Sami Palonen

Extremely high electric field strengths in non-aqueous capillary electrophoresis

The influence of high electric field strength on the separation of basic analytes in non-aqueous alcohol background electrolyte (BGE) solutions was investigated. Increasing the separation voltage in capillary electrophoresis (CE) may be advantageous if the conductivity of the BGE solution is low enough to allow fast separations without excessive Joule heating or band broadening. The voltage range tested was 20-60 kV with methanol and ethanol, and 25-60 kV with propanol and butanol as solvent for BGE. The resulting electric field strengths ranged from 660 V cm(-1) to 2000 V cm(-1). Experiments were made with a special laboratory constructed CE instrument. The separation efficiency vs. voltage curve was found to vary with the alcohol BGE solution. The increase in voltage decreased the separation efficiency in the case of methanol BGE solution, but with the other BGEs a clear efficiency maximum was obtained above 30 kV. The highest separation efficiencies were achieved with propanol BGE solution, where the efficiency maximum was reached at 45 kV. However, reasonable efficiency was achieved even at 60 kV. The extent of Joule heating was determined by calculating the temperature inside the capillary and the observed plate heights were interpreted in terms of the Van Deemter equation. The decrease in the separation efficiency with higher voltage was attributed mainly to Joule heating in the case of methanol and ethanol BGE solution and to the analyte adsorption on the capillary wall with propanol and butanol BGE solutions.

Integration of a contactless conductivity detector into a commercial capillary cassette: Detection of inorganic cations and catecholamines

A contactless conductivity detector integrated into the capillary cassette of Agilent (3D)CE equipment is described. The detector is user-friendly, compact and easily modified. The UV detector of the (3D)CE equipment is available parallel with the contactless conductivity detector increasing the detection power. Two electrolyte solutions, 2-(N-morpholino)ethanesulfonic acid-histidine solution (20 mM, pH 6.0) and ammonium acetate (10 mM, pH 4.0), were used as the separation media for inorganic cations and organic catecholamines, respectively. The detection limit for all metal cations except barium was under 0.5 mg/l, and that for four catecholamines was ca. 10 mg/l. This last value was the same order of magnitude as achieved with parallel UV detection.

Nonaqueous capillary electrophoresis with alcoholic background electrolytes: Separation efficiency under high electrical field strengths

The effect of high voltage on capillary electrophoresis (CE) separations of anionic analytes in nonaqueous separation media was investigated. Methanol, ethanol, 1‐propanol, and 1‐butanol were tested as background electrolyte (BGE) solvents. Experiments were carried out with a laboratory‐built CE instrument suitable for high‐voltage separations. Potentials up to 60 kV were applied with reversed polarity to generate unusually high field strengths (e.g. 2000 Vcm–1) and so achieve fast and efficient separations. Highest separation efficiencies were obtained with propanol as BGE solvent, and the dependency of the efficiency on the separation voltage was more or less linear. With the other alcohols, separation efficiency decreased or remained roughly constant with increasing absolute voltage. The separation efficiencies are discussed in terms of longitudinal diffusion, Joule heating, and analyte interaction with the capillary wall. Capillary preconditioning had a varied effect on the separations in the different BGEs as the BGE and the conditioning process affected the electroosmotic flow (EOF) velocity and direction.

Capillary electrophoresis of methyl-substituted phenols in acetonitrile

The separation of mono- and dimethylphenols by capillary electrophoresis in pure acetonitrile was investigated. In acetonitrile, uncharged phenols interact with background electrolyte anions forming negatively charged complexes, which can be separated from each other by capillary electrophoresis. The background electrolyte anions tested were acetate, bromide and chloride. The calculated formation constants for phenol-anion complexes were highest with acetate and smallest with bromide. Complex formation was found to be sensitive to traces of water in the background electrolyte. The separation of methylphenols was also carried out in acetonitrile at high pH using background electrolytes prepared from diprotic acids and tetrabutylammonium hydroxide. At high pH the phenols were partly dissociated, providing an additional mechanism for the separation. All methylphenols were separated with the use of malonate background electrolyte. However, this approach was prone to interference from methanol resulting from the tetrabutylammonium hydroxide solution.

Compensation of the siphoning effect in nonaqueous capillary electrophoresis by vial lifting

Increasing the sample load in nonaqueous capillary electrophoresis through the use of wide‐bore capillaries is a good way to scale up analytical separations to semipreparative level. However, obtaining high efficiency requires the use of special instrumentation to eliminate siphoning. When wide‐bore capillaries are employed, relatively large solvent volumes are transported from inlet to outlet vial, and due to the difference in liquid levels a siphoning flow from outlet to inlet is established. Siphoning induces a deviation from the plug‐like flow profile and adversely affects the separation efficiency. In this study the use of wide‐bore capillaries in nonaqueous capillary electrophoresis was examined with compensation for siphoning by lifting of the inlet vial. The inlet vial is raised at a speed appropriate for maintaining equal levels of liquid in the inlet and outlet vials. The optimal lift rate was determined empirically from a series of runs in which the lift rate was varied. As well, a simple theoretical model was devised for the calculation of lift rates. The model was successfully applied for the 200 μm and 320 μm ID capillaries but for the 530 μm ID capillary the predicted optimal lift rate was too low. Evidently this was because the theory was unable to account for the effect of siphoning on the migration times. Three model compounds, bumetanide, furosemide and ethacrynic acid, were separated using an acetonitrile‐ethanol mixture (50:50, v/v) with potassium acetate (1 mM) or ammonium acetate (5 mM) as electrolyte. Good separation of bumetadine and ethacrynic acid was obtained even with a 530 μm ID capillary when the lift rate was carefully optimized. Without elimination of siphoning the peaks would not have been detectable. The viscosities and electrical conductivities of the electrolyte solution measured at different temperatures showed that viscosity as well as conductivity decreased with increasing temperature. The temperature dependence of the conductivity was used to estimate the temperature inside the CE capillary.

Interface for coupling nonaqueous wide-bore capillary electrophoresis with mass spectrometry

A liquid‐junction‐type interface where a thin spraying capillary is inserted inside the separation capillary was constructed for coupling nonaqueous wide‐bore capillary electrophoresis (CE) to mass spectrometry (MS). The robust structure of the interface provided fairly easy capillary handling. The study was carried out with uncoated CE capillaries of 200 and 320 νm inner diameter (ID). 1‐Propanol‐acetonitrile (80:20 v/v) with acetate electrolyte provided a low conducting medium for CE and good spraying conditions for electrospray ionization (ESI) without sheath‐flow and drying gas. Methamphetamine, alprenolol, and levorphanol served as model compounds. Approximate detection limits with the 200 νm ID capillary were 35–265 ng/mL.