A capillary-electrode holder was constructed for electrochemical detection in capillary electrophoresis (CE). The device allows for positioning of the working electrode at the end of the capillary column without the aid of micropositioners or microscopes. The design facilitates the exchange of electrodes and capillaries without the need of refabricating the entire capillary-electrode setup. The system can be assembled in a very short period of time. Alignment with the self-guided system proved to be reproducible for the electrodes used (carbon, nickel, copper). The advantages of reduced downtime and low cost, make the device very attractive for the routine analysis of electroactive species by CE with electrochemical detection.

Minute quantities of native cellular fluorophores can be quantitatively assayed using ultraviolet fluorescence detection with microcolumn separations, but spectral diversity of biological chromophores imposes serious limitations on the use of this strategy to investigate biological components. We present an approach for rapid characterization of picoliter samples containing dissimilar cellular fluorophoresincluding amino acids, monoamine neurotransmitters, flavins, and pyridine nucleotidesusing multiphoton excited fluorescence detection coupled to capillary electrophoresis separations. In this highly versatile approach, biological fluorophores are excited through the nearly simultaneous absorption of different numbers of low-energy photons. Because spectrally distinct species all can be excited with a single, long-wavelength laser source, fluorescence throughout the ultraviolet and visible regions can be detected efficiently with extremely low background. Samples containing serotonin, melatonin, FAD, and NADH can be reproducibly analyzed in 5-μm and 2-μm i.d. channels. Detection limits in 5-μm capillaries range from 350 zmols (38 nM) for FAD to 27 amols (1.0 μM) for serotonin. Use of 2-μm channels is shown to improve the mass detection limit for serotonin approximately as the decrease in capillary cross-sectional area (LOD ≈ 4 amol), and further reductions in mass detection limits are projected for analysis with even smaller diameter channels that better match the submicron size of the diffraction-limited multiphoton focal spot.

We have examined the effects of dissolved molecular oxygen on multiphoton-excited (MPE) photochemical derivatization of serotonin (5HT) and related cellular metabolites in various buffer systems and find that oxygen has a profound effect on the formation efficiency of visible-emitting photoproducts. Previously, end-column MPE photoderivatization provided low mass detection limits for capillary electrophoretic analysis of hydroxyindoles, but relied on the use of Good's buffers to generate high-sensitivity visible signal. In the present studies, visible emission from 5HT photoderivatized in different buffers varied by 20-fold under ambient oxygen levels but less than 2-fold in the absence of oxygen; oxygen did not significantly alter the photoproduct excited-state lifetime (approximately 0.8 ns). These results support a model in which oxygen interferes with formation of visible-emitting photoproducts by quenching a reaction intermediate, an effect that can be suppressed by buffer molecules. Deoxygenation of capillary electrophoresis separation buffers improves mass detection limits for 5-hydroxyindoles fractionated in 600-nm channels by approximately 2-fold to < or =30000 molecules and provides new flexibility in identifying separation conditions for resolving 5HT from molecules with similar electrophoretic mobilities, such as the catecholamine neurotransmitters.

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Two-photon excited (2PE) fluorescence detection is demonstrated to be a highly sensitive means for analyzing fluorogen-labeled neurotransmitters fractionated in submicrometer capillary electrophoresis channels. In this approach, fluorescamine-labeled neurotransmitters that have been electrophoretically separated in 620-nm-i.d. capillaries intersect the focused output from a near-infrared mode-locked titanium−sapphire laser positioned at the capillary outlet. Extremely high peak laser intensities (∼1011−1012 W cm-2) facilitate the nearly simultaneous absorption of two near-IR photons (λex ≈ 780 nm) to excite fluorescamine derivatives ordinarily excited with a single, near-ultraviolet photon (λex ≈ 390 nm). Rapid cycling of analytes through the fluorescent excited state and low background from scatter and out-of-sample luminescence combine to make 2PE fluorescence a highly sensitive approach for detecting minute quantities of neurotransmitters. In these studies, mixtures of the fluorescamine derivatives of dopamine, glycine, and glutamate are fractionated reproducibly in several minutes, with instrumental mass detection limits as low as 13 000 molecules (∼20 zmol). These detection levels are ∼100-fold lower than have been achieved previously for fluorescamine-based assays. Analytes can be derivatized at concentrations equal to the limit of quantitation with no loss in sensitivity; hence, characterization of neuronal samples at the zeptomole level appears feasible, provided that efficient on-column labeling procedures can be implemented.