  Protoplanetary disk mass is a key parameter controlling the process of
planetary system formation. CO molecular emission is often used as a tracer of
gas mass in the disk. In this study we consider the ability of CO to trace the
gas mass over a wide range of disk structural parameters and search for
chemical species that could possibly be used as alternative mass tracers to CO.
Specifically, we apply detailed astrochemical modeling to a large set of models
of protoplanetary disks around low-mass stars, to select molecules with
abundances correlated with the disk mass and being relatively insensitive to
other disk properties. We do not consider sophisticated dust evolution models,
restricting ourselves with the standard astrochemical assumption of $0.1~\mu $m
dust. We find that CO is indeed the best molecular tracer for total gas mass,
despite the fact that it is not the main carbon carrier, provided reasonable
assumptions about CO abundance in the disk are used. Typically, chemical
reprocessing lowers the abundance of CO by a factor of 3, compared to the case
of photo-dissociation and freeze-out as the only ways of CO depletion. On
average only 13% C-atoms reside in gas-phase CO, albeit with variations from 2
to 30%. CO$_2$, H$_2$O and H$_2$CO can potentially serve as alternative mass
tracers, the latter two being only applicable if disk structural parameters are
known.
