Steady state microdialysis of microliter volumes of body fluids for monitoring of amino acids by capillary electrophoresis with contactless conductivity detection

BACKGROUND

The availability of dialysis membranes in the form of hollow fibres with diameters compatible with the fused silica capillaries used in capillary electrophoresis is very limited. However, haemodialysis bicarbonate cartridges commonly used in human medicine containing polysulfone hollow fibres are available on the market and are used for the fabrication of coaxial microdialysis probes. The miniature probe design ensures that steady-state conditions are achieved during microdialysis of minimal volumes of body fluids.

RESULTS

A coaxial microdialysis probe with a length of 5 cm and an inner diameter of 200 μm is used for microdialysis of 10 μL of body fluid collected into a sampling fused silica capillary with an inner diameter 430 μm. Microdialysis is performed into 0.01 M HCl as a perfusate at stopped flow and 2 μL of the resulting microdialysate are subjected to analysis by capillary electrophoresis with contactless conductivity detection. Microdialysis pre-treatment is verified by analysis of 11 common amino acids at a 100 μM concentration level, resulting in recoveries of 98.3-102.5%. The electrophoretic separation of amino acids is performed in 8.5 M acetic acid at pH 1.37 as a background electrolyte with analysis time up to 4.5 min and LOD in the range of 0.12-0.28 μM. The reproducibility of the developed technique determined for the peak area ranges from 1.2 to 4.5%. Applicability is tested in the quantification of valine and leucine in plasma during fasting and subsequent reconvalescence.

SIGNIFICANCE

The fabrication of a coaxial microdialysis probe for the laboratory preparation of microliter volumes of various types of clinical samples is described, which is coupled off-line with capillary electrophoretic monitoring of amino acids in 2 μL volumes of microdialysate. The developed methodology is suitable for quantification of 20 amino acids in whole human blood, plasma, tears and has potential for analysis of dry blood spots captured on hollow fibre.

Monitoring of circulating amino acids in patients with pancreatic cancer and cancer cachexia using capillary electrophoresis and contactless conductivity detection

Branched chain amino acids (BCAAs), alanine and glutamine are determined in human plasma by capillary electrophoresis with contactless conductivity detection (CE/C⁴ D). The baseline separation of five amino acids from other plasma components is achieved on the short capillary effective length of 18 cm in 3.2 mol/L acetic acid with addition of 13% v/v methanol as background electrolyte. Migration times range from 2.01 min for valine to 2.84 min for glutamine, and LODs for untreated plasma are in the interval 0.7-0.9 μmol/L. Sample treatment is based on the addition of acetonitrile to only 15 μL of plasma and supernatant is directly subjected to CE/C⁴ D. Circulating amino acids are measured in patients with pancreatic cancer and cancer cachexia during oral glucose tolerance test. It is shown that patients with pancreatic cancer and cancer cachexia syndrome exhibit low basal circulating BCAAs and glutamine levels and loss of their insulin-dependent suppression.

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.

Coupling of CE-MS for protein and peptide analysis

The review is focused on the latest developments in the analysis of proteins and peptides by capillary electrophoresis techniques coupled to mass spectrometry. First, the methodology and instrumentation are overviewed. In this section, recent progress in capillary electrophoresis with mass spectrometry interfaces and capillary electrophoresis with matrix-assisted laser desorption/ionization is mentioned, as well as separation tasks. The second part is devoted to applications-mainly bottom-up and top-down proteomics. It is obvious that capillary electrophoresis with mass spectrometry methods are well suited for peptide and protein analysis (proteomic research) and it is described how these techniques are complementary and not competitive with the often used liquid chromatography with mass spectrometry methods.

Recent progress of sample stacking in capillary electrophoresis (2016-2018)

Electrophoretic sample stacking comprises a group of capillary electrophoretic techniques where trace analytes from the sample are concentrated into a short zone (stack). This paper is a continuation of our previous reviews on the topic and brings a survey of more than 120 papers published approximately since the second quarter of 2016 till the first quarter of 2018. It is organized according to the particular stacking principles and includes chapters on concentration adjustment (Kohlrausch) stacking, on stacking techniques based on pH changes, on stacking in electrokinetic chromatography and on other stacking techniques. Where available, explicit information is given about the procedure, electrolyte(s) used, detector employed and sensitivity reached. Not reviewed are papers on transient isotachophoresis which are covered by another review in this issue.

A decade of microchip electrophoresis for clinical diagnostics - A review of 2008-2017

A core element in clinical diagnostics is the data interpretation obtained through the analysis of patient samples. To obtain relevant and reliable information, a methodological approach of sample preparation, separation, and detection is required. Traditionally, these steps are performed independently and stepwise. Microchip capillary electrophoresis (MCE) can provide rapid and high-resolution separation with the capability to integrate a streamlined and complete diagnostic workflow suitable for the point-of-care setting. Whilst standard clinical diagnostics methods normally require hours to days to retrieve specific patient data, MCE can reduce the time to minutes, hastening the delivery of treatment options for the patients. This review covers the advances in MCE for disease detection from 2008 to 2017. Miniaturised diagnostic approaches that required an electrophoretic separation step prior to the detection of the biological samples are reviewed. In the two main sections, the discussion is focused on the technical set-up used to suit MCE for disease detection and on the strategies that have been applied to study various diseases. Throughout these discussions MCE is compared to other techniques to create context of the potential and challenges of MCE. A comprehensive table categorised based on the studied disease using MCE is provided. We also comment on future challenges that remain to be addressed.

Fundamental aspects of field-amplified electrokinetic injection of cations for enantioselective capillary electrophoresis with sulfated cyclodextrins as selectors

Head-column field-amplified sample stacking of cations from a low conductivity sample followed by enantiomeric separation using negatively charged chiral selectors was studied experimentally and with computer simulation. Aspects investigated include the direct electrokinetic injection of the analytes into the background electrolyte, the use of a selector free buffer plug, the contribution of complexation within the buffer plug and the application of an additional water plug between sample and buffer plug. Attention was paid for changes of ionic strength which is known to have a significant impact on complexation and thus effective mobility. Racemic methadone was selected as a model compound, randomly substituted sulfated β-cyclodextrin as chiral selector and phosphate buffers (pH 6.3) for the background electrolyte and the buffer plug. Results confirm that the buffer plug is providing a spacer between cationic analytes and the negatively charged selector during electrokinetic injection. Simulation predicts the required length and composition of the plug for a given injection time to avoid an interference with the selector. A short water plug added between the low conductivity sample and a high conductivity buffer plug is demonstrated to provide best conditions to achieve high sensitivity in enantioselective drug assays with sulfated cyclodextrins as selectors.

Recent developments in capillary and microchip electroseparations of peptides (2015-mid 2017)

The review brings a comprehensive overview of recent developments and applications of high performance capillary and microchip electroseparation methods (zone electrophoresis, isotachophoresis, isoelectric focusing, affinity electrophoresis, electrokinetic chromatography, and electrochromatography) to analysis, microscale isolation, purification, and physicochemical and biochemical characterization of peptides in the years 2015, 2016, and ca. up to the middle of 2017. Advances in the investigation of electromigration properties of peptides and in the methodology of their analysis (sample preseparation, preconcentration and derivatization, adsorption suppression and EOF control, and detection) are described. New developments in particular CE and CEC methods are presented and several types of their applications to peptide analysis are reported: 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 methods to provide important physicochemical characteristics of peptides are demonstrated.

Insights into head-column field-amplified sample stacking: Part I. Detailed study of electrokinetic injection of a weak base across a short water plug

The fundamentals of electrokinetic injection of the weak base methadone across a short water plug into a phosphate buffer at low pH were studied experimentally and with computer simulation. The current during electrokinetic injection, the formation of the analyte zone, changes occurring within and around the water plug and mass transport of all compounds in the electric field were investigated. The impact of water plug length, plug injection velocity, and composition of sample, plug and background electrolyte are discussed. Experimental data revealed that properties of sample, water plug and stacking boundary are significantly and rapidly altered during electrokinetic injection. Simulation provided insight into these changes, including the nature of the migrating boundaries and the stacking of methadone at the interface to a newly formed phosphoric acid zone. The data confirm the role of the water plug to prevent contamination of the sample by components of the background electrolyte and suggest that mixing caused by electrohydrodynamic instabilities increases the water plug conductivity. The sample conductivity must be controlled by addition of an acid to prevent generation of reversed flow which removes the water plug and to create a buffering environment. Results revealed that a large increase in background electrolyte concentration is not accompanied with a significant increase in stacking.

Analysis of proteins and peptides by electromigration methods in microchips

This review presents the developments and applications of microchip electromigration methods in the separation and analysis of peptides and proteins in the period 2011-mid-2016. The developments in sample preparation and preconcentration, microchannel material, and surface treatment are described. Separations by various microchip electromigration methods (zone electrophoresis in free and sieving media, affinity electrophoresis, isotachophoresis, isoelectric focusing, electrokinetic chromatography, and electrochromatography) are demonstrated. Advances in detection methods are reported and novel applications in the areas of proteomics and peptidomics, quality control of peptide and protein pharmaceuticals, analysis of proteins and peptides in biomatrices, and determination of physicochemical parameters are shown.

Recent progress of sample stacking in capillary electrophoresis (2014-2016)

The term "sample stacking" comprises a relatively broad spectrum of techniques that already form an almost inherent part of the methodology of CZE. Their principles are different but the effect is the same: concentration of a diluted analyte into a narrow zone and considerable increase of the method sensitivity. This review brings a survey of papers on electrophoretic sample stacking published approximately since the second quarter of 2014 till the first quarter of 2016. It is organized according to the principles of the stacking methods and includes chapters aimed at the concentration adjustment principle (Kohlrausch stacking), techniques based on pH changes, micellar methods, and other stacking techniques. Not reviewed are papers on transient ITP that are covered by another review in this issue.

Recent Advances in the Determination of Pesticides in Environmental Samples by Capillary Electrophoresis

Nowadays, owing to the increasing population and the attempts to satisfy its needs, pesticides are widely applied to control the quantity and quality of agricultural products. However, the presence of pesticide residues and their metabolites in environmental samples is hazardous to the health of humans and all other living organisms. Thus, monitoring these compounds is extremely important to ensure that only permitted levels of pesticide are consumed. To this end, fast, reliable, and environmentally friendly methods that can accurately analyze dilute, complex samples containing both parent substances and their metabolites are required. Focusing primarily on research published since 2010, this review summarizes the use of various sample pretreatment techniques to extract pesticides from various matrices, combined with on-line preconcentration strategies for sensitivity improvement, and subsequent capillary electrophoresis analysis.

Improving the sensitivity in chiral capillary electrophoresis

CE is known for being one of the most powerful analytical techniques when performing enantioseparations due to its numerous advantages such as excellent separation efficiency and extremely low solvents and reagents consumption, all of them derived from the capillary small dimensions. Moreover, it is worth highlighting that unlike in chromatographic techniques, in CE the chiral selector is generally within the separation medium instead of being attached to the separation column which makes the method optimization a more versatile task. Despite its numerous advantages, when using UV-Vis detection, CE lacks of sensitivity detection due to its short optical path length derived from the narrow separation capillary. This issue can be overcome by means of different approaches, either by sample treatment procedures or by in-capillary preconcentration techniques or even by employing detection systems more sensitive than UV-Vis, such as LIF or MS. The present review assembles the latest contributions regarding improvements of sensitivity in chiral CE published from June 2013 until May 2015, which follows the works included in a previous review reported by Sánchez-Hernández et al. [Electrophoresis 2014, 35, 12-27].

Gold-nanoparticle extraction and reversed-electrode-polarity stacking mode combined to enhance capillary electrophoresis sensitivity for conjugated nucleosides and oligonucleotides containing thioether linkers

We present a capillary electrophoresis method for determining two different C8-conjugated deoxyadenosines, and for oligonucleotides containing them, in which a psoralen or an acridine molecule is bonded to the base via a short alkyl chain containing sulfur ethers at both ends. The sensitivity of the micellar electrokinetic chromatography (MEKC) method was increased by using two preconcentration techniques, micro solid-phase extraction (μSPE) followed by reversed-electrode-polarity stacking mode (REPSM). Variables that affect the efficiency of the extraction in μSPE and preconcentration by REPSM, including the type and volume of extraction nanoparticle, concentration, and injection time, were investigated. Under the optimum conditions, enrichment factors obtained were in the range 360-400. The limits of detection (LODs) at a signal-to-noise ratio of 3 ranged from 2 to 5 nmol L(-1). The relative recoveries of labelled adenosines from water samples were 95-103%. The proposed method provided high enrichment factors and good precision and accuracy with a short analysis time. On the basis of the advantages of simplicity, high selectivity, high sensitivity, and good reproducibility, the proposed method may have great potential for biochemical applications.