  For the one-loop contribution to the QCD vacuum tunneling amplitude by quarks
of generic mass value, we make use of a calculational scheme exploiting a large
mass expansion together with a small mass expansion. The large mass expansion
for the effective action is given by a series involving higher-order
Seeley-DeWitt coefficients, and we carry this expansion up to order
$1/(m\rho)^8$, where $m$ denotes mass of the quark and $\rho$ the instanton
size parameter. For the small mass expansion, we use the known exact expression
for the particle propagation functions in an instanton background and evaluate
explicitly the effective action to order $(m\rho)^2$. A smooth interpolation of
the results from both expansions suggests that the quark contribution to the
instanton tunneling amplitude have a relatively simple $m\rho$-dependent
behavior.
