  A study of the chemical structure of the envelopes around a sample protostars
is introduced. Physical models for the envelopes derived using 1D radiative
transfer modeling of their dust continuum emission are used as input for Monte
Carlo modeling of single-dish line observations in order to establish the
chemical inventories for the different sources. CO and HCO+ abundances are
found to be correlated with envelope mass: the sources with the most massive
envelopes show the lowest abundances, supporting the idea that the depletion of
these molecules is most efficient in the colder, denser environments. Other
molecules like CS and HCN do not show a similar trend. Deuterated species like
DCO+ and DCN also show signs of correlations with envelope mass and
temperature: the deuterium fractionation of HCO+ is most prominent for the
coldest and most massive envelopes in the sample and at the same time
anti-correlated with the fractionation of HCN. This puts constraints on the low
temperature gas-phase deuterium chemistry.
