs below 5.5%. Relative recoveries in spiked river water were in the range of 95.4–97.5%. A comparison was also made between the proposed approach with sole preconcentration of the field-enhanced sample injection (FASI) and EKS in the absence of the FLM.

Field-enhanced sample injection-micelle to solvent stacking in nonaqueous capillary electrophoresis

The low conductivity of separation electrolytes employed in nonaqueous capillary electrophoresis (NACE) limits the use of on-line sample concentration or stacking by field enhancement. Herein, micelle-to-solvent stacking (MSS) was performed by the simple injection of a micellar solution plug prior to electrokinetic injection of sample prepared under field-enhanced stacking conditions (known as field-enhanced sample injection, FESI). The proposed approach allowed a 214–625-fold improvement in peak signals for targeted anticancer drugs (e.g., tamoxifen) and its major metabolites in NACE using 100% methanol-based separation electrolyte that comprised of 7.5mM deoxycholic acid sodium salt, 15mM acetic acid and 1mM 18-crown-6. These improvements yielded tamoxifen and its metabolites with 2–5 times better stacking efficiency as compared to those obtained without micellar solution injection or FESI only. This is comparable to the results typically achieved when FESI is combined with isotachophoresis (electrokinetic supercharging). The FESI-MSS-NACE was tested for the measuring levels of target drugs in plasma. The analytical figures of merit are also reported.

Three-step stacking by field-enhanced sample injection, sweeping, and micelle to solvent stacking in capillary electrophoresis: Anionic analytes

Three-step stacking by field-enhanced sample injection (FESI), sweeping, and micelle to solvent stacking (MSS) in co-EOF capillary zone electrophoresis (CZE) is presented for anionic analytes. Long FESI produced an overloaded stacked zone of analytes (four model penicillins). Sweeping of the FESI zone was by electrokinetic injection of cetyltrimethylammonium bromide (CTAB) micelles. MSS was by short injection of 60% methanol that released the swept analytes from CTAB micelles. The sensitivity enhancement factors were 146–279 and 519–954 for conductivity ratio of 10 and 100, respectively. The SEF enhancement factors (factor=SEF from three-step stacking/SEF from FESI) were 16–32 and 6–10, correspondingly. The LODs were between 6.6–13.2ng/mL, repeatability (intraday and interday) was %RSD≤5.4%, and linearity was R2≥0.998. Application to real sample was investigated using fortified plasma after liquid–liquid extraction.