Pavel Kubáň

Ten years of axial capacitively coupled contactless conductivity detection for CZE – a review

Contactless conductivity detection is universal for CE in that all charged species can be quantified, and it is particularly attractive for those inorganic and organic ions that are not directly accessible by optical means. It is not necessary to make precise alignments or to create an optical window into the capillary. Commercial detectors that can be retrofitted to existing instruments are available at a cost that is lower than that of any other electrophoresis detector. The approach is also suited for lab‐on‐chip devices. It is attempted in this review to summarize some of the expertise accumulated since the introduction of the axial contactless conductivity detector to CE 10 years ago.

Capacitively coupled contactless conductivity detection for microseparation techniques – recent developments

An overview of the developments of capacitively coupled contactless conductivity detection in CE and related techniques over approximately the last 2 years is given. The method has seen strong growth, and diverse new applications are being reported. Besides more advanced techniques on conventional capillaries, these include further developments of detection on lab‐on‐chip devices as well as in miniaturized chromatographic systems and some methods not involving separations. An increasing number of reports are based on the now readily available commercial detectors, but, while few publications on fundamental studies have appeared recently, interesting new approaches on creating low cost devices have also appeared.

Electromembrane extraction of heavy metal cations followed by capillary electrophoresis with capacitively coupled contactless conductivity detection

Electromembrane extraction (EME) was used as an off‐line sample pre‐treatment method for the determination of heavy metal cations in aqueous samples using CE with capacitively coupled contactless conductivity detection (CE‐C4D). A short segment of porous polypropylene hollow fibre was penetrated with 1‐octanol and 0.5% v/v bis(2‐ethylhexyl)phosphonic acid and constituted a low cost, single use, disposable supported liquid membrane, which selectively transported and pre‐concentrated heavy metal cations into the fibre lumen filled with 100 mM acetic acid acceptor solution. Donor solutions were standard solutions and real samples dissolved in deionized water at neutral pH. At optimized EME conditions (penetration time, 5 s; applied voltage, 75 V; and stirring rate, 750 rpm), 15–42% recoveries of heavy metal cations were achieved for a 5 min extraction time. Repeatability of the EME pre‐treatment was examined for six independent EME runs and ranged from 6.6 to 11.1%. Limits of detection for the EME‐CE‐C4D method ranged from 25 to 200 nM, resulting into one to two orders of magnitude improvement compared with CE‐C4D without sample treatment. The developed EME sample pre‐treatment procedure was applied to the analysis of heavy metal cations in tap water and powdered milk samples. Zinc in the real samples was identified and quantified in a background electrolyte solution consisting of 20 mM L‐histidine and 30 mM acetic acid at pH 4.95 in about 3 min.

Electromembrane extraction of amino acids from body fluids followed by capillary electrophoresis with capacitively coupled contactless conductivity detection

Electromembrane extraction (EME) proved to be a simple and rapid pretreatment method for analysis of amino acids and related compounds in body fluid samples. Body fluids were acidified to the final concentration of 2.5 M acetic acid and served as donor solutions. Amino acids, present as cations in the donor solutions, migrated through a supported liquid membrane (SLM) composed of 1-ethyl-2-nitrobenzene/bis-(2-ethylhexyl)phosphonic acid (85:15 (v/v)) into the lumen of a porous polypropylene hollow fiber (HF) on application of electric field. The HF was filled with 2.5 M acetic acid serving as the acceptor solution. Matrix components in body fluids were efficiently retained on the SLM and did not interfere with subsequent analysis. Capillary electrophoresis with capacitively coupled contactless conductivity detection was used for determination of 17 underivatized amino acids in background electrolyte solution consisting of 2.5 M acetic acid. Parameters of EME, such as composition of SLM, pH and composition of donor and acceptor solution, agitation speed, extraction voltage, and extraction time were studied in detail. At optimized conditions, repeatability of migration times and peak areas of 17 amino acids was better than 0.3% and 13%, respectively, calibration curves were linear in a range of two orders of magnitude (r(2)=0.9968-0.9993) and limits of detection ranged from 0.15 to 10 μM. Endogenous concentrations of 12 amino acids were determined in EME treated human serum, plasma, and whole blood. The method was also suitable for simple and rapid pretreatment and determination of elevated concentrations of selected amino acids, which are markers of severe inborn metabolic disorders.

Contactless conductivity detection for analytical techniques—Developments from 2012 to 2014

The review covers the progress of capacitively coupled contactless conductivity detection over the 2 years leading up to mid‐2014. During this period many new applications for conventional CE as well as for microchip separation devices have been reported; prominent areas have been clinical, pharmaceutical, forensic, and food analyses. Further progress has been made in the development of field portable instrumentation based on CE with contactless conductivity detection. Several reports concern the combination with sample pretreatment techniques, in particular electrodriven extractions. Accounts of arrays of contactless conductivity detectors have appeared, which have been created for quite different tasks requiring spatially resolved information. The trend of the use of contactless conductivity measurements for applications other than CE has continued.

Contactless conductivity detection for analytical techniques: Developments from 2010 to 2012: CE and CEC

The developments in the field of capacitively coupled contactless conductivity detection in the approximate period from July 2010 to June 2012 are traced. Few reports concerning fundamental studies or new detector designs have appeared. On the other hand, applications in standard CZE are flourishing and contactless conductivity measurements are increasingly being employed as part of novel or more sophisticated experimental systems. Work on the lab‐on‐chip devices integrating contactless conductivity detection is continuing. A range of reports on the use of the simple yet powerful detection technique of contactless conductivity measurements in chromatographic separation as well as for analytical methods not including a separation step have also appeared.

Effects of the cell geometry and operating parameters on the performance of an external contactless conductivity detector for microchip electrophoresis

Quantitative data on the effect of the electrode geometry on the signal strength and the signal-to-noise ratio is given. The measurements are affected by the unavoidable presence of stray capacitance. Best results are achieved for short and narrow electrodes arranged in an antiparallel configuration and separated by a minimal gap, which determines the dimensions of the actual detection volume. Limits of detection between 150 and 250 microg l(-1) and separation efficiencies from 13,000 to 17,000 theoretical plates were achieved for six inorganic cations (NH(4)(+), K(+), Ca(2+), Na(+), Mg(2+)and Li(+)) with electrodes of 1 mm width and a detection gap of 0.5 mm (separation channel length: 7.5 cm) when operating the detector at 20 V(pp) and 500 kHz. The analyses of all major inorganic cations in tap and rain water samples were demonstrated for the first time in microchip electrophoresis with contactless conductivity detection.

Evaluation of microchip capillary electrophoresis with external contactless conductivity detection for the determination of major inorganic ions and lithium in serum and urine samples

The determination of inorganic ions in clinical samples in less than 90 seconds was demonstrated for microchip capillary electrophoresis using capacitively coupled contactless conductivity detection (C(4)D). Bare electrophoresis chips were used in combination with external electrodes which were part of the chip holder. In order to achieve the required selectivity and sensitivity, an optimization of the electrode layout was carried out. Limits of detection (LOD) of 1 microM for K(+), 1.5 microM for Ca(2+), 3 microM for Na(+), 1.75 microM for Mg(2+) and 7.5 microM for Li(+) were achieved. The determination of inorganic cations (NH(4)(+), K(+), Na(+), Ca(2+), Mg(2+)) and anions (Cl(-), NO(3)(-), SO(4)(2-), phosphate) in blood serum and urine samples was possible in one common electrolyte solution containing 15 mM L-arginine, 10.75 mM maleic acid and 1.5 mM 18-crown-6 at pH 5.90 by simply switching the separation voltage from positive to negative polarity. Lithium, present at significant levels when used for therapeutic purposes, can also be determined in blood serum using a slightly modified background electrolyte solution.

Rapid and simple pretreatment of human body fluids using electromembrane extraction across supported liquid membrane for capillary electrophoretic determination of lithium.

Electromembrane extraction was used for simultaneous sample cleanup and preconcentration of lithium from untreated human body fluids. The sample of a body fluid was diluted 100 times with 0.5 mM Tris solution and lithium was extracted by electromigration through a supported liquid membrane composed of 1‐octanol into 100 mM acetic acid acceptor solution. Matrix compounds, such as proteins, red blood cells, and other high‐molecular‐weight compounds were efficiently retained on the supported liquid membrane. The liquid membrane was anchored in pores of a short segment of a polypropylene hollow fiber, which represented a low cost, single use, disposable extraction unit and was discarded after each use. Acceptor solutions were analyzed using capillary electrophoresis with capacitively coupled contactless conductivity detection (CE‐C4D) and baseline separation of lithium was achieved in a background electrolyte solution consisting of 18 mM L‐histidine and 40 mM acetic acid at pH 4.6. Repeatability of the electromembrane extraction‐CE‐C4D method was evaluated for the determination of lithium in standard solutions and real samples and was better than 0.6 and 8.2% for migration times and peak areas, respectively. The concentration limit of detection of 9 nM was achieved. The developed method was applied to the determination of lithium in urine, blood serum, blood plasma, and whole blood at both endogenous and therapeutic concentration levels.

Electromembrane extraction using stabilized constant d.c. electric current--a simple tool for improvement of extraction performance.

This contribution presents an experimental approach for improvement of analytical performance of electromembrane extraction (EME), which is based on the use of stabilized constant d.c. electric current. Extractions were performed using a high voltage power supply, which provided stabilized constant d.c. current down to 1μA and facilitated current-controlled transfer of ions of interest from a donor solution through a supported liquid membrane (SLM) into an acceptor solution. Repeatability of the extraction process has significantly improved for EME at constant electric current compared to EME at constant voltage. The improved repeatability of the extraction process was demonstrated on EME-capillary electrophoresis (EME-CE) analyses of selected basic drugs and amino acids in standard solutions and in human urine and serum samples. RSD values of peak areas of the analytes for EME-CE analyses were about two-fold better for EME at constant electric current (2.8-8.9%) compared to EME at constant voltage (3.6-17.8%). Other analytical parameters of the EME-CE methods, such as limits of detection, linear ranges and correlation coefficients were not statistically different for the two EME modes. Moreover, EME at constant electric current did not suffer from SLM instabilities frequently observed for EME at constant voltage.