The present invention provides for an injection device including a syringe with a proximal end, a distal end, a needle extending from the distal end, and a plunger inserted into the proximal end. The drive member engages the drive screw and has a first end with an outer periphery and an end surface. The optical head includes a window having a convex surface facing away from the sample, so that light beams crossing this convex surface do so at or near normal incidence and are therefore not significantly aberrated.