A method of fluid testing includes pressurizing a fluid testing system, disposed at a subterranean location under high pressure compared to a surface pressure, to achieve a desired pressure differential between the high pressure and an internal pressure of the fluid testing system. The fluid testing system includes a capillary electrophoresis system and one or more test fluid reservoirs. The method also includes directing test fluid from the one or more test fluid reservoirs into capillaries to condition the capillaries. The method further includes directing sample fluid into the capillaries for testing while at the subterranean location.