Supersonic injection of a neutral, ground state gas through a microcapillary nozzle into a low density, weakly ionized, flowing plasma is employed to obtain information on the plasma characteristics. The ''zone of silence'' of the injected free-jet constitutes a pristine pocket of injected gas, adjustable in size by varying nozzle pressure and diameter of the injector. The shock waves bounding this region exclude massive particles yet transmit electrons, which excite the injected gas to yield radiative emission on lines specific to the gas. Other possible uses of microcapillary injection are discussed.