The Physical Properties of High-Mass Star-Forming Clumps: A Systematic
  Comparison of Molecular Tracers
  We present observations of HCO+ and H^13CO+, N2H+, HCS+, HCN and HN^13C, SO
and ^34SO, CCH, SO_2, and CH_3OH-E towards a sample of 27 high-mass clumps
coincident with water maser emission. All transitions are observed with or
convolved to nearly identical resolution (30"), allowing for inter-comparison
of the clump properties derived from the mapped transitions. We find N2H+
emission is spatially differentiated compared to the dust and the other
molecules towards a few very luminous cores (10 of 27) and the N2H+ integrated
intensity does not correlate well with dust continuum flux. We calculate the
effective excitation density, n_eff, the density required to excite a 1 K line
in T_kin=20 K gas for each molecular tracer. The intensity of molecular tracers
with larger effective excitation densities (n_eff > 10^5 cm^-3) appear to
correlate more strongly with the submillimeter dust continuum intensity. The
median sizes of the clumps are anti-correlated with the n_eff of the tracers
(which span more than three orders of magnitude). Virial mass is not correlated
with n_eff, especially where the lines are optically thick as the linewidths
may be broadened significantly by non-virial motions. The median mass surface
density and median volume density of the clumps is correlated with n_eff
indicating the importance of understanding the excitation conditions of the
molecular tracer when deriving the average properties of an ensemble of cores.
