A plunger for a syringe used to inject gas and liquid chromatography samples has an end portion surrounded by a flexible sleeve. The sleeve forms a seal in the bore of a barrel traversed by the plunger, the bore being in fluid communication with a cannula attached to the end of the barrel. The plunger has an end surface that is exposed and is flush with an annular end surface of the sleeve. The plunger end surface, and not the sleeve, contacts the cannula upon injection and avoids imposing cyclical hoop stresses on the barrel which tend to crack the barrel after a finite number of cycles. In a further embodiment, a thin layer of sleeve material covers the end surface of the plunger. The thin layer avoids inducing significant barrel stress.