Sample stacking in laboratory-on-a-chip devices.

Sample stacking is a very important sample concentration technique. It has been used widely in capillary electrophoresis (CE). There are many different stacking techniques. One of the most popular techniques is called "field-amplified sample stacking" where an electric field discontinuity is set up across a concentration boundary. Charged analytes will then automatically stacked due to velocity changes after they cross the concentration boundary. There are several different strategies to perform sample stacking in microfluidic laboratory-on-a-chip devices. One could simply inject a plug of low concentration buffer containing sample into a channel surrounded by high concentration buffer. The electric field is then applied to stack the sample and move the whole plug into the separation channel. One could also stack the sample in a side channel adjacent to the separation channel. The disadvantage of this sample stacking technique is the difficulty in control of the precise location of stacked sample. We present a new sample stacking technique applied specifically to microfluidic laboratory-on-a-chip devices. Up to hundreds of fold increases in sample concentration can be achieved. We have also combined this stacking with electrophoretic separation in the same device.