A new technique for probing the chemical composition of low pressure environments, including plasmas, is described. The technique is based on the expansion of a high pressure pulse of noble gas into the plasma region. As the expansion takes place, stable and radical gas-phase species are incorporated into the developing shock wave and carried to the mass spectrometer in a matrix of frozen noble gas atoms. This method of plasma sampling has been applied to electron cyclotron resonance microwave plasmas as the chemical nature of the carbon species is varied, within the range of conditions known to deposit diamond films. In each of these experiments, a rapid pseudoequilibrium is observed to exist in the plasma environment, covering a range of carbon species from C2H2 to C2H5 and favoring the formation of acetylene. These results are consistent with the broad spectrum of previously observed phenomena in diamond deposition and provide insight into the empirically observed species independence of diamond growth.