A novel injector for microchip electrophoresis (MCE) has been designed and evaluated that achieves very high repeatability of injection volume suitable for quantitative analysis. It eliminates the injection biases in electrokinetic injection and the dependence on pressure and sample properties in hydrodynamic injection. The microfluidic injector, made of poly(dimethylsiloxane) (PDMS), operates similarly to an HPLC injection valve. It contains a channel segment (chamber) with a well-defined volume that serves as an ?injection loop?. Using on-chip microvalves, the chamber can be connected to the sample source during the ?loading? step, and to the CE separation channel during the ?injection? step. Once the valves are opened in the second state, electrophoretic potential is applied to separate the sample. For evaluation and demonstration purposes, the microinjector was connected to a 75 ?m ID capillary and UV absorbance detector. For single compounds, a relative standard deviation (RSD) of peak area as low as 1.04% (n = 11) was obtained, and for compound mixtures, RSD as low as 0.40% (n = 4) was observed. Using the same microchip, the performance of this new injection technique was compared to hydrodynamic injection and found to have improved repeatability and less dependence on sample viscosity. Furthermore, a non-radioactive version of the positron-emission tomography (PET) imaging probe, FLT, was successfully separated from its known 3 structurally-similar byproducts with baseline resolution, demonstrating the potential for rapid, quantitative analysis of impurities to ensure the safety of batches of short-lived radiotracers. Both the separation efficiency and injection repeatability were found to be substantially higher when using the novel volumetric injection approach compared to electrokinetic injection (performed in the same chip). This novel microinjector provides a straightforward way to improve the performance of hydrodynamic injection and enables extremely repeatable sample volume injection in MCE. It could be used in any MCE application where volume repeatability is needed, including the quantitation of impurities in pharmaceutical or radiopharmaceutical samples.