Maria A. Schwarz

Improved capacitively coupled conductivity detector for capillary electrophoresis

The new features of the capacitively coupled contactless conductivity detector for capillary electrophoresis described are higher peak-to-peak excitation voltages for the detector cell of up to 250 V, a pick-up amplifier in close proximity to the electrode and synchronous detection. The electrical performance of the cell was characterized and found to follow readily predictable patterns. The alterations led to a higher signal strength, a better signal-to-noise ratio (S/N) and improved stability. The 3 × S/N detection limits obtained for inorganic cations and anions are in the range 0.1–0.2 μM. For the indirect detection of model compounds of organic cations and anions (aliphatic amines and sulfonates), detection limits of typically 1 μM were achieved.

A two-electrode configuration for simplified amperometric detection in a microfabricated electrophoretic separation device

The simplified amperometric detection scheme demonstrated is based on the amperometric working and electrophoretic ground electrodes only. The latter serves as counter and pseudo-reference as well. It is shown via the successful determination of neurotransmitters, ascorbic acid and phenols on gold or platinum working electrodes that this approach is feasible for detection on a channel based electrophoretic separation device. Also presented is the detection of carbohydrates and amino acids with copper electrodes. The results were found to be similar to those obtained with conventional capillary systems with amperometric detection, albeit at much reduced analysis times.

Portable capillary electrophoresis instrument with amperometric, potentiometric and conductometric detection

Abstract The prototype of a field-portable battery-powered capillary electrophoresis instrument described here includes a high voltage supply capable of delivering the standard 30 kV at both polarities. The instrument has dimensions of 340 mm×175 mm×175 mm (w×h×d) and a weight of 7.5 kg. Data acquisition is carried out with a portable laptop or palmtop computer. Electrochemical detection was chosen as the straightforward signal transduction methods are relatively easily implemented. For robustness, the amperometric and potentiometric detection modes are carried out in a fixed wall-jet cell without decoupler. Both methods rely on the electrophoretic ground electrode as reference and counter electrode. For conductometric detection the only recently reported contactless version was implemented. The availability of the three complementary electrochemical detection methods allows for great versatility, which includes the ability to determine inorganic cations and anions as well as many organic species of interest.

Rapid chiral on-chip separation with simplified amperometric detection

The enantiomers of adrenaline, noradrenaline, ephedrine and pseudoephedrine were separated by capillary electrophoresis on a micromachined device. Detection was carried out with a new two-electrode amperometric detector, eliminating the need for individual counter and reference electrodes. Separation of the isomers was achieved by employing carboxymethyl-beta-cyclodextrin as chiral selector in the buffer, partly with the additional inclusion of the crown ether 18-crown-6. Plate numbers of up to 20,000, chiral resolutions of 2.5 and detection limits of the order of 10(-7) M were achieved. All separations were completed in less then 3 min.

Application of capillary electrophoresis for characterizing interactions between drugs and bile salts. Part I

Capillary electrophoresis offers a new way of characterizing interactions between different bile salts and drugs. The observed interactions were characterized with modified model functions known from affinity capillary electrophoresis (ACE) an micellar electrokinetic capillary electrophoresis (MECC). The methodical background of both methods is the change of the ionic mobility of the drug caused by partition between phases and aggregation with the bile salt molecules, respectively. This phenomenon is described by two different physicochemical models. A parameter estimation was carried out in order to obtain the partition coefficients KP as well as constants for the aggregate formation KA. Furthermore, an expression about the specific molar volume of the micelles and stoichiometric coefficients can be given.

Pharmaceutical applications of isoelectric focusing on microchip with imaged UV detection

For the first time, the application of a commercial Shimadzu microchip electrophoresis system MCE-2010 equipped with an imaging UV detector for isoelectric focusing (IEF) of therapeutic proteins is reported. By proper adjustment of the pH gradient, samples with pI values ranging from 2.85 to 10.3 can be focused to the imaged part of the separation channel. Three therapeutic proteins (hirudin, erythropoietin, and bevacizumab) have been successfully focused on the microchip, and the results have been compared to conventional capillary IEF in terms of peak profile, pI values, and reproducibility.

Characterization of interactions between bile salts and drugs by micellar electrokinetic capillary chromatography. Part I.

AbstractPurpose. The general properties of micellar electrokinetic capillary chromatography (MECC) were utilized to characterize the strength of interactions between bile salts and biological active substances. Methods. For that purpose various bile salts were used as micellar pseudostationary phase in the background electrolyte. Furthermore, a physicochemical model was applied and the effective partition coefficients between micellar and water phase were calculated in order to evaluate the strength of interactions between bile acids and the drugs. Results. It was found that the interactions between the selected drugs and bile salts depend both on the lipohilicity of the drugs and on the charge of the components. Only hydrophobic, cationic drugs such as quinine and propranolol are able to interact with these surface-active agents. Conclusions. MECC is a valuable methode to characterize interactions such occurring between drugs and bile salts.

Investigation of the interactions between drugs and mixed bile salt/ lecithin micelles a characterization by micellar affinity capillary electrophoresis (MACE). Part III

Mixed micelles, which mimic the bile in the gastrointestinal tract, were used as a pseudostationary phase in capillary electrophoresis. The mixed micellar system studied contained sodium glycodeoxycholate (NaGDC) or sodium taurocholate (NaTC) as bile salt and lecithin (LC) or dipalmitoylphosphatidylcholine (DPPC) as phospholipid. The determination of the changing mobilities of ionic analytes in the presence of mixed micelles reflected interactions between the used drugs and the mixed micelles. These were estimated according to the lipid concentration in the bile salt (BS)/phosphatidylcholine (PC) micelles and the capacity factors k for the partition were calculated whenever possible. For the calculation of k it is important to characterize the formation of the mixed micellar phase and to estimate the ionic mobility of the mixed micelles. In doing so two different methods were applied in order to determine this micellar phase. The partition equilibrium of basic and acidic drugs depends considerably on size and shape of the micelles as well as on the lipophilicity of the drug. This paper also outlines the use of electrospray mass spectrometry and MS-MS for the characterization of mixed micelle composition.

Application of capillary zone electrophoresis for analyzing biotin in pharmaceutical formulations—a comparative study

The detection limit and reproducibility of capillary zone electrophoresis (CZE) measurement of biotin were compared to those of a spectrophotometric method and those of the determination of the sulphur content by combustion of the biotin sample followed by coulometric titration of the formed SO2. Drug analysis showed that all three methods gave consistent results and were suitable for the determination of biotin. CZE was found to be the best method for the determination of pharmaceutical formulations containing biotin because of its high separation efficiency, short analysis time, ease of instrumentation and sample preconditioning, and good precision.

Microchip Affinity Capillary Electrophoresis: Applications and Recent Advances

Abstract This review gives the basic principle of affinity capillary electrophoresis (ACE) and examines its utilization in bioscience on microchips. ACE on chip is used as a separation tool as well as for study of molecular interactions. MEKC and chiral separations on microfluidic systems are described. Applications for measuring bioaffinity are focused on enzyme assay and immunoassay that demonstrate a further development of classical ACE in capillaries.