Microdialysis sampling amino acids

Piepponen TP, Skujins A. 2001. Rapid and sensitive step gradient assays of glutamate, glycine, taurine and y-amino-butyric acid by high-performance liquid chromatography-fluorescence detection with o-phthalaldehyde-mercap-toethanol derivatization with emphasis on microdialysis samples. J Chromatogr B 757 277-283. 

Zhou SY, Zuo H, Stobaugh JE, Lunte CE, Lunte SM. 1995. Continuous in vivo monitoring of amino acid neurotransmitters by microdialysis sampling with on-line derivatization and capillary electrophoresis separation. Anal Chem 67(3) 594-599. 

Figure 27.17 Separation and determination of amino acids by reversed phase gradient LCEC of isoindole derivatives. The sample was obtained from an awake monkey using a microdialysis sampling probe to collect amino acids from the extracellular fluid of the brain.

Most electrode materials that are employed in LCEC can also be used for CEEC. The most commonly employed working electrode is a carbon fiber. Carbon fibers come in many different sizes and can also be etched to smaller diameters. Common applications of CEEC with carbon fiber electrodes are the detection of catecholamines in single neuronal cells and amino acids in brain microdialysis samples following derivatization with NDA/CN. 

De Lange ECM, De Boer AG, Breimer DD (2000) Methodological issues in microdialysis sampling for pharmacokinetic studies. Adv Drug Deliv Rev 45 125-148 Elmquist WF, Sawchuk RJ (1997) Application of microdialysis in pharmacokinetic studies. Pharm Res 14 267-288 Evrard PA, Deridder G, Verbeeck RK (1996) Intravenous microdialysis in the mouse and the rat development and pharmacokinetic application of a new probe. Pharm Res 13 12-17 Jacobson I, Sandberg M, Hamberger A (1985) Mass transfer in brain dialysis devices a new method for the estimation of extracellular amino acids concentration. J Neurosci Methods 15 263-268... 

The Kennedy lab has been at the forefront of using rapid capillary electrophoresis to measure neurochemical changes. Optically gated injection of OPA-derivatized amino acids has been achieved in less than 2 s. However, the high salt concentrations of physiological samples, such as cerebral spinal fluid, can reduce EOF, and make separations slower. Still, 10 s temporal resolution for online monitoring of directly sampled and microdialysis samples were obtained. An instrument has been developed in the Kennedy lab for online analysis of microdialysis samples after precolumn... 

Huynh, B. H., et al., A microchip electrophoresis device with on-line microdialysis sampling and on-chip sample derivatization by naphthalene 2,3-dicarboxaldehyde/2-mercaptoethanol for amino acid and peptide analysis, J. Pharm. Biomed. Anal., 42, 529, 2006. 

One of the advantages of CE is that it can be employed for the analysis of very small samples. Generally, only a few nanoliters of sample are required for analysis. Hence, CE has been employed for the analysis of amino acids in single cells, small tissue extracts, and microdialysis samples. Figure 5 shows a separation of NDA amino acids from a rat brain di-alysate using CE with electrochemical detection. 

In a separate study, amino acids were detected in a brain microdialysis sample by CEEC without derivatization. A copper electrode was employed as the working electrode and the CE separation was accomplished using a zwitterionic buffer at a high pH. Under these conditions, Zhou and Lunte were able to detect several amino acids in microdialysis samples [30]. 

Hogan, B.L. Lunte, S.M. Stobaugh, J.F. Lunte, C.E. On-line Coupling of Microdialysis Sampling and Capillary Electrophoresis. Anal. Chem. 1994 66, 596-602. Zhou, S.Y. Zuo, H. Stobaugh, J.F. Lunte, C.E. Lunte, S.M. Continuous In Vivo Monitoring of Amino Acid Neurotransmitters by Microdialysis Sampling, On-line Derivatization and CE Separation. Anal. Chem. 1995 67, 594-599. 

However, the run time was approximately 24 minutes, which is still longer than desired for analysis of microdialysis samples. Commercial availabUity and improvements in monolithic column technology will increase the possibly of attaining faster separations with amino acid analysis, leading to the higher throughput and enhanced temporal resolution that has been demonstrated only by CE methods. 

Taken together, amino acid analysis in the brain can be achieved through numerous LC and detection schemes. With respect to amino acid sampling using microdialysis, LC separation combined with fluorescence detection... 

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