This thesis reports on the development of technologies and methods for a microfiuidic device for continuous pressure-driven flow-injection analysis. Chip based, pressure-driven liquid chromatographic (LC) systems are often avoided due to the difficulties encountered when it comes to flow control. The complexity involved precludes, for the time being commercializable, on-chip integration of valves and micropumps for microfluidic devices. However, LC is still the workhorse analytical method in most laboratories for small molecule analysis. There is a growing demand from the industry to have LC sensors being capable of direct sampling on the production lines, hence obviating the time-consuming detour to the lab. As a possible solution to this demand, the concept of a chromatographic device is presented. It incorporates continuously flowing analyte and mobile phase streams and is capable of aliquoting samples from meso scale flows. Plugs are injected by momentarily change the flow distribution within the microfluidic channels by exploiting the method of gated flow injection in pressure-driven microchannels. The theoretical concept and experimental evidence is shown for two possible implementations; thermal plug injection through local homogenous bubble nucelation and pressure-drop plug injection. For the microfabricated device, a sol-gel technique is adapted to coat the open-tubular column walls with a C8 stationary phase. Multi-compound phenolic solutions and vitamins are successfully separated to characterize the performance and limits of the chromatographic columns. The last chapter of this work introduces the concept of size-exclusion enhanced, open-tubular chromatography (SOHS) on chip, where a porous column wall is excluding larger molecules and thus increases the separation power of the hydrodynamic separation generated through the parabolic flow profile of the pressure driven flow. The encouraging results are very promising for future applications such as polymer separations or DNA separation.