  The ratio of mass and magnetic flux determines the relative importance of
magnetic and gravitational forces in the evolution of molecular clouds and
their cores. Its measurement is thus central in discriminating between
different theories of core formation and evolution. Here we discuss the effect
of chemical depletion on measurements of the mass-to-flux ratio using the same
molecule (OH) both for Zeeman measurements of the magnetic field and the
determination of the mass of the region. The uncertainties entering through the
OH abundance in determining separately the magnetic field and the mass of a
region have been recognized in the literature. It has been proposed however
that, when comparing two regions of the same cloud, the abundance will in both
cases be the same. We show that this assumption is invalid. We demonstrate that
when comparing regions with different densities, the effect of OH depletion in
measuring changes of the mass-to-flux ratio between different parts of the same
cloud can even reverse the direction of the underlying trends (for example, the
mass-to-flux ratio may appear to decrease as we move to higher density
regions). The systematic errors enter primarily through the inadequate
estimation of the mass of the region.
