  The knowledge of absolute fluxes of reactive species such as radicals or
energetic ions to the surface is crucial for understanding the growth or
etching of thin films. These species have due to their high reactivity very low
densities and their detection is therefore a challenging task. Mass
spectrometry is a very sensitive technique and it will be demonstrated that it
is a good choice for the study of plasma chemistry. Mass spectrometry measures
the plasma composition directly at the surface and is not limited by existence
of accessible optical transitions. When properly designed and carefully
calibrated mass spectrometry provides absolute densities of the measured
species. It can even measure internally excited metastable species. Here,
measurement of neutral species and positive ions generated in an atmospheric
pressure plasmas jet operated with He, hexamethyldisiloxane and O2 will be
presented.
