An apparatus that couples automated injection with flow feedback to provide nanoliter accuracy in controlling microliter volumes. The apparatus comprises generally a source of hydraulic fluid pressure, a fluid isolator joined to the outlet of the hydraulic pressure source and a flow sensor to provide pressure-driven analyte metering. For operation generally and particularly in microfluidic systems the hydraulic pressure source is typically an electrokinetic (EK) pump that incorporates gasless electrodes. The apparatus is capable of metering sub-microliter volumes at flowrates of 1-100 μL/min into microsystem load pressures of up to 1000-50 psi, respectively. Flowrates can be specified within 0.5 μL/min and volumes as small as 80 nL can be metered.