I do not have access to the full-text, but the abstract of Giggenbach (1996) suggests that
The variability in the composition of volcanic gases accessible to sampling, therefore, is largely due to shallow processes, such as reequilibration in response to cooling and dilution by meteoric water, and interaction with fluids of associated hydrothermal systems.
The section Gas Compositions and Tectonic Setting in , from Oregon State University has quite a nice table showing that the different volcanoes do seem to have different gas compositions, depending on where they are found and consequently what type of tectonic setting they occur in. I am not sure if these variations are actually significant though, not being a vulcanologist.
This is certainly not a complete answer, but it does look like it might not be that easy to use the volatiles as a diagnostic feature. Hope that this helps somewhat....