The selective adsorption of heavy isotopes in narrow nanotubes, known as quantum sieving, is studied using a simple approximate theory for several different potential models. We address the reasons for wide disagreement among previously published results for quantum sieving. We analyze the sensitivity of quantum sieving to perturbations in the potential parameters used in the calculations. The selectivities are very sensitive to changes in the atomic diameter parameter and less sensitive to changes in the potential well depth. We present an approximate method for accounting for rotational-translational coupling that is computationally efficient and accurate for the narrowest nanotubes. For wide nanotubes, the estimation of rotational-transational coupling becomes inaccurate because of neglect of the effect of rotational states on the translational degrees of freedom.