Influence of the sample volume and the position of the electrode and the capillary-end in the sample vial on the electrokinetic injection in capillary electrophoresis

Isotachophoresis: Theory and Microfluidic Applications

Isotachophoresis (ITP) is a versatile electrophoretic technique that can be used for sample preconcentration, separation, purification, and mixing, and to control and accelerate chemical reactions. Although the basic technique is nearly a century old and widely used, there is a persistent need for an easily approachable, succinct, and rigorous review of ITP theory and analysis. This is important because the interest and adoption of the technique has grown over the last two decades, especially with its implementation in microfluidics and integration with on-chip chemical and biochemical assays. We here provide a review of ITP theory starting from physicochemical first-principles, including conservation of species, conservation of current, approximation of charge neutrality, pH equilibrium of weak electrolytes, and so-called regulating functions that govern transport dynamics, with a strong emphasis on steady and unsteady transport. We combine these generally applicable (to all types of ITP) theoretical discussions with applications of ITP in the field of microfluidic systems, particularly on-chip biochemical analyses. Our discussion includes principles that govern the ITP focusing of weak and strong electrolytes; ITP dynamics in peak and plateau modes; a review of simulation tools, experimental tools, and detection methods; applications of ITP for on-chip separations and trace analyte manipulation; and design considerations and challenges for microfluidic ITP systems. We conclude with remarks on possible future research directions. The intent of this review is to help make ITP analysis and design principles more accessible to the scientific and engineering communities and to provide a rigorous basis for the increased adoption of ITP in microfluidics.

Amplification-free detection of DNA in a paper-based microfluidic device using electroosmotically balanced isotachophoresis

We present a novel microfluidic paper-based analytical device (μPAD) which utilizes the native high electroosmotic flow (EOF) in nitrocellulose to achieve stationary isotachophoresis (ITP) focusing. This approach decouples sample accumulation from the length of the channel, resulting in significant focusing over short channel lengths. We provide a brief theory for EOF-balanced ITP focusing under continuous injection from a depleting reservoir and present the design of a short (7 mm) paper-based microfluidic channel, which allows a 200 μL sample to be processed in approximately 6 min, resulting in a 20 000-fold increase in concentration - a full order of magnitude improvement compared to previous paper-based ITP devices. We show the stability of the assay over longer (40 min) durations of time, and using Morpholino probes, we present the applicability of the device for amplification-free detection of nucleic acids, with a limit-of-detection (LoD) of 5 pM in 10 min. Finally, we utilize the small footprint of the channel and show a multiplexed platform in which 12 assays operate in parallel in a 24-well plate format.

A new nonpeptide substrate of human sirtuin in a capillary electrophoresis-based assay. Investigation of the binding mode by docking experiments

Sirtuins are nicotinamide dinucleotide-dependent class III histone deacetylases catalyzing various physiological processes involved in cell proliferation, differentiation, apoptosis, and ageing. This makes them attractive targets in drug research. In order to simplify sirtuin substrates for assay development, two N(ɛ)-acetyllysine derivatives, N(ɛ)-acetyl-N(α)-(4-methyl-7-methoxycoumarin)lysine amide, and N(ɛ)-acetyl-N(α)-(4-methyl-7-methoxycoumarin)lysine methyl ester were synthesized and evaluated as substrates for human SIRT1 in a capillary electrophoresis-based enzyme assay. Substrate, deacetylated product, and the coproduct nicotinamide were separated in a 200 mM phosphate/Tris buffer at pH 2.85. Field-amplified sample injection was employed to achieve sufficient assay sensitivity. While the ester derivative was not recognized by the enzyme, the amide substrate was effectively converted to the deacetylated product. The assay was subsequently validated with respect to range, linearity, limit of detection, and limit of quantification. Michaelis-Menten kinetic parameters, K(m) = 83 μM and V(max) = 6.8 μM/min were determined. The applicability of the assay for inhibitor screening was demonstrated using the known inhibitors sirtinol and the suramin derivate NF258. Resveratrol did not increase the deacetylation rate at concentrations of up to 200 μM. Docking experiments revealed the necessity of an amide function at the C-terminus of nonpeptide substrates while more structural freedom is tolerated at the N-terminus of N(ɛ) -acetyllysine.

Development and validation of a capillary electrophoresis method for the determination of phenothiazines in human urine in the low nanogramper milliliter concentration range using field-amplified sample injection

A capillary zone electrophoresis (CZE) method with ultraviolet-visible detection has been established and validated for the determination of five phenothiazines: thiazinamium methylsulfate, promazine hydrochloride, chlorpromazine hydrochloride, thioridazine hydrochloride, and promethazine hydrochloride in human urine. Optimum separation was obtained on a 64.5 cm x 75 microm bubble cell capillary using a buffer containing 150 mM tris(hydroxymethyl)aminomethane and 25% acetonitrile at pH 8.2, with temperature and voltage of 25 degrees C and 20 kV, respectively. Naphazoline hydrochloride was used as an internal standard. Field-amplified sample injection (FASI) has been applied to improve the sensitivity of the detection. Considering the influence of parameters affecting the on-line preconcentration (nature of preinjection plug, sample solvent composition, injection times, and injection voltage) and due to the significant interactions among them, in this paper we propose for the first time the application of a multivariate approach to carry out the study. The optimized conditions were as follows: preinjection plug of water for 7 s at 50 mbar, electrokinetic injection for 40 s at 6.2 kV, and 32 microm of H3PO4 in the sample solvent. Also, a solid-phase extraction (SPE) procedure is developed to obtain low detection limits and an adequate selectivity for urine samples. The combination of SPE and FASI-CZE-UV allows adequate linearities and recoveries, low detection limits (from 2 to 5 ng/mL), and satisfactory precisions (3.0-7.2% for an intermediate RSD %).

Fast, efficient capillary electrophoresis method for measuring nucleotide degradation and metabolism

An easy and fast method for the quantitative analysis of nucleotides by capillary zone electrophoresis was developed. The method employing a neutral-bonded capillary and reversed polarity mode provided a good resolution and a short analysis time of less than 5 min. The samples were injected electrokinetically using -6 kV voltage for 30 s and detected by their UV absorbance at 254 nm. Constant current (-45 microA) was applied, and a phosphate buffer, pH 7.4, was used. The detection limits for ATP, UDP, and UTP ranged between 0.14 and 0.28 microM. This method was required for the investigation of the purity of the commercially available nucleotides used in pharmacological studies. In addition, the analytical method was applied to study the metabolism of nucleotides in a cell line, neuroblastoma x glioma hybrid cells (NG108-15), which is used in pharmacological studies with nucleotides, since it contains purine- and pyrimidine-sensitive nucleotide receptors. Furthermore, we used the new method for monitoring enzymatic studies using the enzyme hexokinase to convert nucleotide triphosphates to diphosphates.

Selectivity in capillary electrokinetic separations

This review gives a survey of selectivity modes in capillary electrophoresis separations in pharmaceutical analysis and bioanalysis. Despite the high efficiencies of these separation techniques, good selectivity is required to allow quantitation or identification of a particular analyte. Selectivity in capillary electrophoresis is defined and described for different separation mechanisms, which are divided into two major areas: (i) capillary zone electrophoresis and (ii) electrokinetic chromatography. The first area describes aqueous (with or without organic modifiers) and nonaqueous modes. The second area discusses all capillary electrophoretic separation modes in which interaction with a (pseudo)stationary phase results in a change in migration rate of the analytes. These can be divided in micellar electrokinetic chromatography and capillary electrochromatography. The latter category can range from fully packed capillaries, via open-tubular coated capillaries to the addition of microparticles with multiple or single binding sites. Furthermore, an attempt is made to differentiate between methods in which molecular recognition plays a predominant role and methods in which the selectivity depends on overall differences in physicochemical properties between the analytes. The calculation of the resolution for the different separation modes and the requirements for qualitative and quantitative analysis are discussed. It is anticipated that selectivity tuning is easier in separation modes in which molecular recognition plays a role. However, sufficient attention needs to be paid to the efficiency of the system in that it not only affects resolution but also detectability of the analyte of interest.

Modelling of conditions for the enantiomeric separation of beta2-adrenergic sympathicomimetics by capillary electrophoresis using cyclodextrins as chiral selectors in a polyethylene glycol gel

A two-factor central composite design was used to determine a mathematical model for prediction of the optimal conditions for the separation of the enantiomers of some widely used beta2-sympathicomimetic drugs (beta2-agonists) by capillary electrophoresis using cyclodextrins (CD) as a chiral selector in a polyethylene glycolgel. The effects of the chemical structure of these drugs along with the addition of polyethylene glycol to the cyclodextrin solution on the resolution of their enantiomers were studied. To allow impurity studies down to 0.1% (distomer eutomer) a resolution of 2.5 should be warranted. Those beta2-agonists containing two hydroxylic groups in the aromatic ring structure show the highest enantiomeric separation, due to the fact that one of their enantiomers has a better geometric structure to fit into the beta-cyclodextrin cavity.

Electrokinetic injection in capillary electrophoresis and its application to the analysis of inorganic compounds

In capillary electrophoresis, electrokinetic injection is a highly controversial sampling technique. It is a simple mode of sample introduction which is suitable for on-line preconcentration of the analytes, but its precision and accuracy are more strongly affected by experimental conditions compared to hydrodynamic injection. In the first part of this paper the features of electrokinetic and hydrodynamic injections are compared, followed by a detailed discussion on the different biases of electrokinetic injection and on how to reduce them. Finally, applications of the electrokinetic injection are reviewed with special emphasis on the analysis of inorganic compounds.

Determination of the enantiomeric purity of (-) terbutaline by capillary electrophoresis using cyclodextrins as chiral selectors in a polyethylene glycol gel

A method was developed for determination of the enantiomeric purity of the therapeutic-pharmacological active (-)-enantiomer of terbutaline using cyclodextrins as a chiral selector dissolved in a removable liquid polyethylene glycol gel by use of capillary electrophoresis. The effect of temperature, type and concentration of polyethylene glycol and cyclodextrins was studied on the resolution between the two enantiomers. Best results were obtained with 10 mM hydroxyethyl-beta-cyclodextrin dissolved in a 10% polyethylene glycol-2000 solution at 15 degrees C. Under these conditions, an impurity of 0.1% (distomer/eutomer) can be readily detected.