The dissertation examines two different options to separate DNA based on differences in size by utilising electric fields. Each of the techniques is based on a new approach and faces several fundamental problems concerning electrokinetics. A microfluidic environment is chosen to experimentally investigate DNA electrophoresis at a small scale. A sophisticated setup is employed that on the one hand enables a multiphase flow, while at the same time it stabilises two immiscible polymer phases in a microfluidic compartment. An aqueous two-phase system consisting of poly(ethylene glycol) and dextran provides a stable liquid-liquid interface under quiescent conditions. Such a setup allows the application of an electric field perpendicular to the liquid-liquid interface. In doing so, DNA accumulates at the interface. The parameters influencing the electrophoretic adsorption process are examined in detail. A highlight of the experimental investigations is desorption of DNA from the interface that is triggered by increasing the electric field strength. The latter phenomenon affords a separation of different sized DNA fragments across the liquid-liquid interface. Smaller DNA fragments desorb at lower field amplitudes while larger ones desorb at larger field strengths. Although liquid-liquid interfacial phenomena in aqueous two-phase systems are complex, a preliminary understanding is achieved addressing basic theoretical issues. In the following the reader is introduced into a second and alternative setup to yield a size separation of DNA. The approach is based on traditional capillary electrophoresis. The novelty is examined by combining several preconcentration techniques with a gel-based size separation of DNA in a preparative manner. The DNA migrates due to the application of an electric field. The preconcentration is accomplished by electrokinetic trapping at a charged membrane embedded into a poly(methyl methacrylate) microchip. It has been found that a fluidic counter flow supports DNA trapping at a membrane. A subsequent DNA size separation is exploited to separate free fetal DNA from maternal DNA in blood of pregnant women providing preliminary results to afford a basis for non-invasive prenatal diagnosis.