Chapter 12
Microchip chemical separations

Chemical separationsare a critical component of analytical and synthetic chemistry. In all cases, a sample comprising multiple chemical species is separated spatially into individual components by inducing the components of a sample to move at differing velocities in a microchannel. This is shown schematically in Figure 12.1 and a sample experimental result is shown in Figure 12.2. Separations are achieved by inserting a sample fluid bolus into a microchannel, inducing motion of these species with velocities that differ from species to species, and detecting the concentration of species as a function of time as these species elute (i.e., arrive) at the location of the detector (Figure 12.1). Many microfluidic separations are modified from capillary or column-based techniques, and draw advantage from more optimal fluid transport, thermal dissipation, or system integration.

One example of a chemical separation is an electrophoresisseparation, which can be used to separate species that have differentelectrophoretic mobilities. In this case, species motion is induced by an electric field aligned along the axis of the microchannel, which induces electroosmosis and electrophoresis. Because this technique requires only that electric fields be applied, it integrates easily into microsystem designs, and a large fraction of the microchip analyses developed in the last 15 years use microchip electrophoresis. This is true for both protein analysis (Section 12.5) and DNA analysis and sequencing (Chapter 14).

In this chapter, we outline the basic experimental setup and techniques used to realize microchip separations, discuss some modes of separation, and identify transport issues related to these separations. In particular, separations motivate discussion of how a discrete bolus of fluid travels through a long straight channel, as well as the diffusive and dispersive effects of the flow on the bolus. Since our focus is on the fluid mechanical impact on these separations, we dwell on the separations themselves only long enough to motivate the discussion, and use the Exercises to encourage implementation of topics described in earlier chapters to these chemical separation-motivated flows.

Related work on chemical separations from our research group can be found here.

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