A rectangular electophoresis chamber has been constructed whereby electroosmotic fluid mobility can be determined at small non-transparent sample plates. A hydrodynamic description of fluid flow in the chamber is forwarded, and verified, for accurate determination of particle mobility and electroosmosis at sample plates. Advantages of the chamber design and technique include the ability to determine electroosmotic fluid mobility at sample plates from a single particle mobility measurement within the chamber. As demonstration of the usefulness of the technique, characterization of several polymer surfaces deposited as a thin film from a cold gas plasma was performed by determining the pH dependence of electroosmotic fluid flow. The plasma polymer surfaces include those of plasma polymerized 1,2-diaminocyclohexane, acrylic acid, and hexamethyldisiloxane. Results of the characterization include quantification of ionizable surface groups with concomitant determination of pK values of these groups. The values obtained have proven useful to explain trends in protein adsorption at these surfaces.