On-line Sample Preconcentration Techniques Research Articles

Application of on-line sample preconcentration by large-volume dual preconcentration by isotachophoresis and stacking (LDIS) on straight-channel microchips.

In this study, large-volume dual preconcentration by isotachophoresis and stacking (LDIS) which is an on-line sample preconcentration technique coupling large-volume sample stacking with an electroosmotic flow pump (LVSEP) with transient isotachophoresis (tITP) was applied to microchip electrophoresis (MCE) for improving both detection sensitivities and peak shapes. To realize LDIS in MCE, we investigated experimental procedures for injecting a short plug of a leading electrolyte (LE) solution into a straight microchannel without any sophisticated injector apparatus. We found that a short LE plug could be injected into a sample-filled straight-channel only by making the liquid level of the LE solution in an outlet reservoir higher than that in an inlet one. By applying a reversed-polarity voltage to the microchip, anionic analytes injected throughout the microchannel were first enriched by LVSEP, followed by tITP. Through the second preconcentration effect by tITP in LDIS, sensitivity enhancement factor (SEF) and asymmetry factor for a standard dye were improved from 878 and 0.62 to 1330 and 1.14, respectively, relative to those in conventional LVSEP. It should be noted that more viscous running buffer containing sieving polymers could be employed to the LDIS analysis, which was effective for improving the SEF and the separation efficiencies, especially for bio-polymeric compounds. Finally, LDIS was applied to the oligosaccharide and protein analyses in MCE, resulting in the SEFs of 1410 and ca. 50 for maltotriose and bovine milk casein, respectively.

Innovative analysis of diazepam, zolpidem and their main metabolites in human urine by micelle-to-solvent stacking in capillary electrophoresis

Diazepam and zolpidem are the most widely used medications for managing insomnia. However, significant concerns regarding the potential risks of misuse and abuse problems arose in many literatures. While urine analysis is a valuable diagnostic tool, a challenge arises from the fact that some parent drugs may remain undetectable in urine. This necessitates concurrent monitoring of their metabolites. Here, we described an innovative on-line sample preconcentration technique known as micelle to solvent stacking (MSS) for the analysis of diazepam, zolpidem, and their main metabolites in urine. Several key parameters warrant further discussion to optimize the MSS model, enhancing its performance in terms of sensitivity and resolution. After optimizing the conditions, we conducted a validation test, achieving high correlation coefficients (greater than 0.9977) for intra-day and inter-day regression lines. Additionally, both the relative standard deviation (RSD) and relative error (RE) remained below 6.10% and 12.55%, respectively. The limits of detection (LODs, S/N = 3) for all five analytes ranged from 2.0 to 56 ng/mL. Compared to the conventional capillary zone electrophoresis method, this new approach exhibited remarkable sensitivity enhancements, ranging from 123 to 235-fold. Upon applying this method to actual urine samples from patients, we successfully detected nordiazepam, zolpidem, and its metabolites. This simple and sensitive approach has promising applications in supporting patient medication safety and bolstering forensic investigations.

On-line sample preconcentration by LVSEP-FASI with simple voltage control on Y-channel chips.

To eliminate complicated voltage controls for highly sensitive microchip electrophoresis (MCE) analyses on the basis of combining two online sample preconcentration techniques, large-volume sample stacking with an electroosmotic flow (EOF) pump (LVSEP) and field-amplified sample injection (FASI), cross-channel microchips and a multichannel high-voltage power supply were replaced to Y-channel chips and a conventional power supply designed for capillary electrophoresis, respectively. By simple switching of the electric circuit after the LVSEP-FASI sample enrichments, the focused analytes could be separated during anodic migration in a separation channel. In the LVSEP-FASI analysis of fluorescein using the Y-channel microchip, the maximum sensitivity enhancement factor (SEF) of 7400 was achieved, resulting in a 30-fold detectability increase compared to the conventional LVSEP. The developed method was applied to the oligosaccharide analysis in MCE. As a result, the SEF for maltotriose was improved from 450 to 2300 and the baseline separation of the oligosaccharides was achieved without any complicated voltage control in LVSEP-FASI on the Y-channel chips.

Recent Applications of Dynamic On-Line Sample Preconcentration Techniques in Capillary Electrophoresis

This review highlights recent applications of on-line sample preconcentration techniques in capillary electrophoresis (CE) from 2019 to 2021. Various preconcentration techniques based on the analyte velocity change in two or three discontinuous solutions system including field-amplified sample stacking, transient isotachophoresis, pH-mediated stacking, sweeping, and their modified and combined techniques have been employed to enrich and separate biological, environmental, food samples, and so on, in CE. More than 60 published research articles collected from Scopus databases described the dynamic on-line sample preconcentration techniques. This review provides comprehensive tables listing the applications of the on-line sample preconcentration techniques.

Development of transient trapping micellar electrokinetic chromatography coupled with mass spectrometry for steroids analysis.

An on-line sample preconcentration technique based on transient trapping (tr-trapping) in micellar electrokinetic chromatography (MEKC) was applied for steroid detection with UV (tr-trapping-UV) and electrospray ionization mass spectrometry detection (tr-trapping-ESI-MS). ESI-MS was used to improve the sensitivity in MEKC. The MEKC separation was carried out using volatile ammonium formate as a background solution to facilitate the coupling with ESI-MS. The partial introduction of a sodium dodecyl sulfate (SDS) micellar solution before the introduction of a sample solution to the capillary provided the effective preconcentration of analytes. At the same time, the SDS micelle would not enter the ESI-MS system, so its interference in ESI-MS detection was suppressed under the optimal condition, then five steroids can be separated by the developed method. In tr-trapping-ESI-MS, an acidic condition of pH 3.5 was employed to suppress the electroosmotic flow, which can avoid micellar solution migrating to the MS instrument. The developed method showed that the micellar solution requires a twofold slower time than the sample to migrate along the column, which can prohibit the cause of the problem with the MS instrument and interference signal of SDS in the steroid's detection. The tr-trapping-ESI-MS protocol showed up to 540-fold enhancements of the peak intensity and 50-fold improvement of the limit of detection compared with capillary zone electrophoresis using androsterone as a model sample.

Online sample preconcentration techniques in nonaqueous capillary and microchip electrophoresis

One of the major drawbacks of electrophoresis in both capillary and microchip is the unsatisfactory sensitivity. Online sample preconcentration techniques can be regarded as the most common and powerful approaches commonly applied to enhance overall detection sensitivity. While the advances of various online preconcentration strategies in capillary and microchip employing aqueous background electrolytes are well-reviewed, there has been limited discussion of the feasible preconcentration techniques specifically developed for capillary and microchip using nonaqueous background electrolytes. This review provides the first consolidated overview of various online preconcentration techniques in nonaqueous capillary and microchip electrophoresis, covering the period of the last two decades. It covers developments in the field of sample stacking, isotachophoresis, and micellar-based stacking. Attention is also given to multi-stacking strategies that have been used for nonaqueous electrophoresis.

Recent Advances in Trace Bioanalysis by Capillary Electrophoresis.

Recently, single cell analysis is becoming more and more important to elucidate cellular heterogeneity. Except for nucleic acid that can be amplified by PCR, the required technical level for single cell analysis is extremely high and the appropriate design of sample preparation and a sensitive analytical system is necessary. Capillary/microchip electrophoresis (CE/MCE) can separate biomolecules in nL-scale solution with high resolution, and it is highly compatible with trace samples like a single cell. Coupled with highly sensitive detectors such as laser-induced fluorescence and nano-electrospray ionization-mass spectrometry, zmol level analytes can be detected. For further enhancing sensitivity, online sample preconcentration techniques can be employed. By integrating these high-sensitive techniques, single cell analysis of metabolites, proteins, and lipids have been achieved. This review paper highlights successful research on CE/MCE-based trace bioanalysis in recent 10 years. Firstly, an overview of basic knowledge on CE/MCE including sensitivity enhancement techniques is provided. Applications to trace bioanalysis are then introduced with discussion on current issues and future prospects.

Sample Preconcentration Protocols in Microfluidic Electrophoresis.

Electrophoretic on-line sample preconcentration techniques in microfluidic channels improve the sensitivity prior to the separation. Among various techniques, the most important field-amplified sample stacking and sweeping on cross-channel microchips are demonstrated. As a novel microfluidic preconcentration approach, a large-volume sample stacking with electroosmotic flow pump (LVSEP) on straight-channel chips is also presented, which can omit a complicated voltage program for sample injection processes. In this chapter, we describe how to prepare and how to run these on-line sample preconcentration methods in microchip electrophoresis.

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.

Imidazolium ionic liquids as dynamic and covalent modifiers of electrophoretic systems for determination of catecholamines

The subject of this study is comparison of imidazolium based dynamic and covalent coatings of the quartz capillary wall on the example of catecholamines determination. A way of synthesis of covalent coatings was proposed. For the first time different type of on-line sample preconcentration techniques (field-amplified sample stacking (FASS), head-column field amplified sample stacking (HC FASS), electrostacking, sweeping) were performed for catecholamines determination in N-alkylsubstituted imidazolium coated capillary.It was found that long chain imidazolium ionic liquids (C12MImCl and C16MImCl) in background electrolyte content create a dynamic coating of the quartz capillary walls and generate anode electroosmotic flow. Same is true for covalent coatings based on N-alkylsubstituted imidazolium. Both coatings prevent sorption of catecholamines on the internal surface of the quartz capillary. As a result efficiency and peak symmetry are increased.The maximum stacking efficiency factor (SEF) values for dynamic coatings were in range of 70–85, and limits of detection (LODs) were about 0.05 µg/ml under sweeping condition (micelle forming agent – C16MImCl) when the electric conductivity of sample matrix was higher than electric conductivity of BGE. In case of covalent coating, the maximum SEF values were higher than 1000, and LODs were about 1–2 ng/ml under sweeping condition (micelle forming agent – sodium dodecyl sulphate) in combination with electrostacking.

Sensitive arsenic speciation by capillary electrophoresis using UV absorbance detection with on-line sample preconcentration techniques

The World Health Organization (WHO) guideline states that the total arsenic concentration in drinking water must not exceed 10 ppb. However, arsenic toxicity varies significantly, with inorganic arsenic species being more toxic than organic species. Arsenic speciation is therefore important for evaluating the health risks from arsenic-contaminated drinking water. Capillary electrophoresis provides the necessary high performance separation to determine arsenic species in water, but its sensitivity with absorbance detection is far below than needed. Using a coated capillary, several on-line sample preconcentration techniques such as large volume sample stacking with an electroosmotic flow pump, field amplified sample injection (FASI), transient isotachophoresis (tITP), electrokinetic supercharging (EKS) combining FASI and tITP, and counter flow (CF)-EKS, were therefore investigated. With CF-EKS using phosphate and N-cyclohexyl-2-aminoethanesulfonate as leading and terminating electrolytes, respectively, standard samples of arsenite, arsenate, monomethylarsonic acid, and dimethylarsinic acid were preconcentrated from 6,300- to 45,000-fold. The limits of detection obtained with UV absorbance detection were 0.08–0.3 ppb As. For a spring water sample spiked with the four arsenic species, LODs of 2–9 ppb As were obtained, which are lower than the WHO guideline of 10 ppb total As.

Sample Stacking Provides Three Orders of Magnitude Sensitivity Enhancement in SDS Capillary Gel Electrophoresis of Adeno-Associated Virus Capsid Proteins.

Size-based protein analysis utilizing only 25 ng of total proteins has been realized by sodium dodecyl sulfate capillary gel electrophoresis (SDS CGE) with head-column field-amplified sample stacking as an online sample preconcentration technique. This method has been used as a replacement of SDS-PAGE for purity analysis of adeno-associated virus (AAV) therapeutic products of different serotypes and transgenes. A limit of detection of 0.2 ng/mL (3.3 pM) capsid proteins was achieved with convenient UV absorbance detection at 214 nm, equivalent to 20 pg of protein (330 attomole) loaded in the autosampler vial. For purity analysis, only 25 ng of total AAV capsid proteins (4.3 femtomole virus particles) were loaded to the autosampler vial. The sensitivity is comparable to silver-stained SDS-PAGE. The RSD of purity measurement was 0.0-0.8%, comparable to conventional SDS CGE utilizing 0.1-0.5 mg proteins. The new method provided 3 orders of magnitude sensitivity enhancement as compared to conventional SDS CGE. It shares all the advantages of conventional SDS CGE (labor-saving, easy automation, and convenient quantitation) and also the high sensitivity of silver stained SDS-PAGE. The sample stacking SDS CGE technique can be adopted for size-based analysis of other types of proteins. It is especially useful when protein quantity or concentration is not sufficient for regular SDS CGE or SDS-PAGE assay.

Ionic liquids in enhancing the sensitivity of capillary electrophoresis: Off-line and on-line sample preconcentration techniques.

The popularity of ionic liquids (ILs) has grown during the last decade in enhancing the sensitivity of CE through different off-line or on-line sample preconcentration techniques. Water-insoluble ILs were commonly used in IL-based liquid phase microextraction, in all its variants, as off-line sample preconcentration techniques combined with CE. Water-soluble ILs were rarely used in IL-based aqueous two phase system (IL-ATPS) as an off-line sample preconcentration approach combined with CE in spite of IL-ATPS predicted features such as more compatibility with CE sample injection due to its relatively low viscosity and more compatibility with CE running buffers avoid, in some cases, anion exchange precipitation. Therefore, the attentions for the key parameters affecting the performance of IL-ATPSs were generally presented and discussed. On-line CE preconcentration techniques containing IL-based surfactants at nonmicellar or micellar concentrations have become another interesting area to improve CE sensitivity and it is likely to remain a focus of the field in the endeavor because of their numerous to create rapid, simple and sensitive systems. In this article, significant contributions of ILs in enhancing the sensitivity of CE are described, and a specific overview of the relevant examples of their applications is also given.

Recent developments in chiral analysis of β-blocker drugs by capillary electromigration techniques.

The latest developments in chiral analysis of β-blocker drugs by capillary electromigration techniques are reviewed in this article. Following the previous review by Aturki et al. [Electrophoresis 2011, 32, 2602-2628], this review includes the papers published during the period from January 2011 to December 2013. During this time, some novel chiral selectors were reported and applied to improve the enantioseparation of β-blocker drugs and structurally related compounds. These chiral selectors include CDs and their derivatives, macrocyclic antibiotics, tartrate complexs, the monolithic molecularly imprinted polymer, and the polymeric surfactants. In addition, this article summarizes the methodological improvements for enhancing sensitivity in chiral analysis of β-blockers and structurally related compounds by CE. The involved authors described the use of online sample preconcentration techniques to increase the detection sensitivity in the enantiomeric analysis of a broad range of samples.

Sensitive enantioanalysis of β-blockers via field-amplified sample injection combined with water removal in microemulsion electrokinetic chromatography.

In this study, an on-line sample preconcentration technique, field-amplified sample injection combined with water removal by electroosmotic flow (EOF) pump, was applied to realize a highly sensitive chiral analysis of β-blocker enantiomers by MEEKC. The introduction of a water plug in capillary before the electrokinetic injection provided the effective preconcentration of chiral compounds. And then the water was moving out of the column from the injection end under the effect of the EOF, which avoided dilution of the stacked β-blocker enantiomers concentration suffering from the presence of water in separation buffer. Moreover, the addition of H3 PO4 and methanol in the sample solution greatly improved the enhancement efficiency further. Under optimized conditions, more than 2700-fold enhancement in sensitivity was obtained for each enantiomer of bupranolol (BU), alprenolol (AL), and propranolol (PRO) via electrokinetic injection. LODs were 0.10, 0.10, 0.12, 0.11, 0.02, and 0.02 ng/mL for S-BU, R-BU, S-AL, R-AL, S-PRO, and R-PRO, respectively. Eventually, the proposed method was successfully applied to the determination of BU, AL, and PRO in serum samples with good recoveries ranging from 93.4 to 98.2%.

Recent Advances in Enhancing the Sensitivity and Resolution of Capillary Electrophoresis

An extensive search of published research and review articles indicates that enhancing the sensitivity and resolution of capillary electrophoresis (CE) is a very active area of interest. This review focuses on developments and applications in this field over several years, especially from 2009-present. It first reviews developments in the fields of online sample preconcentration and highly sensitive detection for sensitivity enhancement. The online sample preconcentration techniques cover all methods, from electrophoretic preconcentration, extraction-based preconcentration, micelle-based preconcentration and hybrid preconcentration to nanoparticle-based preconcentration. Attention is also given to multiple dimension separations, additives in buffer solution and capillary inner surface modifications that have been used to enhance the resolution and separation efficiency of complex samples in CE. The additives include nanoparticles, chiral selectors and other buffer additives that have been added to the buffer solution.

Preconcentration of 3-nitrotyrosine in urine by transient isotachophoresis in MEKC

Transient isotachophoresis (tITP), usually performed in CZE, is a useful on-line sample preconcentration technique for high saline samples. However, only a few papers have applied tITP in MEKC especially SDS system. This study compared tITP in MEKC with that in CZE for 3-nitro-tyrosine (NT) analysis in urine. No sample clean-up was required. Self-stacking occurred simultaneously as chloride in urine acted as a leading ion. HEPES–Tris buffer at pH 8.2 was used as a terminating electrolyte and separation buffer. UV detection wavelength was set at 426nm. Experiments showed that tITP in MEKC exhibited much higher peak efficiency (up to 1000000), and improved 4-fold sensitivity, compared with those by tITP in CZE. The limit of detection was 0.07μM for 3-nitrotyrosine in urine.

Headspace‐single drop microextraction with a commercial capillary electrophoresis instrument

Automated coupling of headspace-single drop microextraction (HS-SDME) and CE has been demonstrated using a commercial CE instrument. When a drop hanging at the inlet tip of a capillary for CE is used as the acceptor phase, HS-SDME becomes a simple but powerful sample pretreatment technique for CE before injection to facilitate sample cleanup and enrichment. By combining HS-SDME with an on-line sample preconcentration technique, large volume sample stacking using an electroosmotic flow pump, the sensitivity can be improved further. The overall enrichment factors for phenolic compounds were from 1900 to 3400. HS-SDME large volume sample stacking using an electroosmotic flow pump was successfully applied to a red wine sample to obtain an LOD of 4 nM (0.8 ppb) for 2,4,6-trichlorophenol which is a precursor for 2,4,6-trichloroanisole causing the foul odor in wine called cork taint.

Review of recent developments of on-line sample stacking techniques and their application in capillary electrophoresis

Abstract Capillary electrophoresis (CE) has become one of the most useful tools in separation science because of its high separation efficiency, low cost, versatility, ease of sample preparation and automation. However, some limitations of CE, such as poor concentration sensitivity due to its lower sample loading and shorter optical path length, limits its further applications in separation science. In order to solve this problem, various on-line sample preconcentration techniques such as transient isotachophoresis preconcentration, field-enhanced sample stacking, micelle to solvent stacking, micelle collapse, dynamic pH junction, sweeping, solid phase extraction, single drop microextraction and liquid phase microextraction have been combined with CE. Recent developments, applications and some variants together with different combinations of these techniques integrating in CE are reviewed here and our discussions will be confined to the past three years (2008–2011).

Recent Progress of On-line Sample Preconcentration Techniques in Microchip Electrophoresis

This review highlights recent developments and applications of on-line sample preconcentration techniques to enhance the detection sensitivity in microchip electrophoresis (MCE); references are mainly from 2008 and later. Among various developed techniques, we focus on the sample preconcentration based on the changes in the migration velocity of analytes in two or three discontinuous solutions system, since they can provide the sensitivity enhancement with relatively easy experimental procedures and short analysis times. The characteristic features of the on-line sample preconcentration applied to microchip electrophoresis (MCE) are presented, categorized on the basis of "field strength-" or "chemically" induced changes in the migration velocity. The preconcentration techniques utilizing field strength-induced changes in the velocity include field-amplified sample stacking, isotachophoresis and transient-isotachophoresis, whereas those based on chemically induced changes in the velocity are sweeping, transient-trapping and dynamic pH junction.