Unlike fluorescence detection, MS-based detection methods maintain their sensitivity when moving from normal-bore chromatography columns to capillary and nano LC systems. MS-based bioassays are therefore particularly suited for miniaturization. Conventional assays are operated at reagent flow rates of 20-50 mL min-1. By using electrospray MS as readout, flow rates of 1 mL min-1 and lower could be envisaged, which is particularly useful for assays comprising expensive reagents.
We have demonstrated the feasibility of miniaturized MS assays by converting the cathepsin B assay described in Section 5.2.2 to a chip format, using the same substrate and products for the MS-based readout . The assay set-up is identical to the format described in Fig. 5.1. The advantages of chips as microreactors over fused silica capillaries are in their compactness, strength, greater degrees of freedom in design and material, and the presence of hair-pin curves to increase the diffusion rate.
Miniaturizing a conventional-flow screening system (macro-scale system) to a chip-based system comprises a number of changes, such as flow rates, reagent supply, and the material. While the conventional system with the open tubular reactors is restricted to polymer reactors, the choice of materials for the chip is
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