A sample injection diaphragm valve is provided, as well as a plunger assembly and a method of operating the valve. The valve includes a valve head and a valve body. The valve includes a diaphragm positioned between the valve head and the valve body. The diaphragm has a process groove seated in the recess of the valve body for circulating fluid therein. Plungers are slidably fitted in plunger passages of the valve body. Each plunger is movable between a closed position wherein the plunger deforms the diaphragm and blocks fluid circulation between two process ports, and an open position wherein the plunger is retracted away from the diaphragm, thereby allowing fluid to circulate in the process groove. The plungers have linear lips extending across the top face of the plungers for pressing against the diaphragm when in the closed position.