Advances in amperometric and conductometric detection in capillary and chip-based electrophoresis

Strategies for capillary electrophoresis: Method development and validation for pharmaceutical and biological applications-Updated and completely revised edition

This review is in support of the development of selective, precise, fast, and validated capillary electrophoresis (CE) methods. It follows up a similar article from 1998, Wätzig H, Degenhardt M, Kunkel A. "Strategies for capillary electrophoresis: method development and validation for pharmaceutical and biological applications," pointing out which fundamentals are still valid and at the same time showing the enormous achievements in the last 25 years. The structures of both reviews are widely similar, in order to facilitate their simultaneous use. Focusing on pharmaceutical and biological applications, the successful use of CE is now demonstrated by more than 600 carefully selected references. Many of those are recent reviews; therefore, a significant overview about the field is provided. There are extra sections about sample pretreatment related to CE and microchip CE, and a completely revised section about method development for protein analytes and biomolecules in general. The general strategies for method development are summed up with regard to selectivity, efficiency, precision, analysis time, limit of detection, sample pretreatment requirements, and validation.

Rapid determination of NSAIDs by capillary and microchip electrophoresis with capacitively coupled contactless conductivity detection in wastewater

A simple, rapid method using CE and microchip electrophoresis with C4 D has been developed for the separation of four nonsteroidal anti-inflammatory drugs (NSAIDs) in the environmental sample. The investigated compounds were ibuprofen (IB), ketoprofen (KET), acetylsalicylic acid (ASA), and diclofenac sodium (DIC). In the present study, we applied for the first time microchip electrophoresis with C4 D detection to the separation and detection of ASA, IB, DIC, and KET in the wastewater matrix. Under optimum conditions, the four NSAIDs compounds could be well separated in less than 1 min in a BGE composed of 20 mM His/15 mM Tris, pH 8.6, 2 mM hydroxypropyl-beta-cyclodextrin, and 10% methanol (v/v) at a separation voltage of 1000-1200 V. The proposed method showed excellent repeatability, good sensitivity (LODs ranging between 0.156 and 0.6 mg/L), low cost, high sample throughputs, portable instrumentation for mobile deployment, and extremely lower reagent and sample consumption. The developed method was applied to the analysis of pharmaceuticals in wastewater samples with satisfactory recoveries ranging from 62.5% to 118%.

Fast determination of paracetamol and its hydrolytic degradation product p-aminophenol by capillary and microchip electrophoresis with contactless conductivity detection

Paracetamol (PAC) is one of the most extensively used analgesics and antipyretic drugs to treat mild and moderate pain. P-aminophenol (PAP), the main hydrolytic degradation product of PAC, can be found in environmental water. Recently, capillary electrophoresis (CE) has been developed for the detection of a wide variety of chemical substances. The purpose of this study is to develop a simple and fast method for the detection and separation of PAC and its main hydrolysis product PAP, using CE and microchip electrophoresis (ME) with capacitively coupled contactless conductivity detection (C⁴ D). The determination of these compounds using ME with C⁴ D is being reported for the first time. The separation was run for all analytes using a background electrolyte (BGE) (20 Mm β-alanine, pH 11) containing 14% (v/v) methanol. The RSDs obtained for migration time were less than 0.05%, and RSDs obtained for peak area were less than 3%. The detection limits (S/N = 3) that were achieved ranged from 0.3 to 0.6 mg/L without sample preconcentration. The presented method showed rapid analysis time (less than 1 min), high efficiency and precision, low cost, and a significant decrease in the consumption of reagents. The microchip system has proved to be an excellent analytical technique for fast and reliable environmental applications. This article is protected by copyright. All rights reserved.

Application of Capillary Electrophoresis with Capacitively Coupled Contactless Conductivity Detection (CE-C4D): 2017-2020

Capacitively coupled contactless conductivity detection (C⁴D) has emerged as influential to detect analytes that do not have chromogenic or fluorogenic functional group. Since our last review several new capillary electrophoresis (CE) methods coupled with (CE-C⁴D) have been communicated. The aim of this review is to give an update of the almost all the new applications of CE-C⁴D in the field of pharmaceutical, food and biomedical analysis covering the period from 2017 to April 2020. The utilization of CE with C⁴D in the areas of pharmaceutical, food and biomedical analysis is presented. Finally, concluding remarks and outlooks are discussed.

Overview of Optical and Electrochemical Alkaline Phosphatase (ALP) Biosensors: Recent Approaches in Cells Culture Techniques

Alkaline phosphatase (ALP), which catalyzes the dephosphorylation process of proteins, nucleic acids, and small molecules, can be found in a variety of tissues (intestine, liver, bone, kidney, and placenta) of almost all living organisms. This enzyme has been extensively used as a biomarker in enzyme immunoassays and molecular biology. ALP is also one of the most commonly assayed enzymes in routine clinical practice. Due to its close relation to a variety of pathological processes, ALP's abnormal level is an important diagnostic biomarker of many human diseases, such as liver dysfunction, bone diseases, kidney acute injury, and cancer. Therefore, the development of convenient and reliable assay methods for monitoring ALP activity/level is extremely important and valuable, not only for clinical diagnoses but also in the area of biomedical research. This paper comprehensively reviews the strategies of optical and electrochemical detection of ALP and discusses the electrochemical techniques that have been addressed to make them suitable for ALP analysis in cell culture.

Dual-channel Microchip Electrophoresis with Amperometric Detection System for Rapid Analysis of Cefoperazone and Sulbactam

A dual-channel microchip electrophoresis (ME) with in-channel amperometric detection was developed for cefoperazone and sulbactam determination simultaneously. In this study, a microelectrode detector was made of gold nanoparticles (GNPs) modified indium tin oxide (ITO)-coated poly-ethylene terephthalate (PET) film. The parameters including detection potential applied on working electrode, buffer concentration and pH value were optimized to improve the detection sensitivity and separation efficiency of cefoperazone and sulbactam. Under the optimal conditions, sensitive detection of cefoperazone and sulbactam was obtained with limits of detection (LODs) (S/N = 3) of 0.52 and 0.75 μg/mL, respectively. The plasma sample, which was from a patient with a brain injury taking Sulperazone, was successfully detected with a simple sample pretreatment process by dual-channel ME amperometric detection. This rapid and sensitive method possesses practical potential in clinical applications, and could provide a guidance for clinical rational drug use.

Contactless conductivity detection for analytical techniques: Developments from 2016 to 2018

The publications concerning capacitively coupled contactless conductivity detection for the 2-year period from mid-2016 to mid-2018 are covered in this update to the earlier reviews of the series. Relatively few reports on fundamental investigations or new designs have appeared in the literature in this time interval, but the development of new applications with the detection method has continued strongly. Most often, contactless conductivity measurements have been employed for the detection of inorganic or small organic ions in conventional capillary electrophoresis, less often in microchip electrophoresis. A number of other uses, such as detection in chromatography or the gauging of bubbles in streams have also been reported.

Liquid Beam Desorption Mass Spectrometry for the Investigation of Continuous Flow Reactions in Microfluidic Chips

In this work, we present the combination of microfluidic chips and mass spectrometry employing laser-induced liquid beam ionization/desorption. The developed system was evaluated with respect to stable beam generation and laser parameters as well as solvent compatibility. The device was exemplarily applied to study a vinylogous Mannich reaction performed in continuous flow on chip. Fast processes can be observed with this technique which in the future could be beneficial for studying intermediates or contribute to the elucidation of reaction mechanisms.

Dual-opposite injection capillary electrophoresis: Principles and misconceptions

Dual-opposite injection capillary electrophoresis (DOI-CE) is a separation technique that utilizes both ends of the capillary for sample introduction. The electroosmotic flow (EOF) is suppressed to allow all ions to reach the detector quickly. Depending on the individual electrophoretic mobilities of the analytes of interest and the effective length that each analyte travels to the detection window, the elution order of analytes in a DOI-CE separation can vary widely. This review discusses the principles, applications, and limitations of dual-opposite injection capillary electrophoresis. Common misconceptions regarding DOI-CE are clarified.

Integration of a graphite/poly(methyl-methacrylate) composite electrode into a poly(methylmethacrylate) substrate for electrochemical detection in microchips

Traditional fabrication methods for polymer microchips, the bonding of two substrates together to form the microchip, can make the integration of carbon electrodes difficult. We have developed a simple and inexpensive method to integrate graphite/PMMA composite electrodes (GPCEs) into a PMMA substrate. These substrates can be bonded to other PMMA layers using a solvent-assisted thermal bonding method. The optimal composition of the GPCEs for electrochemical detection was determined using cyclic voltammetry with dopamine as a test analyte. Using the optimized GPCEs in an all-PMMA flow cell with flow injection analysis, it was possible to detect 50 nM dopamine under the best conditions. These electrodes were also evaluated for the detection of dopamine and catechol following separation by MCE.

Ultracompact three-dimensional tubular conductivity microsensors for ionic and biosensing applications

We present ultracompact three-dimensional tubular structures integrating Au-based electrodes as impedimetric microsensors for the in-flow determination of mono- and divalent ionic species and HeLa cells. The microsensors show an improved performance of 2 orders of magnitude (limit of detection = 0.1 nM for KCl) compared to conventional planar conductivity detection systems integrated in microfluidic platforms and the capability to detect single HeLa cells in flowing phosphate buffered saline. These highly integrated conductivity tubular sensors thus open new possibilities for lab-in-a-tube devices for bioapplications such as biosensing and bioelectronics.

Electro-driven extraction across a polymer inclusion membrane in a flow-through cell

A flow-through arrangement for electrodriven extraction across a polymer inclusion membrane was developed. Sample introduction into the donor chamber was continuous, while the acceptor solution was stagnant. By adjustment of the total volume of the donor solution pumped through the cell the best compromise between enrichment factor and extraction time can be set. The enriched extract was analyzed by capillary electrophoresis with contactless conductivity detection. Membranes of 20μm thickness were employed which consisted of 60% cellulose triacetate as base polymer, 20% o-nitrophenyl octyl ether as plasticizer, and 20% Aliquat 336. By passing through 10mL of sample at a flow rate of 1mL/min the model analytes glyphosate (a common herbicide) and its major metabolite aminomethylphosphonic acid could be transported from the aqueous donor solution to the aqueous acceptor solution with efficiencies >87% in 10min at an applied voltage of 1500V. Enrichment factors of 87 and 95 and limits of detection down to 43 and 64pg/mL were obtained for glyphosate and aminomethylphosphonic acid, respectively. The intra- and interday reproducibilities for the extraction of the two compounds from spiked river water were about 6 and 7% respectively when new membranes were used for each experiment. For consecutive extractions of batches of river water with a single piece of membrane a deterioration of recovery by about 16% (after 20 runs) was noted, an effect not observed with purely aqueous standards.

Determination of PCR products by CE with contactless conductivity detection

The use of CE with contactless conductivity detection for the determination of PCR products is demonstrated for the first time. The separation of specific length PCR products according to their size could be achieved using 5% PVP as a sieving medium in a separation buffer consisting of 20 mM Tris and 20 mM 2-(cyclohexylamino)ethansulphonic acid (pH 8.5). A fused silica capillary of 60 cm length and 50 μm id and an applied separation voltage of -15 kV were employed and separations could be completed within 20-50 min. PCR amplified DNA fragments of different sizes obtained from different bacterial plasmid templates as well as a fragment from genomic DNA of genetically modified soybeans could be successfully identified.

Electrochemical methods in conjunction with capillary and microchip electrophoresis

Electromigrative techniques such as capillary and microchip electrophoresis (CE and MCE) are inherently associated with various electrochemical phenomena. The electrolytic processes occurring in the buffer reservoirs have to be considered for a proper design of miniaturized electrophoretic systems and a suitable selection of buffer composition. In addition, the control of the electroosmotic flow plays a crucial role for the optimization of CE/MCE separations. Electroanalytical methods have significant importance in the field of detection in conjunction with CE/MCE. At present, amperometric detection and contactless conductivity detection are the predominating electrochemical detection methods for CE/MCE. This paper reviews the most recent trends in the field of electrochemical detection coupled to CE/MCE. The emphasis is on methodical developments and new applications that have been published over the past five years. A rather new way for the implementation of electrochemical methods into CE systems is the concept of electrochemically assisted injection which involves the electrochemical conversions of analytes during the injection step. This approach is particularly attractive in hyphenation to mass spectrometry (MS) as it widens the range of CE-MS applications. An overview of recent developments of electrochemically assisted injection coupled to CE is presented.

Recent progress for capillary electrophoresis with electrochemical detection

Capillary electrophoresis (CE) is an attractive technique in separation science because of its high separation performance, short analysis time and low cost. Electrochemical detection (EC) is a powerful tool for CE because of its high sensitivity. In this review, developments of CE-EC from 2008 to August, 2011 are reviewed. We choose papers of innovative and novel results to demonstrate the newest and most important progress in CE-EC.

Label-free analysis in chip electrophoresis applying deep UV fluorescence lifetime detection

Herein we introduce deep UV fluorescence lifetime detection in microfluidics applied for label-free detection and identification of various aromatic analytes in chip electrophoresis. For this purpose, a frequency quadrupled Nd:YAG (neodymium-doped yttrium aluminum garnet) picosecond laser at 266  nm was incorporated into an inverse fluorescence microscope setup with time-correlated single photon counting detection. This allowed recording of photon timing with sub-nanosecond precision. Thereby fluorescence decay curves are gathered on-the-fly and average lifetimes can be determined for each substance in the electropherogram. The aromatic compounds serotonin, propranolol, 3-phenoxy-1,2-propanediol and tryptophan were electrophoretically separated using a fused-silica microchip. Average lifetimes were independently determined for each compound via bi-exponential tail fitting. Time-correlated single photon counting also allows the discrimination of background fluorescence in the time domain. This results in improved signal-to-noise-ratios as demonstrated for the above model analytes. Microchip electrophoretic separations with fluorescence lifetime detection were also performed with a protein mixture containing lysozyme, trypsinogen and chymotrypsinogen emphasizing the potential for biopolymer analysis.

New trends in quantification of acrylamide in food products

Methods applied in acrylamide quantification in foods have been reviewed in this paper. Novel analytical techniques like capillary electrophoresis (CE), immunoenzymatic test (ELISA) and electrochemical biosensors, which can replace traditional methods like high performance liquid chromatography (HPLC) and gas chromatography (GC) were presented. Short time of analysis and high resolution power of electrophoretic techniques caused that they became routinely used in food analysis apart from high performance liquid chromatography and gas chromatography. Application of modern chromatography methods like ultra performance liquid chromatography (UPLC) in acrylamide quantification considerably shortened the time of analysis and decreased the consumption of indispensable reagents. The most promising approaches to acrylamide quantification in foods are electrochemical biosensors and immunoenzymatic tests. In contrast to chromatography and electrophoretic methods they require neither expensive equipment nor time consuming sample preparation and allow for fast screening of numerous samples without the usage of sophisticated apparatuses. Because of many advantages such as miniaturization, rapid and simple analysis, and high sensitivity and selectivity, biosensors are thought to replace conventional methods of acrylamide quantification in food.

Comparison of the performance characteristics of two tubular contactless conductivity detectors with different dimensions and application in conjunction with HPLC

Two tubular capacitively coupled contactless conductivity detection (C(4)D) cells with different geometric dimensions were evaluated with regard to their main analytical characteristics under non-separation and separation conditions in conjunction with liquid chromatography. A comparison of the performance of the tubular cells to a previously tested thin-layer detection cell was drawn. Additionally, using a theoretical model the experimental results were compared with sets of calculated values and partially enabled to model the complex behavior of C(4)D detection in combination with high-performance liquid chromatography (HPLC). While cell 1 is characterized by a geometric cell volume of 0.6 μL, a wall thickness of 675 μm, and an inner diameter of 125 μm, the respective values for cell 2 are 2.3 μL, 200 μm, and 250 μm. The main analytical parameters were evaluated using a potassium chloride (KCl) solution. The limits of detection were 0.4 μM KCl (5.7 × 10(-6) S m(-1)) for cell 1 and 0.2 μM KCl (3.2 × 10(-6) S m(-1)) for cell 2, which compares well to the previously found 0.2 μM for the thin-layer cell. A pair of linear ranges was found for both cells in a concentration interval ranging from 1 × 10(-6) to 1 × 10(-4) M (corresponding to 1.5 × 10(-5) to 1.5 × 10(-3) S m(-1)) KCl, respectively. Furthermore, the detector cells were applied to the HPLC separation of a model compound system consisting of benzoic acid, lactic acid, octanesulphonic acid, and sodium capronate. Separation of the compounds was achieved with a Biospher PSI 100 C18 column using 60% aqueous acetonitrile mobile phase. Calibration curves for the examined model system were well correlated (r² > 0.997), and it was found that under separation conditions the arrangement with the lower cell volume (cell 1) yields higher sensitivity and respectively lower limits of detection for all model compounds. Compared with the thin-layer cell, the tubular cells show better overall performance in regard to the determined analytical characteristics.