  The aim of this study is to investigate systematic chemical differentiation
of molecules in regions of high mass star formation. We observed five prominent
sites of high mass star formation in HCN, HNC, HCO+, their isotopes, C18O, C34S
and some other molecular lines, for some sources both at 3 and 1.3 mm and in
continuum at 1.3 mm. Taking into account earlier obtained data for N2H+ we
derive molecular abundances and physical parameters of the sources (mass,
density, ionization fraction, etc.). The kinetic temperature is estimated from
CH3C2H observations. Then we analyze correlations between molecular abundances
and physical parameters and discuss chemical models applicable to these
species. The typical physical parameters for the sources in our sample are the
following: kinetic temperature in the range ~ 30-50 K (it is systematically
higher than that obtained from ammonia observations and is rather close to dust
temperature), masses from tens to hundreds solar masses, gas densities ~ 10^5
cm^{-3}, ionization fraction ~ 10^{-7}. In most cases the ionization fraction
slightly (a few times) increases towards the embedded YSOs. The observed clumps
are close to gravitational equilibrium. There are systematic differences in
distributions of various molecules. The abundances of CO, CS and HCN are more
or less constant. There is no sign of CO and/or CS depletion as in cold cores.
At the same time the abundances of HCO+, HNC and especially N2H+ strongly vary
in these objects. They anti-correlate with the ionization fraction and as a
result decrease towards the embedded YSOs. For N2H+ this can be explained by
dissociative recombination to be the dominant destroying process. N2H+, HCO+,
and HNC are valuable indicators of massive protostars.
