Microfluidic sample injection, which is based on a mechanical valve rather than electrokinetic injection into an integrated separation channel or a discrete separation column, can provide improved sample injections, enhanced capabilities, and can eliminate the need for changing the electric field in the separation channel to induce sample injection. An interface allowing the use of a discrete separation column easily allows for flexibility to utilize the microfluidic injector with existing analytical techniques. Multiple sample channels and/or sample sources can be utilized with the microfluidic sample injector.