  We study the fractionation of two components between a well-mixed gas and a
saturated convecting porous layer. Motivated by geological carbon dioxide
(CO$_2$) storage we assume that convection is driven only by the dissolved
concentration of the first component, while the second acts as a tracer with
increased diffusivity. Direct numerical simulations for convection at high
Rayleigh numbers reveal that the partitioning of the components, in general,
does not follow a Rayleigh fractionation trend, as commonly assumed. Initially,
increases in tracer diffusivity also increase its flux, because the diffusive
boundary layer penetrates deeper into the flow. However, for $D_2\geq 10\,
D_1$, where $D_1$ and $D_2$ are, respectively, the diffusion coefficients of
CO$_2$ and the tracer in water, the transverse leakage of tracer between up-
and down-welling plumes reduces the tracer flux. Rayleigh fractionation between
components is only realized in the limit of two gases with very large
differences in solubility and initial concentration in the gas.
