Binding characteristics of various neurochemicals to glassy carbon

The determination of acetylcholine and choline using liquid chromatography with electrochemical detection using the approach of Potter et al. [J. Neurochem., 41 (1983) 188] normally requires isolation of the desired species before an analysis of tissue samples can be undertaken due to coeluting interferences afforded by other neutrochemicals. We have recently shown that this problem can be overcome by the use of a glassy carbon precolumn to effectively trap the interfering species [Ikarashi et al., J. Chromatogr., 575 (1992) 29]. We now report on the nature and mechanism of this adsorption onto glassy carbon for norepinephrine, dopamine, serotonin, 3,4-dihydroxyphenylacetic acid, homovanillic acid and 5-hydroxyindoleacetic acid. For both the acidic and basic compounds which comprise this group, distinct Langmuir adsorption processes appear to be involved for both the neutral and ionic forms of the individual compounds. Using various data fitting approaches, we have attempted to derive appropriate adsorption constants for the two forms of each compound. Theoretical predictions employing these derived constants provided results which match reasonably well with the observed adsorption data in most cases.