  Microchip electrokinetic methods are capable of increasing the sensitivity of
molecular assays by enriching and purifying target analytes. However, their use
is currently limited to assays that can be performed under a high external
electric field, as spatial separation and focusing is lost when the electric
field is removed. We present a novel method that uses two-phase encapsulation
to overcome this limitation. The method uses passive filling and pinning of an
oil phase in hydrophobic channels to encapsulate electrokinetically separated
and focused analytes with a brief pressure pulse. The resulting encapsulated
sample droplet maintains its concentration over long periods of time without
requiring an electric field and can be manipulated for further analysis, either
on- or off- chip. We demonstrate the method by encapsulating DNA
oligonucleotides in a 240 pL aqueous segment after isotachophoresis (ITP)
focusing, and show that the concentration remains at 60% of the initial value
for tens of minutes, a 22-fold increase over free diffusion after 20 minutes.
Furthermore, we demonstrate manipulation of a single droplet by selectively
encapsulating amplicon after ITP purification from a polymerase chain reaction
(PCR) mix, and performing parallel off-chip detection reactions using the
droplet. We provide geometrical design guidelines for devices implementing the
encapsulation method, and show how the method can be scaled to multiple analyte
zones.
