Composition distortion in MBMS sampling

Notwithstanding the advantages of molecular-beam mass-spectrometer (MBMS) sampling from combustion systems, including from flames, experience has shown that care is required either to minimize or to correct for composition and temperature distortions arising from the presence and operation of a MBMS sampling system. Hence the origins and handling of such distortions are reviewed here. In the main body of this review, radical recombination and heat transfer at the external probe surface, acceleration into the probe orifice, chemical relaxation in the free jet, radial diffusion in the free jet, skimmer interference, Mach-number focusing, and fragmentation during detection are considered. A section on miscellaneous possible origins of distortions reviews briefly condensation, preferential scattering of beam molecules by background molecules, background invasion of the free jet, overun of slower molecules by faster molecules when sampling from an unsteady source, and contaminating the time-averaged signal for a given sampling location by the stored time-averaged signal for the prior sampling location. It is concluded that the increased understanding of MBMS sampling has provided the means for either avoiding or handling the possible distortions sufficiently so that MBMS sampling has graduated from an object of research and development to a tool for studies of complicated combustion systems.