Monitoring of met-enkephalin in vivo with 5-min temporal resolution using microdialysis sampling and capillary liquid chromatography with electrochemical detection

Fabrication and Optimization of a Molecularly Imprinted Carbon Fiber Microelectrode for Selective Detection of Met-enkephalin Using Fast-Scan Cyclic Voltammetry

Methionine-enkephalin (Met-Enk) is an endogenous opioid peptide that is involved in various physiological processes including memory. A technological gap in the understanding of Met-Enk's role in memory is the lack of rapid measurement tools to selectively quantify Met-Enk concentrations in situ. Here, we integrate molecularly imprinted polymers (MIPs) with carbon fiber microelectrodes (CFMs) to selectively detect Met-Enk by using fast-scan cyclic voltammetry (FSCV). We report two MIP conditions that yield 2-fold and 5-fold higher selectivity toward Met-Enk than the tyrosine-containing hexapeptide fragment angiotensin II (3-8). We demonstrate that MIP technology can be combined with FSCV at CFMs to create rapid and selective sensors for Met-Enk. This technology is a promising platform for creating selective sensors for other peptides and biomarkers.

Optimized Sawhorse Waveform for the Measurement of Oxytocin Release in Zebrafish

Oxytocin is a nonapeptide hormone involved in numerous physiological functions. Real-time electrochemical measurements of oxytocin in living tissue are challenging due to electrode fouling and the large potentials needed to oxidize the tyrosine residue. Here, we used fast-scan cyclic voltammetry at carbon-fiber microelectrodes and flow injection analysis to optimize a waveform for the measurement of oxytocin. This optimized waveform employed an accumulation potential of -0.6 V, multiple scan rates, and a 3 ms holding potential at a positive, oxidizing potential of +1.4 V before linearly scanning the potential back to -0.6 V (versus Ag/AgCl). We obtained a limit of quantitation of 0.34 ± 0.02 μM, and our electrodes did not foul upon multiple injections. Moreover, to demonstrate the utility of our method, we measured the release of oxytocin, evoked by light application and mechanical perturbation, in whole brains from genetically engineered adult zebrafish that express channelrhodopsin-2 selectively on oxytocinergic neurons. Collectively, this work expands the toolkit for the measurement of peptides in living tissue preparations.

Dynorphin and its role in alcohol use disorder

The dynorphin / kappa opioid receptor (KOR) system has been implicated in many aspects that influence neuropsychiatric disorders. Namely, this system modulates neural circuits that primarily regulate reward seeking, motivation processing, stress responsivity, and pain sensitivity, thus affecting the development of substance and alcohol use disorder (AUD). The effects of this system are often bidirectional and depend on projection targets. To date, a majority of the studies focusing on this system have examined the KOR function using agonists and antagonists. Indeed, there are studies that have examined prodynorphin and dynorphin levels by measuring mRNA and tissue content levels; however, static levels of the neuropeptide and its precursor do not explain complete and online function of the peptide as would be explained by measuring dynorphin transmission in real time. New and exciting methods using optogenetics, chemogenetics, genetic sensors, fast scan cyclic voltammetry are now being developed to detect various neuropeptides with a focus on opioid peptides, including dynorphin. In this review we discuss studies that examine dynorphin projections in areas involved in AUD, its functional involvement in AUD and vulnerability to develop AUD at various ages. Moreover, we discuss dynorphin's role in promoting AUD by dysregulation motivation circuits and how advancements in opioid peptide detection will further our understanding.

Characterization of a Multiple-Scan-Rate Voltammetric Waveform for Real-Time Detection of Met-Enkephalin

Opioid peptides are critically involved in a variety of physiological functions necessary for adaptation and survival, and as such, understanding the precise actions of endogenous opioid peptides will aid in identification of potential therapeutic strategies to treat a variety of disorders. However, few analytical tools are currently available that offer both the sensitivity and spatial resolution required to monitor peptidergic concentration fluctuations in situ on a time scale commensurate with that of neuronal communication. Our group has developed a multiple-scan-rate waveform to enable real-time voltammetric detection of tyrosine containing neuropeptides. Herein, we have evaluated the waveform parameters to increase sensitivity to methionine-enkephalin (M-ENK), an endogenous opioid neuropeptide implicated in pain, stress, and reward circuits. M-ENK dynamics were monitored in adrenal gland tissue, as well as in the dorsal striatum of anesthetized and freely behaving animals. The data reveal cofluctuations of catecholamine and M-ENK in both locations and provide measurements of M-ENK dynamics in the brain with subsecond temporal resolution. Importantly, this work also demonstrates how voltammetric waveforms can be customized to enhance detection of specific target analytes, broadly speaking.

Carbon Nanoelectrodes for the Electrochemical Detection of Neurotransmitters

Carbon-based electrodes have been developed for the detection of neurotransmitters over the past 30 years using voltammetry and amperometry. The traditional electrode for neurotransmitter detection is the carbon fiber microelectrode (CFME). The carbon-based electrode is suitable for in vivo neurotransmitter detection due to the fact that it is biocompatible and relatively small in surface area. The advent of nanoscale electrodes is in high demand due to smaller surface areas required to target specific brain regions that are also minimally invasive and cause relatively low tissue damage when implanted into living organisms. Carbon nanotubes (CNTs), carbon nanofibers, carbon nanospikes, and carbon nanopetals among others have all been utilized for this purpose. Novel electrode materials have also required novel insulations such as glass, epoxy, and polyimide coated fused silica capillaries for their construction and usage. Recent research developments have yielded a wide array of carbon nanoelectrodes with superior properties and performances in comparison to traditional electrode materials. These electrodes have thoroughly enhanced neurotransmitter detection allowing for the sensing of biological compounds at lower limits of detection, fast temporal resolution, and without surface fouling. This will allow for greater understanding of several neurological disease states based on the detection of neurotransmitters.

Dynamic measurement of extracellular opioid activity: status quo, challenges, and significance in rewarded behaviors

Opioid peptides are the endogenous ligands of opioid receptors, which are also the molecular target of naturally occurring and synthetic opiates, such as morphine and heroin. Since their discovery in the 1970s, opioid peptides, which are found widely throughout the central nervous system and the periphery, have been intensely studied because of their involvement in pain and pleasure. Over the years, our understanding of opioid peptides has widened to cover a multitude of functions, including learning and memory, affective state, gastrointestinal transit, feeding, immune function, and metabolism. Unsurprisingly, aberrant opioid activity is implicated in numerous pathologies, including drug addiction, overeating, pain, depression, and obesity. To date, virtually all preclinical and clinical studies aimed at understanding the function of endogenous opioids have relied upon manipulating endogenous opioid fluxes using opioid receptor ligands or genetic manipulations of opioid receptors and endogenous opioids. Difficulties in directly monitoring endogenous opioid fluxes, particularly in the central nervous system, have presented a major obstacle to fully understanding endogenous opioid function. This review summarizes these challenges and offers suggestions for future goals while focusing on the neurobiology of reward, specifically drawing attention to studies that have succeeded in dynamically measuring opioid peptides.

Rapid preconcentration for liquid chromatography-mass spectrometry assay of trace level neuropeptides

Measurement of neuropeptides in the brain through in vivo microdialysis sampling provides direct correlation between neuropeptide concentration and brain function. Capillary liquid chromatography-multistage mass spectrometry (CLC-MS(n)) has proven to be effective at measuring endogenous neuropeptides in microdialysis samples. In the method, microliter samples are concentrated onto nanoliter volume packed beds before ionization and mass spectrometry analysis. The long times required for extensive preconcentration present a barrier to routine use because of the many samples that must be analyzed and instability of neuropeptides. In this study, we evaluated the capacity of 75 μm inner diameter (i.d.) capillary column packed with 10 μm reversed phase particles for increasing the throughput in CLC-MS(n) based neuropeptide measurement. Coupling a high injection flow rate for fast sample loading/desalting with a low elution flow rate to maintain detection sensitivity, this column has reduced analysis time from ∼30 min to 3.8 min for 5 μL sample, with 3 pM limit of detection (LOD) for enkephalins and 10 pM LOD for dynorphin A1-8 in 5 μL sample. The use of isotope-labeled internal standard lowered peptide signal variation to less than 5 %. This method was validated for in vivo detection of Leu and Met enkephalin with microdialysate collected from rat globus pallidus. The improvement in speed and stability makes CLC-MS(n) measurement of neuropeptides in vivo more practical.

The absolute quantification of endogenous levels of brain neuropeptides in vivo using LC-MS/MS

Neuropeptides seem to play an important role when the CNS is challenged. In order to obtain better insights into the central peptidergic effects, it is essential to monitor their concentration in the brain. Quantification of neuropeptides in dialysates is challenging due to their low extracellular concentrations (low pM range), their low microdialysis efficiencies, the need for acceptable temporal resolution, the small sample volumes, the complexity of the matrix and the tendency of peptides to stick to glass and polymeric materials. The quantification of neuropeptides in dialysates therefore necessitates the use of very sensitive nano-LC-MS/MS methods. A number of LC-MS/MS and microdialysis parameters need to be optimized to achieve maximal sensitivity. The optimized and validated methods can be used to investigate the in vivo neuropeptide release during pathological conditions, in this way initiating new and immense challenges for the development of new drugs.

Review of recent advances in analytical techniques for the determination of neurotransmitters

Methods and advances for monitoring neurotransmitters in vivo or for tissue analysis of neurotransmitters over the last five years are reviewed. The review is organized primarily by neurotransmitter type. Transmitter and related compounds may be monitored by either in vivo sampling coupled to analytical methods or implanted sensors. Sampling is primarily performed using microdialysis, but low-flow push-pull perfusion may offer advantages of spatial resolution while minimizing the tissue disruption associated with higher flow rates. Analytical techniques coupled to these sampling methods include liquid chromatography, capillary electrophoresis, enzyme assays, sensors, and mass spectrometry. Methods for the detection of amino acid, monoamine, neuropeptide, acetylcholine, nucleoside, and soluble gas neurotransmitters have been developed and improved upon. Advances in the speed and sensitivity of these methods have enabled improvements in temporal resolution and increased the number of compounds detectable. Similar advances have enabled improved detection at tissue samples, with a substantial emphasis on single cell and other small samples. Sensors provide excellent temporal and spatial resolution for in vivo monitoring. Advances in application to catecholamines, indoleamines, and amino acids have been prominent. Improvements in stability, sensitivity, and selectivity of the sensors have been of paramount interest.

Time-resolved microdialysis for in vivo neurochemical measurements and other applications

Monitoring changes in chemical concentrations over time in complex environments is typically performed using sensors and spectroscopic techniques. Another approach is to couple sampling methods, such as microdialysis, with chromatographic, electrophoretic, or enzymatic assays. Recent advances of such coupling have enabled improvements in temporal resolution, multianalyte capability, and automation. In a sampling and analysis method, the temporal resolution is set by the mass sensitivity of the analytical method, analysis time, and zone dispersion during sampling. Coupling methods with high speed and mass sensitivity to microdialysis sampling help to reduce some of these contributions to yield methods with temporal resolution of seconds. These advances have been primarily used in monitoring neurotransmitters in vivo. This review covers the problems associated with chemical monitoring in the brain, recent advances in using microdialysis for time-resolved in vivo measurements, sample applications, and other potential applications of the technology such as determining reaction kinetics and process monitoring.

Highly sensitive assay for the measurement of serotonin in microdialysates using capillary high-performance liquid chromatography with electrochemical detection

A highly sensitive isocratic capillary high-performance liquid chromatographic (HPLC) method with electrochemical detection (ED) for the simultaneous measurement of serotonin (5-hydroxytryptamine, 5-HT) and its metabolite 5-hydroxyindole-3-acetic acid (5-HIAA) in microdialysates has been developed using a 0.5 mm i.d. capillary column and a 11-nL detection cell. This method, validated on both pharmacological and analytical bases, can be performed using injection volumes as low as 1 microL. The limits of detection were 5.6 x 10(-11)mol/L and 3.0 x 10(-9)mol/L for 5-HT and 5-HIAA. Several applications of the present method are given on microdialysates from rodent brain and human spinal cord.

Capillary and nano-liquid chromatography-tandem mass spectrometry for the quantification of small molecules in microdialysis samples: comparison with microbore dimensions

Enhanced sensitivity is a well known benefit of miniaturised LC-electrospray (ESI)-MS/MS methods. The suitability of miniaturised LC-MS/MS for quantification of small molecules in dialysates was investigated using the anti-epileptic drug oxcarbazepine, its active metabolite, 10,11-dihydro-10-hydroxycarbamazepine, and the internal standard for microdialysis probe calibration, 2-methyl-5H-dibenz(b,f)azepine-5-carboxamide, as test compounds. ESI-MS detection is sensitive to matrix effects. Therefore, dialysate matrix effects were investigated by comparing the responses of standards made in water, Ringer's solution (salt solution used as perfusion fluid) and blank dialysate matrix. Due to the occurrence of ion suppression or enhancement, direct injection of dialysis samples onto the analytical column could not be applied for quantification of small molecules in dialysis samples. Column switching was necessary for desalting and preconcentration of the dialysates. However, this approach was not able to completely eliminate salt effects when the injection volume exceeded 1 microL. No differences in response between Ringer's solution and dialysate matrix were detected at capillary and nano-dimensions. Calibration standards should be prepared with Ringer's solution instead of water for quantitative analysis of microdialysates. A microbore, capillary and nano-LC-ESI-MS/MS method were compared in terms of method feasibility, linearity, sensitivity, accuracy and precision. Downscaling to capillary and nano-dimensions resulted in a gain in detection sensitivity of 5 and 50, respectively. Miniaturised LC-MS/MS was found to be fit for quantification of small molecules in dialysates with acceptable accuracy and method precision.

Separation and determination of enkephalin-related peptides using capillary electrophoresis

CZE with UV-absorption detection has been used for the separation and determination of enkephalin-related peptides. The experimental conditions, such as pH and concentration of running buffer, applied voltage, injection method, and time, were investigated in detail. Excellent separation efficiency could be obtained for ten enkephalin-related peptides with a 50 microm (ID) x 58 cm capillary using sodium dihydrogen phosphate as the running buffer (pH 3.11) when 20 kV of applied voltage was used. The concentration detection limits were found to be in the range of 0.31-1.94 microg/mL (defined as S/N = 3). The proposed method has been applied to analyze the spiked cerebrospinal fluid (CSF) sample, and the results showed that CZE is a powerful technique for separation and detection of the above biological peptides.

Microfluidic Electrophoresis Chip Coupled to Microdialysis for in Vivo Monitoring of Amino Acid Neurotransmitters

Microfluidic electrophoresis devices were coupled on-line to microdialysis for in vivo monitoring of primary amine neurotransmitters in rat brain. The devices contained a sample introduction channel for dialysate, a precolumn reactor for derivatization with o-phthaldialdehyde, a flow-gated injector, and a separation channel. Detection was performed using confocal laser-induced fluorescence. In vitro testing revealed that the initial device design had detection limits for amino acids of approximately 200 nM, relative standard deviation of peak heights of 2%, and separations within 95 s with up to 30,200 theoretical plates when applying an electric field of 370 V/cm. A second device design that allowed electric fields of 1320 V/cm to be applied while preserving the reaction time allowed separations within 20 s with up to 156,000 theoretical plates. Flow splitting into the electrokinetic network from hydrodynamic flow in the sample introduction channel was made negligible for sampling flow rates from 0.3 to 1.2 microL/min by placing a 360-microm-diameter fluidic access hole that had flow resistance (0.15-7.2) x 10(8)-fold lower than that of the electrokinetic network at the junction of the sample introduction channel and the electrokinetic network. Using serial injections, the device allowed the dialysate stream to be analyzed at 130-s intervals. In vivo monitoring was demonstrated by using the microdialysis/microfluidic device to record glutamate concentrations in the striatum of an anesthetized rat during infusion of the glutamate uptake inhibitor l-trans-pyrrolidine-2,4-dicarboxylic acid. These results prove the feasibility of using a microfabricated fluidic system coupled to sampling probes for chemical monitoring of complex media such as mammalian brain.

Capillary liquid chromatography with MS3 for the determination of enkephalins in microdialysis samples from the striatum of anesthetized and freely-moving rats

In vivo microdialysis sampling was coupled to capillary liquid chromatography (LC)/electrospray ionization quadrupole ion trap mass spectrometry (MS) to monitor [Met]enkephalin and [Leu]enkephalin in the striatum of anesthetized and freely-moving rats. The LC system utilized a high-pressure pump to load 2.5 microl samples and desalt the 25 microm i.d. by 2 cm long column in 12 min. Samples were eluted with a separate pump at approximately 100 nl min(-1). A rapid gradient effectively separated the endogenous neuropeptides in 4 min. A comparison was made for operating the mass spectrometer in the MS2 and MS3 modes for detection of the peptides. In standard solutions, the detection limits were similar at 1-2 pM (2-4 amol injected); however, the reproducibility was improved with MS3 as the relative standard deviation was <5% compared with 20% for MS2 for 60 pM samples. For dialysate solutions, reconstructed ion chromatograms and tandem mass spectra had much higher signal-to-noise ratios in the MS3 mode, resulting in more confident detection at in vivo concentrations. The method was successfully used to monitor the peptides under basal conditions and with stimulation of peptide secretion by infusion of elevated K+ concentration.

Discovery and Neurochemical Screening of Peptides in Brain Extracellular Fluid by Chemical Analysis of in Vivo Microdialysis Samples

Endogenous peptides from brain extracellular fluid of live rats were analyzed using capillary liquid chromatography (LC)-tandem mass spectrometry (MS2). A 4-mm-long microdialysis probe perfused at 0.6 microL/min implanted into the striatum of anesthetized male rats was used to collect 3.6 microL dialysate fractions that were injected on-line into the capillary LC-MS2 system for analysis. A total of 3349 MS2 spectra were collected from 13 different animals under basal conditions and during localized depolarization evoked by infusion of a high-K+ solution through the microdialysis probe. Subtractive analysis revealed a total of 859 MS2 spectra that were observed only during depolarization. From these spectra, 29 peptide sequences (25 were peptides not previously observed) from 6 different protein precursors were identified using database searching software. Proteins identified include precursors to neuropeptides, synaptic proteins, blood proteins, and transporters. The identified peptides represent candidates for neurotransmitters, neuromodulators, and markers of synaptic activity or brain tissue damage. A screen for neuroactivity of novel proenkephalin fragments that were found was performed by infusing the peptides into the brain while monitoring amino acid neurotransmitters by microdialysis sampling combined with capillary electrophoresis. Three of the six tested peptides evoked significant increases in various neuroactive amino acids. These results demonstrate that this combination of methods can identify novel neurotransmitter candidates and screen for potential neuroactivity.

Assaying protein unbound drugs using microdialysis techniques

Compared with traditional sampling methods, microdialysis is a technique for protein unbound drug sampling without withdrawal of biological fluids and involving minimal disturbance of physiological function. Conventional total drug sample consists of unbound drugs and protein bound drugs, which are loosely bound to plasma proteins such as albumin and alpha-1 acid glycoprotein, forming an equilibrium ratio between bound and unbound drugs. However, only the unbound fraction of drug is available for absorption, distribution, metabolism and elimination, and delivery to the target sites for pharmacodynamic actions. Although several techniques have been used to determine protein unbound drugs from biological fluids, including ultrafiltration, equilibrium dialysis and microdialysis, only microdialysis allows simultaneous sampling of protein unbound chemicals from plasma, tissues and body fluids such as the bile juice and cerebral spinal fluid for pharmacokinetic and pharmacodynamic studies. This review article describes the technique of microdialysis and its application in pharmacokinetic studies. Furthermore, the advantages and limitations of microdialysis are discussed, including the detailed surgical techniques in animal experiments from rat blood, brain, liver, bile duct and in vitro cell culture for unbound drug analysis.

Gradient liquid chromatography of leucine-enkephalin peptide and its metabolites with electrochemical detection using highly boron-doped diamond electrode

Boron-doped diamond thin film (BDD) electrodes have been used to study the oxidation reactions and to detect leucine-enkephalinamide (LEA) and its metabolites, tyrosine (T), tyrosyl-alanine (TA), tyrosyl-alanine-glycine (TAG) and leucine-enkephalin (LE) using cyclic voltammetry (CV), flow-injection analysis (FIA), and gradient liquid chromatography (LC) with amperometric detection. At diamond electrodes, well-defined and highly reproducible cyclic voltammograms were obtained with signal-to-background (S/B) ratios 5-10 times higher than those observed for glassy carbon (GC) electrodes. The analytical peaks of LC for LEA and its metabolites were well resolved. No deactivation of BDD electrodes was found after several experiments with standard as well as plasma samples, indicating high stability of the electrode. Calibration curves were linear over a wide range from 0.06 to 30 microM with regression coefficients of 0.999 for all compounds. The limits of detection obtained based on a signal-to-noise ratio of 3:1 were 3, 2.2, 2.7, 20 and 11 nM for T, TA, TAG, LE and LEA, respectively. These values were at least one order lower than those obtained at GC electrodes, which has given limits of detection of 22.88, 20.64, 89.57, 116.04 and 75.67 for T, TA, TAG, LE and LEA, respectively. Application of this method to real samples was demonstrated and validated using rabbit serum samples. This work shows the promising use of conducting diamond as an amperometric detector in gradient LC, especially for the analysis of enkephalinamide and its metabolites.

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

In recent years capillary chromatography has gained popularity for trace analyses. Most often UV or electrochemical detection is employed because the small peak volumes make post-column derivatization challenging. We have developed a simple method based on flow injection for determining contributions to peak broadening from post-column reactors. The only requirement for application of our methodology is that diffusion be in the Taylor regime so that radial concentration gradients are relaxed enabling mixing purely by diffusion.

Capillary LC-MS2 at the attomole level for monitoring and discovering endogenous peptides in microdialysis samples collected in vivo

Fused-silica capillary LC columns (25-microm i.d.) with 3-microm-i.d. integrated electrospray emitters interfaced to a quadrupole ion trap mass spectrometer were evaluated for high-sensitivity LC-MS2. Column preparation involved constructing frits by in situ photopolymerization of glycidyl methacrylate and trimethylolpropane trimethacrylate, preparing the electrospray emitter by pulling the column outlet to a fine tip with a CO2 laser puller, and slurry-packing the column with 5-microm reversed-phase particles. Large-volume injections were facilitated by an automated two-pump system that allowed high-flow rates for sample loading and low-flow rates for elution. Small electrospray emitters, low elution flow rates, and optimization of gradient steepness allowed a detection limit of 4 amol, corresponding to 2 pM for 1.8 microL injected on-column, for a mixture of peptides dissolved in artificial cerebral spinal fluid. The system was coupled on-line to microdialysis sampling and was used to monitor and discover endogenous neuropeptides from the globus pallidus of anesthetized male Sprague-Dawley rats. Time-segmented MS2 scans enabled simultaneous monitoring of Met-enkephalin, Leu-enkephalin, and unknown peptides. Basal dialysate levels of Met-enkephalin and Leu-enkephalin were 60 +/- 30 and 70 +/- 20 pM while K+-stimulated levels were 1,900 +/- 500 and 1,300 +/- 300 pM, respectively (n = 7). Data-dependent and time-segmented MS2 scans revealed several unknown peptides that were present in dialysate. One of the unknowns was identified as peptide I(1-10) (SPQLEDEAKE), a novel product of preproenkephalin A processing, using MS2, MS3, and database searching.

Detection of native amino acids and peptides utilizing sinusoidal voltammetry

Native amino acids and peptides were detected at a copper microelectrode using sinusoidal voltammetry (SV). Traditionally, these molecules can only be measured after derivatization with either a fluorescent or electroactive tag. In this work, an electrocatalytic oxidation reaction at copper is used to detect underivatized peptides and amino acids. The oxidation reaction is somewhat independent of peptide structure (i.e., it is not limited to the detection of aromatic amino acids) and is therefore able to produce nanomolar detection limits for all amino acids and peptides tested. A scanning technique, sinusoidal voltammetry, is used to provide the sensitivity of constant-potential techniques but also provide selectivity gained through utilization of the frequency domain. The frequency spectrum due to the oxidation of each molecule has a unique "fingerprint" response resulting from the kinetics of oxidation at the electrode surface. Through examination of the frequency spectra, even structurally similar molecules can be easily distinguished from one another. Flow injection analysis is used to demonstrate the sensitive and selective detection of a variety of amino acids and peptides. This technique can also be easily coupled to a separation step, i.e., high-performance liquid chromatography or capillary electrophoresis without electrode fouling from the adsorption of the analytes.

Recent developments in microcolumn liquid chromatography

An overview of the most recent developments in microcolumn liquid chromatography (LC) is presented. A short theoretical discussion on chromatographic dilution and extracolumn bandbroadening is given and also the recent progress and advances in column technology and instrumentation are reviewed. However, the emphasis of this review is on miniaturized sample clean-up, sample introduction techniques and on both established and more recent detection techniques for microcolumn LC. The hyphenation of miniaturized LC columns with other techniques, specifically on multidimensional chromatography and the coupling of microcolumn LC to mass spectrometry is discussed in detail. Both the on-line and automated off-line interfacing to other separation and detection techniques will also be addressed. Finally, a number of typical microcolumn LC applications are presented in order to demonstrate the potential of microcolumn LC methods in a variety of scientific areas.

Separation and quantification of ropinirole and some impurities using capillary liquid chromatography

Ropinirole, 4-[2-(dipropylamino)ethyl]-1,3-dihydro-2H-indol-2-one, is a potent anti-Parkinson's disease drug developed by SmithKline Beecham Pharmaceuticals. Capillary liquid chromatography (CLC) was used for the separation and quantification of ropinirole and its five related impurities, potentially formed during its synthesis. A simultaneous optimization of three mobile phase parameters, i.e., pH, buffer concentration and acetonitrile content was performed employing an experimental design approach which proved a powerful tool in method development. The retention factors of the investigated substances in different mobile phases were determined. Baseline resolution of the six substances on a C18 reversed stationary phase was attained using a mobile phase with an optimized composition [acetonitrile-8.7 mM 2-(N-morpholino)ethanesulfonic acid adjusted to pH 6.0 (55:45, v/v)]. It was shown that CLC, operated in the isocratic mode under the mobile phase flow-rate of 4 microl/min, can determine the level of these impurities, down to a level of 0.06% of the main component within 25 min.

Detection of peptides by precolumn derivatization with biuret reagent and preconcentration on capillary liquid chromatography columns with electrochemical detection

The separation and detection of biuret complexes of neuropeptides by capillary liquid chromatography with electrochemical detection was explored. Capillaries of 25-micron inner diameter packed with base-resistant, polymer-based reversed-phase particles were used for separation, and C-fiber electrodes were used for detection. Detection at the C-fiber electrode was found to have some differences in relative sensitivity for peptides compared to glassy carbon electrodes used previously. On-column preconcentration of preformed complexes allowed up to 1-microL samples to be injected with minimal band broadening resulting in a 100-fold improvement in concentration detection limit with no effect on mass detection limit. Concentration detection limits ranged from 5 to 59 pM, depending upon the peptide, corresponding to 5-59 amol injected. The low concentration detection limit was possible because of minimal baseline disturbances, minimal formation of unwanted products, and high efficiency of complex formation associated with biuret derivatization. The method was applied to determination of vasopressin and bradykinin in dialysates collected with 5-min sampling frequency from the rat supraoptic nucleus.