Disclosed herein is an apparatus for supplying reagent ions, for example ETD or PTR reagent ions, to a mass spectrometer. The apparatus includes a reagent material reservoir, coupled to a carrier gas supply, which delivers an entrained reagent vapor flow to an inlet of a mixing junction through a first flow restrictor. A control gas flow of carrier gas is delivered to another inlet of the mixing junction via a variable pressure regulator and a second flow restrictor. The outlet of the mixing junction is coupled via a third flow restrictor and a reagent transfer junction to an inlet of an ionizer, such as a glow-discharge ionizer. By dynamic adjustment of the output pressure of the variable pressure regulator, the flow rate of reagent vapor may be controlled over a broad range, even for reagent materials of relatively high volatility.