Mass spectrometric sampling of a flame

Some of the more important perturbations of the gas composition in samples taken for mass spectrometric analysis are considered. In particular, the flame is cooled by passing up to the inlet hole at the tip of a relatively cool sampling nozzle; cooling occurs in thermal boundary layers before the sample reaches the local speed of sound at the throat of the sampling orifice. Certainly, the sample also cools just because of attaining its sonic velocity. Finally, there is a sudden fall in temperature and density in the supersonic expansion into the first vacuum chamber. It is shown here that perturbations in the external boundary layer are much reduced by using a larger inlet hole. However, a bigger sampling orifice causes the supersonic expansion to last for a longer time, thereby increasing the likelihood of a sampling perturbation inside the expansion duct. These effects are demonstrated experimentally in studies of ions in various flames, where the cooling in the boundary layer was measured to be up to 400 K. This means that the total cooling can be up to at least 700 K. In addition, the effects of burning a flame at a reduced pressure and also of changing the ratio of the principal specific heats of the flame gas are considered. If there is a falsification of a mass spectrometric measurement, corrections can be made by extrapolating the measurements to that for either an infinitely large sampling orifice or one of zero diameter, depending on whether the sample is perturbed in the boundary layer or in the supersonic expansion. It also has proved possible to deduce the rate constant of reactions occurring during supersonic expansion, e.g., for H$_3$O$^+$ + H$_2$O + M $\to$ H$_3$O$^+$ $\cdot$ H$_2$O + M.