Simple method for the quantitative examination of extra column band broadening in microchromatographic systems

In vivo monitoring of serotonin in the striatum of freely moving rats with one minute temporal resolution by online microdialysis-capillary high-performance liquid chromatography at elevated temperature and pressure

Online monitoring of serotonin in striatal dialysate from freely moving rats was carried out for more than 16 h at 1 min time resolution using microdialysis coupled online to a capillary HPLC system operating at about 500 bar and 50 °C. Several aspects of the system were optimized toward robust, in vivo online measurements. A two-loop, eight-port rotary injection valve demonstrated better consistency of continuous injections than the more commonly used two-loop, 10-port valve. A six-port loop injector for introducing stimulating solutions (stimulus injector) was placed in-line between the syringe pump and microdialysis probe. We minimized solute dispersion by using capillary tubing (75 μm inside diameter, 70 cm long) for the probe inlet and outlet. In vitro assessment of concentration dispersion during transport with a 30 s time resolution showed that the dispersion standard deviation for serotonin was well within the desired system temporal resolution. Each 30 or 60 s measurement reflects the integral of the true time response over the measurement time. We have accounted for this mathematically in determining the concentration dispersion during transport. The delay time between a concentration change at the probe and its detection is 7 min. The timing of injections from the stimulus injector and the cycle time for the HPLC monitoring of the flow stream were controlled. The electrochemical detector contained a 13 μm spacer to minimize detector dead volume. During in vivo experiments, retention time and separation efficiency were stable and reproducible. There was no statistically significant change over 5.5 h in the electrochemical detector sensitivity factor for serotonin. Dialysate serotonin concentrations change significantly in response to a 120 mM K(+) stimulus. Release of serotonin evoked by a 10 min, 120 mM K(+) stimulation, but not for other K(+) stimuli, exhibited a reproducible, oscillating profile of dialysate serotonin concentration versus time. Infusion of fluoxetine, a serotonin uptake inhibitor, increased dialysate serotonin concentrations and stimulated release magnitude. Transient serotonin increases were observed in response to the stress associated with unexpected handling. This system is simple, efficient, reliable, and suitable for the study of serotonin neurochemistry associated with emotion and behavior.

Synthesis and characterization of a hydrogel with controllable electroosmosis: a potential brain tissue surrogate for electrokinetic transport

Electroosmosis is the bulk fluid flow initiated by application of an electric field to an electrolyte solution in contact with immobile objects with a nonzero ζ-potential such as the surface of a porous medium. Electroosmosis may be used to assist analytical separations. Several gel-based systems with varying electroosmotic mobilities have been made in this context. A method was recently developed to determine the ζ-potential of organotypic hippocampal slice cultures (OHSC) as a representative model for normal brain tissue. The ζ-potential of the tissue is significant. However, determining the role of the ζ-potential in solute transport in tissue in an electric field is difficult because the tissue's ζ-potential cannot be altered. We hypothesized that mass transport properties, namely the ζ-potential and tortuosity, could be modulated by controlling the composition of a set of hydrogels. Thus, poly(acrylamide-co-acrylic acid) gels were prepared with three compositions (by monomer weight percent): acrylamide/acrylic acid 100/0, 90/10, and 75/25. The ζ-potentials of these gels at pH 7.4 are distinctly different, and in fact vary approximately linearly with the weight percent of acrylic acid. We discovered that the 25% acrylic acid gel is a respectable model for brain tissue, as its ζ-potential is comparable to the OHSC. This series of gels permits the experimental determination of the importance of electrokinetic properties in a particular experiment or protocol. Additionally, tortuosities were measured electrokinetically and by evaluating diffusion coefficients. Hydrogels with well-defined ζ-potential and tortuosity may find utility in biomaterials and analytical separations, and as a surrogate model for OHSC and living biological tissues.

The Rotating Ring-Disk Electrochemistry of the Copper(II) Complex of Thyrotropin-releasing Hormone

Thyrotropin-releasing Hormone (TRH) forms an electroactive Cu(II) complex in aqueous solution. Rotating ring-disk electrochemistry reveals oxidation at the disk electrode and reduction at the ring electrode. The plot of limiting current vs. square root of rotation frequency deviates from the Levich equation, indicating both preceding and following chemical reactions. The reaction following the oxidation is a multiple-electron ECE-type of process that has been seen before in Cu(II)-peptide electrochemistry. The preceding reaction is unusual. The deviation from diffusion-controlled behavior is more pronounced at higher initial concentration of Cu(II) and peptide. We propose that a non-electroactive dimer, Cu(II)(2)-TRH(2), is in a slow equilibrium with the electroactive Cu(II)-TRH. Simulation of the RRDE behavior of the postulated Cu(II)-TRH system has succeeded in matching experimental data. Capillary electrophoresis indicates that there is a negative charge on the dimer. It is suggested that a hydroxo-bridge may link the two Cu(II) centers. Calculations verify that bi-nuclear Cu(II)(2)-TRH(2) complexes are possible.

Use of tris(2,2'-bipyridine)osmium as a photoluminescence-following electron-transfer reagent for postcolumn detection in capillary high-performance liquid chromatography

The photoluminescence-following electron-transfer (PFET) technique, developed in our laboratory, is a sensitive chromatographic detection method for oxidizable analytes. Because the oxidations are homogeneous, the technique avoids the problem of electrode fouling. A liquid-phase oxidant reacts with the electrochemically active analytes after separation, becoming capable of photoluminescence. Laser-induced photoluminescence is measured to quantitate the analytes. Thus, the electrochemical properties of the oxidant determine the detection selectivity, and the spectroscopic properties define the sensitivity. The properties of tris(2,2'-bipyridine)osmium (1) were investigated for use as the liquid-phase oxidant in the PFET system. The redox potential of the complex is less positive than that of tris(2,2'-bipyridine)ruthenium (2); thus, on-line generation of 1(3+) by reaction with PbO2, and selective oxidation of catechols by 1(3+), was possible. The mild oxidizing power of 1(3+) led to a lower background signal (compared to 2(3+)) when mixed with acidic mobile phases. Photoluminescence from 1(2+) was much weaker than that from 2(2+); nonetheless, the system achieved subnanomolar detection limits for dopamine, 3-methoxytyramine, and serotonin. Dopamine and 3-methoxytyramine in rat brain striatal dialysates were determined before and after the injection of nomifensine. The pH of the mobile phase can govern the detection selectivity, since oxidation of most organics is accompanied by proton transfer. Reaction of 1 with catechols showed pH-dependent sensitivity resulting from pH-dependent reaction rate changes. Since the reaction rate is also temperature dependent, increased temperature at the mixer resulted in higher sensitivity. However, the noise level also increased at elevated temperature; thus, the detection limit did not improve.

Influence of chemical kinetics on postcolumn reaction in a capillary Taylor reactor with catechol analytes and photoluminescence following electron transfer

Postcolumn derivatization reactions can enhance detector sensitivity and selectivity, but their successful combination with capillary liquid chromatography has been limited because of the small peak volumes in capillary chromatography. A capillary Taylor reactor (CTR), developed in our laboratory, provides simple and effective mixing and reaction in a 25-microm-radius postcolumn capillary. Homogenization of reactant streams occurs by radial diffusion, and a chemical reaction follows. Three characteristic times for a given reaction process can be predicted using simple physical and chemical parameters. Two of these times are the homogenization time, which governs how long it takes the molecules in the analyte and reagent streams to mix, and the reaction time, which governs how long the molecules in a homogeneous solution take to react. The third characteristic time is an adjustment to the reaction time called the start time, which represents an estimate of the average time the analyte stream spends without exposure to reagent. In this study, laser-induced fluorescence monitored the extent of the postcolumn reaction (reduction of Os(bpy)3(3+) by analyte to the photoluminescent Os(bpy)3(2+)) in a CTR. The reaction time depends on the reaction rates. Analysis of product versus time data yielded second-order reaction rate constants between the PFET reagent, tris(2,2'-bipyridine)osmium, and standards ((ferrocenylmethyl)trimethylammonium cation and p-hydroquinone) or catechols (dopamine, epinephrine, norepinephrine, 3, 4-dihydroxyphenylacetic acid. The extent of the reactions in a CTR were then predicted from initial reaction conditions and compared to experimental results. Both the theory and experimental results suggested the reactions of catechols were generally kinetically controlled, while those of the standards were controlled by mixing time (1-2 s). Thus, the extent of homogenization can be monitored in a CTR using the relatively fast reaction of the reagent and p-hydroquinone. Kinetically controlled reactions of catechols, however, could be also completed in a reasonable time at increased reagent concentration. A satisfactory reactor, operating at 1.7 cm/s (2 microL/min) velocity with solutes having diffusion coefficients in the 5 x 10(-6) cm2/s range, can be constructed from 8.0 cm of 25-microm-radius capillary. Slower reactions require longer reaction times, but theoretical calculations expect that a CTR does not broaden a chromatographic peak (N = 14 000) from a 100-microm-capillary chromatography column by 10% if the pseudo-first-order rate constant is larger than 0.1 s(-1).

Analysis of the performance of a flow reactor for use with microcolumn HPLC

Postcolumn derivatization reactions can be used to improve detector sensitivity or selectivity. The advantages of capillary chromatography for trace analysis could be augmented if there were postcolumn reactors suitable for microchromatographic systems. However, postcolumn derivatization is a challenge because of the small peak volumes associated with capillary columns. We have developed a postcolumn flow reactor from microchannels formed in fluorinated ethylene propylene and 50-microm fused-silica tubing for use with capillary HPLC analyses. Theoretical and experimental evidence show that the reactor, which operates in the Taylor dispersion regime, enables contact of analyte and derivatization streams purely by diffusion. Reactor lengths as short as 2 cm allow formation of copper(II)-peptide complexes that are detected electrochemically at a carbon fiber microelectrode. The reactor has been used with 100-microm-i.d. columns with insignificant effects (i.e., <3%) on peak band spreading. Theoretical calculations indicate that even smaller i.d. columns can be used with little effect on chromatographic resolution.

Extracolumn band broadening in capillary liquid chromatography

A commercially available capillary LC instrument was modified to investigate and control the contribution of different instrument components on extracolumn band broadening. Quantitative estimations of dispersion induced by several equipmental parts were carried out. Injection parameters could be optimized to achieve the theoretical value of 12 for a profile factor describing a rectangular sample profile. Additionally, an additive injector flow channel dependent dispersion effect was found. A practical approach for minimizing instrumental effects in capillary LC is suggested. The results were compared with those obtained with an HPLC instrument designed for conventional size columns.

Techniques for neuropeptide determination

Because immunoassay responds to epitopes, and many molecules share the same peptide epitope, it is very difficult to obtain an accurate understanding of peptides, their creation and hydrolysis, in biological systems. Separate-and-detect approaches have merit in that the many active peptides and inactive fragments of a particular system can be separately determined. This review discusses the separation, by chromatography and capillary electrophoresis, and detection, by absorbance, fluorescence, electrochemistry, and immunoassay techniques. When separation pre-concentration is accompanied by laser-induced fluorescence or biuret-based electrochemical detection, nM-pM detection limits are obtained.