A molecular-beam-sampling apparatus utilizing both a time-of-flight mass spectrometer with vacuum ultraviolet (VUV) photoionization and a quadrupole mass spectrometer with electron impact ionization is described. These systems are used to analyze reactants and products, as well as reactive intermediates, from flow tube experiments at reduced pressures and elevated temperatures. It is shown that the quadrupole signal sensitivity can be predicted using literature cross sections which are based upon the atom hybridization within the various molecules. Measurements of the VUV photoionization sensitivity for stable species led to development of a group additivity approach to predict cross sections. This approach was extended to reactive intermediates, and detailed comparisons of quadrupole and TOF data shows a very good agreement between the two techniques. The two techniques are compensation of each other and provide the full information we need. It is also shown how the minimal fragmentation achieved with the VUV source leads to much more accurate species concentration measurements for reactive intermediates.