Abstract: A new approach for the determination of organic acids in soft drinks based on pH modulation and Fourier transform infrared spectroscopic detection is presented. The analytical readout was taken from the spectral changes induced by the pH change of the sample. The pH modulation was carried out in a novel sheath-flow cell which was connected to a sequential injection analysis system. The sheath-flow cell comprised three stream lines flowing adjacent to each other in a strongly laminar fashion, The sample (pH similar to 9) was introduced in the central channel whereas reagent (hydrochloric acid) was introduced in the outer channels. As a consequence of the laminar flow profile, hardly any mixing between the stream lines was observed while maintaining the flow, hence allowing the measurement of the sample spectrum at alkaline pH (similar to 9). Upon stopping the flow, diffusion of protons from the outer stream lines into the central line occurred resulting in a complete protonation of the analyte (pH similar to 2). The spectral changes were calculated and the region between 1400 and 1180 cm-1 used to set-up a partial least squares (PLS) calibration model. For the PLS model only standards containing the analytes but no matrix molecules were used. By evaluation of the spectral changes induced by the pH modulation the PLS model could successfully be applied to test samples containing sugars as well as to natural soft drinks.

Abstract: A microfabricated flow chamber (MFC) suitable for performing liquid- based fluorimetric assays is introduced. Precision delivery of microliter volumes of sample and reagent to the MFC is accomplished by a double-syringe-pump flow injection analysis (FIA) apparatus. The FIA- MFC system also combines the 'sheath flow' technique (traditionally used in flow cytometry) and stopped-flow FIA as a way to allow sample and reagent streams to be mixed reproducibly. The applicability of this FIA-MFC system to bioanalytical assays is demonstrated by performing an enzymatic assay with an artificial fluorigenic substrate to determine the activity of Savinase, a proteolytic enzyme. When coupled to a fluorescence microscope platform, quantitative analysis of the reaction product is possible. Experiments showed that the FIA-MFC system was capable of performing the assay with good reproducibility of injection (1.5%), and linearity of response (r2 = 0.9997) in activity ranges of analytical interest. Owing to the incorporation of flow cytometry sheath flow principles into an FIA format, the FIA-MFC system is a suitable tool for cytometric studies.

Abstract: A system for post-column fluorescence derivatization in capillary electrophoresis is described. The post-column reactor uses a sheath flow detection cell where the reagents, o-phthaldialdehyde and β- mercaptoethanol, are added to the sheath buffer and mix by diffusion with the analytes effusing from the separation capillary. Reaction progress is monitored and optimized by imaging a large portion of the sheath flow cuvette using an extended UV source and a CCD camera. Significantly, this design provides the ability to switch between the analysis of pre- and post-column derivatized amino acids and peptides easily and without sacrificing system performance. The lack of turbulent flow in this system minimizes post-separation band broadening. The limit of detection for glycine is 9.4 x 10^-8 M (110 amol) with a separation efficiency of 190,000 theoretical plates, without stacking. The performance of the system for a series of amino acids was evaluated using post-column and pre-capillary derivatization.