Schematic and Operational Diagram of the Microchip

The microchips used in our experiments were obtained from PerSeptive Biosystems (Framingham, Massachusetts). Using standard photolithographic techniques and chemical wet etching, the injection and separation channels are formed on a glass substrate in the shape of a cross. Another unetched glass substrate is glued on top of the etched piece. Then, holes are made for connection to the cylindrical reservoirs that contain the buffer and sample.

The microchip has two modes of operation: loading or injection mode and running or separation mode. To switch from loading to running mode, a high voltage relay circuit was constructed to control the applied voltage at each reservoir. Injection is accomplished by applying the appropriate high voltages to the reservoirs. A pinching process occurs from the flow of the other reservoirs into the analyte waste reservoir. This process is important because it allows for the analyte to be shaped into a small well-defined volume (pL) at the microchip's intersection. If this process did not occur, the analyte would flow out into the separation channel causing degradation of resolving power. Once injection is complete, the voltages are instantly switched to a running configuration. The small volume of analyte, i.e. the "plug", is directed from the intersection down towards the waste reservoir. Since various components present in the sample analyte have different shapes, sizes, and ionic charges, they will separate in the separation channel according to their electrophoretic velocities (electrophoresis is the migration of charged particles suspended in a nonconducting medium in an applied electric field).