  Recent observations suggest that the behaviour of tracer species such as
N_2H+ and CS is significantly different in regions of high and low mass star
formation. In the latter, N_2H+ is a good tracer of mass, while CS is not.
Observations show the reverse to be true in high-mass star formation regions.
We use a computational chemical model to show that the abundances of these and
other species may be significantly altered by a period of accelerated collapse
in high mass star forming regions. We suggest these results provide a potential
explanation of the observations, and make predictions for the behaviour of
other species.
