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

Capillary zone electrophoresis of lipoarabinomannan by multi-layered concentration

The present paper describes a capillary zone electrophoresis method which relies on a multi-layered water-alkali solvent stacking with on-line ionization to enhance detection of mannose, arabinose, and their oligosaccharides, which are used as the migration profile standards but are also the distinctive structural components of lipoarabinomannan. Lipoarabinomannan is detected in patients having tuberculosis. The CE method with ionization of the whole saccharides without degradation in alkaline solution inside the capillary is based structural deprotonation of the molecules under ultrahigh pH conditions. The validation of the CE parameters revealed that the 15-fold electrolyte - water -injection plug allowed detection of one third lower concentrations than detected without on-line concentration. For the first time, the better detectability was seen especially for highly polymerized, but otherwise poorly ionized, arabino-octaose. The applicability of the method for detecting whole large biological saccharide complexes was confirmed by the glycolipid lipoarabinomannan. For the first time also, the migration of the indestructible lipoarabinomannan was detected together with oligosaccharides used representing the capping units, namely mannose, mannobiose and mannotriose. The myo-inositol-phosphate-lipid unit was seen to migrate separately from the arabinomannan, since it was hydrolyzed from one lipoarabinomannan product under alkaline conditions in CE. This article is protected by copyright. All rights reserved.

Recent advances in enhancing the sensitivity of electrophoresis and electrochromatography in capillaries and microchips (2016-2018)

One of the most cited limitations of capillary and microchip electrophoresis is the poor sensitivity. This review continues to update this series of biannual reviews, first published in Electrophoresis in 2007, on developments in the field of online/in-line concentration methods in capillaries and microchips, covering the period July 2016-June 2018. It includes developments in the field of stacking, covering all methods from field-amplified sample stacking and large-volume sample stacking, through to isotachophoresis, dynamic pH junction, and sweeping. Attention is also given to online or in-line extraction methods that have been used for electrophoresis.

Chiral Measurement of Aspartate and Glutamate in Single Neurons by Large-Volume Sample Stacking Capillary Electrophoresis

d-Amino acids (d-AAs) are endogenous molecules found throughout the metazoan, the functions of which remain poorly understood. Measurements of low abundance and heterogeneously distributed d-AAs in complex biological samples, such as cells and multicellular structures of the central nervous system (CNS), require the implementation of sensitive and selective analytical approaches. In order to measure the d- and l-forms of aspartate and glutamate, we developed and applied a stacking chiral capillary electrophoresis (CE) with laser-induced fluorescence detection method. The achieved online analyte preconcentration led to a 480-fold enhancement of detection sensitivity relative to capillary zone electrophoresis, without impacting separation resolution or analysis time. Additionally, the effects of inorganic ions on sample preconcentration and CE separation were evaluated. The approach enabled the relative quantification of d-aspartate and d-glutamate in individual neurons mechanically isolated from the CNS of the sea slug Aplysia californica, a well characterized neurobiological model. Levels of these structurally similar d-AAs were significantly different in subpopulations of cells collected from the investigated neuronal clusters.

Analysis of endogenous nucleotides by single cell capillary electrophoresis-mass spectrometry

Analytical technologies that enable investigations at the single cell level facilitate a range of studies; here a lab-fabricated capillary electrophoresis-electrospray ionization-mass spectrometry (CE-ESI-MS) platform was used to analyze anionic metabolites from individual Aplysia californica neurons. The system employs a customized coaxial sheath-flow nanospray interface connected to a separation capillary, with the sheath liquid and separation buffer optimized to ensure a stable spray. The method provided good repeatability of separation and reliable detection sensitivity for 16 mono-, di- and triphosphate nucleosides. For a range of anionic analytes, including cyclic adenosine monophosphate (cAMP), adenosine diphosphate (ADP) and adenosine triphosphate (ATP), the detection limits were in the low nanomolar range (<22 nM). A large Aplysia R2 neuron was used to demonstrate the ability of CE-ESI-MS to quantitatively characterize anionic metabolites within individual cells, with 15 nucleotides and derivatives detected. Following the method validation process, we probed smaller, 60 μm diameter Aplysia sensory neurons where sample stacking was used as a simple on-line analyte preconcentration approach. The calculated energy balance ([ATP] + 0.5 × [ADP])/([AMP] + [ADP] + [ATP]) of these cells was comparable with the value obtained from bulk samples.

Advance of CE and CEC in phytochemical analysis (2012–2013)

This article presents an overview of the advance of CE and CEC in phytochemical analysis, based on the literature not mentioned in our previous review papers [Chen, X. J., Zhao, J., Wang, Y. T., Huang, L. Q., Li, S. P., Electrophoresis 2012, 33, 168–179], mainly covering the years 2012–2013. In this article, attention is paid to online preconcentration, rapid separation, and sensitive detection. Selected examples illustrate the applicability of CE and CEC in biomedical, pharmaceutical, environmental, and food analysis. Finally, some general conclusions and future perspectives are given.

Recent developments in capillary and microchip electroseparations of peptides (2011-2013)

The review presents a comprehensive survey of recent developments and applications of capillary and microchip electroseparation methods (zone electrophoresis, ITP, IEF, affinity electrophoresis, EKC, and electrochromatography) for analysis, isolation, purification, and physicochemical and biochemical characterization of peptides. Advances in the investigation of electromigration properties of peptides, in the methodology of their analysis, including sample preseparation, preconcentration and derivatization, adsorption suppression and EOF control, as well as in detection of peptides, are presented. New developments in particular CE and CEC modes are reported and several types of their applications to peptide analysis are described: conventional qualitative and quantitative analysis, determination in complex (bio)matrices, monitoring of chemical and enzymatical reactions and physical changes, amino acid, sequence and chiral analysis, and peptide mapping of proteins. Some micropreparative peptide separations are shown and capabilities of CE and CEC techniques to provide relevant physicochemical characteristics of peptides are demonstrated.

Novel moving reaction boundary-induced stacking and separation of human hemoglobins in slab polyacrylamide gel electrophoresis

We developed a novel polyacrylamide gel electrophoresis (PAGE) method to stack and separate human hemoglobins (Hbs) based on the concept of moving reaction boundary (MRB). This differs from the classic isotachophoresis (ITP)-based stacking PAGE in the aspect of buffer composition, including the electrode buffer (pH 8.62 Tris-Gly), sample buffer (pH 6.78 Tris-Gly), and separation buffer (pH 8.52 Tris-Gly). In the MRB-PAGE system, a transient MRB was formed between alkaline electrode buffer and acidic sample buffer, being designed to move toward the anode. Hbs carried partial positive charges in the sample buffer due to its pH below pI values of Hbs, resulting in electromigrating to the cathode. Hbs would carry negative charges quickly when migrated into the alkaline electrode buffer and be transported to the anode until meeting the sample buffer again. Thus, Hbs were stacked within a MRB until the transient MRB reached the separation buffer and then separated by zone electrophoresis with molecular sieve effect of the gel. The experimental results demonstrated that there were three clear and sharp protein zones of Hbs (HbA1c, HbA0, and HbA2) in MRB-PAGE, in contrast to only one protein zone (HbA0) in ITP-PAGE for large-volume loading (≥15 μl), indicating high stacking efficiency, separation resolution, and good sensitivity of MRB-PAGE. In addition, MRB-PAGE was performed in a conventional slab PAGE device, requiring no special device. Thus, it could be widely used in separation and analysis of diluted protein in a standard laboratory.

Large-volume sample stacking for in vivo monitoring of trace levels of γ-aminobutyric acid, glycine and glutamate in microdialysates of periaqueductal gray matter by capillary electrophoresis with contactless conductivity detection

A new variant of large-volume sample stacking injection (LVSS) was used in the capillary electrophoresis with capacitively coupled contactless conductivity detection (CE/C(4)D) determination of the neurotransmitters γ-aminobutyric acid (GABA), glycine (Gly) and glutamate (Glu) in microdialysates of periaqueductal gray matter (PAG). The separation capillary was filled to 98% from the injection side with a sample of microdialysate in acetonitrile. Simultaneously with turning on the separation voltage, the sample zone was forced out by the background electrolyte by increasing the pressure in the terminal capillary outlet vessel. As a consequence of the stacking effect, the analyte was concentrated from the large sample volume into a narrow zone at the sample/background electrolyte boundary close to the injection end of the capillary. Under these conditions, LOD values of 9, 10 and 15nM were determined in the model samples for GABA, Gly and Glu, respectively; RSD equalled 0.5% for the migration times and 1.0-1.9% for the peak areas, respectively. In analysis of microdialysates of PAG, LOD values of 29, 29 and 37nM were determined for GABA, Gly and Glu, respectively; RSD equalled 0.5-0.7% for the migration times and 2.6-8.2% for the peak areas, respectively. The determined basal levels of the neurotransmitters in PAG microdialysates are 0.08, 4.7 and 0.8μM for GABA, Gly and Glu, respectively. Carrageenan-induced hyperalgesia increases the Gly and Glu levels and reduces GABA in PAG microdialysate. Peroral administration of paracetamol in hyperalgesia effectively reduces the Gly value and has no effect on Glu and GABA.

Contemporary sample stacking in analytical electrophoresis

Sample stacking is a term denoting a multifarious class of methods and their names that are used daily in CE for online concentration of diluted samples to enhance separation efficiency and sensitivity of analyses. The essence of these methods is that analytes present at low concentrations in a large injected sample zone are concentrated into a short and sharp zone (stack) in the separation capillary. Then the stacked analytes are separated and detected. Regardless of the diversity of the stacking electromigration methods, one can distinguish four main principles that form the bases of nearly all of them: (i) Kohlrausch adjustment of concentrations, (ii) pH step, (iii) micellar methods, and (iv) transient ITP. This contribution is a continuation of our previous reviews on the topic and brings an overview of papers published during 2010-2012 and relevant to the mentioned principles (except the last one which is covered by another review in this issue).