Electrophoretic separation of DNA using capillaries typically
requires coating the capillary to minimize electro-osmotic flow
(EOF) as well as the addition of a sieving matrix to the buffer to
achieve moderate resolution between different size DNA fragments.
By contrast, using microfluidic chips, we demonstrate the
possibility of separating small DNA fragments in free solution and
with uncoated capillaries. The microfluidic chip has the standard
T configuration where the sample and separation channels are
approximately 20 by 50 microns in cross-section. To diminish EOF
in these channels, CE buffers of considerably higher concentration
than normal need to be used. Separation of fluorophore-labelled
oligomers containing 21, 15 and 9 bases is achieved.
Interestingly, certain pure single-stranded oligomers exhibit
peak-splitting in this experiment, indicating that they may
actually be present in two (or more) slightly different structural
configurations. To model this behavior, Aris's method-of-moments,
which was developed in the context of Taylor dispersion theory, is
applied to a model system which considers the transport of two
reversibly reacting species through a long channel with different
electrophoretic mobilities and diffusivities. By taking various
moments of the original convection-reaction-diffusion equation, a
set of coupled ordinary differential equations is derived
describing the moments. Analytical results are obtained for the
long-time behavior of such a system, which does indeed exhibit
peak-splitting. However, some of the features observed in the
experiment are not explained by this simple model; therefore,
further research into the mechanisms of peak-splitting is
underway. [Joint work with E. Fabrizio, Y. Daneshbod, and J.
Sterling]
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