Process control requires sensitive and selective on-line monitors for species concentrations. This applies in particular if feeds of varying compositions are involved. Current REMPI (= Resonance Enhanced Multiphoton Ionization) mass spectrometers would be suitable instruments in terms of selectivity and on-line capability, their sensitivities, however, are generally too low By contrast, the DLR-Jet REMPI geometry provides a major sensitivity improvement. An extended test series shows that this is achieved without paying a price in terms of selevtivity. The paper provides some of these results and lists up the development steps needed to make Jet REMPI an instrument suitable for industrial use.