Dean S. Burgi

Field amplified sample injection in high-performance capillary electrophoresis

Abstract A simple on-column concentration technique in high-performance capillary electrophoresis (HPCE) is reported. In conventional electro-injection in HPCE, samples are prepared in a buffer solution which has the same concentration as that inside the capillary column. The amount of ions injected into the column under this condition is limited. By preparing samples in a low-conductivity solution, e.g., water, and injecting the sample solution electroosmotically into the column, one can achieve a field enhancement at the injection point. The amount of ions injected will then be proportional to this enhancement factor. However, if one samples by switching the column directly from the high-conductivity buffer reservoir to the low-conductivity sample solution, the buffer boundary at the end of the column is disturbed and the electric field at the injection point might not be amplified properly. By injecting a short plug of water before sample introduction, one can provide a high electric field strength from the beginning of the injection. Several hundred-fold enhancements in the amount of injection were confirmed experimentally.

Field-amplified polarity-switching sample injection in high-performance capillary electrophoresis

Abstract In conventional electro-injection in high-performance capillary electrophoresis, although one is able to inject both positive and negative ions into the column, the number of negative ions injected is rather limited because of their movement against the electric field, assuming the column wall is negatively charged. If one simply reverses the polarity of the field, the electroosmotic flow will deter all positive and most of negative ions from injecting into the column. In the case of field-amplified sample injection, where samples are prepared in a low-conductivity buffer and injected electrically into the column, the number of positive ions injected is porportional to the field enhancement factor at the injection point. The negative ions will not be enhanced, but will be pushed away from the column by this high field strength. However, since the electroosmotic velocity of the bulk solution is much slower than the electrophoretic velocity of sample ions under the enhanced field, one can inject and concentrate both positive and negative ions into the column by switching the polarity of the electrodes at the proper time. Furthermore, one can also achieve selected charge discrimination.

Evaluation of fundamental properties of a silica capillary used for capillary electrophoresis

Abstract A model was developed that accounts for the decrease in the electroosmotic flow in a capillary electrophoresis system when the buffer concentration is increased. Important parameters are: the initial charge per unit area at the silica capillary wall, Qo; a compact layer of molecules of constant thickness do that exists between the capillary wall and the buffer; and the equilibrium constant, Kwall, between the cations in the buffer and adsorption sites on the silica capillary. An excellent fit of an equation derived from the model to experimental results was obtained. Values for the above parameters were determined in a number of different buffers and the effect of pH, buffer composition and column coatings on these parameters was evaluated.

Improvement in the method of sample stacking for gravity injection in capillary zone electrophoresis

A method that allows capillary electrophoresis to be used as a microconcentrating technique is presented. An 85-fold improvement in the amount of material that can be injected into a capillary column without loss of resolution is shown. The method can be used for negative- and positive-charged species but it cannot be used for both species simultaneously.

Separation of seven tricyclic antidepressants using capillary electrophoresis

Abstract Seven tricyclic antidepressants, protriptyline, desipramine, nortriptyline, nordoxepin, imipramine, amitriptyline and doxepin, were separated using capillary electrophoresis. Because the tricyclic antidepressants were similar in structure and mass, careful manipulation of the electroosmotic flow and the electrophoretic mobilities was required for an optimal separation. In the systematic approach we have developed, the differential electrophoretic mobilities were first maximized by adjusting pH. Next, increasing the buffer concentration improved the separation at the expense of migration times by reducing the electroosmotic flow. Full resolution was achieved by the addition of methanol to the buffer which decreased both the electroosmotic flow and the electrophoretic mobilities of the samples.

On-line sample pre-concentration in microfluidic devices: a review.

On-line sample preconcentration is an essential tool in the development of microfluidic-based separation platforms. In order to become more competitive with traditional separation techniques, the community must continue to develop newer and more novel methods to improve detection limits, remove unwanted sample matrix components that disrupt separation performance, and enrich/purify analytes for other chip-based actions. Our goal in this review is to familiarize the reader with many of the options available for on-chip concentration enhancement with a focus on those manuscripts that, in our assessment, best describe the fundamental principles that govern those enhancements. Sections discussing both electrophoretic and nonelectrophoretic modes of preconcentration are included with a focus on device design and mechanisms of preconcentration. This review is not meant to be a comprehensive collection of every available example, but our hope is that by learning how on-line sample concentration techniques are being applied today, the reader will be inspired to apply these techniques to further enhance their own programs.

Methods For Calculating the Internal Temperature of Capillary Columns During Capillary Electrophoresis

Abstract The internal temperature, the buffer viscosity and the efficiency of heat removal from a silica capillary can be calculated by measuring the differential electroosmostic mobility at a low voltage and a high voltage. the calculated temperature is plotted vs the power generated by the electrophoretic instrument and yields a linear relationship when the power is below 3.0 W. the temperature can also be calculated using the conductivity of the solution. the two methods provide the same temperature, which compares well with literature values. A rule of thumb for a quick calculation of internal temperatures is that a power of 0.1 W

Direct injection of seawater for the analysis of nitroaromatic explosives and their degradation products by micellar electrokinetic chromatography.

Practical considerations for the injection and separation of nitroaromatic explosives in seawater sample matrices are discussed. The use of high surfactant concentrations and long electrokinetic injections allows for improved detection limits. Sensitivity was enhanced by two mechanisms, improved stacking at the detector-side of the sample plug and desorption of analyte from the capillary wall by surfactant-containing BGE from the inlet side of the sample plug. Calculated limits of detection (S/N=3) for analytes prepared in pure seawater were 70-800 ppb with injection times varying from 5 to 100 s.