Understanding deuterium fractionation is currently one of the greatest challenges in astrochemistry. In this contribution deuteration experiments of the series CH$_n^+$, n=2–5, in a low temperature 22-pole ion trap are used to systematically test a simple chemical rule predicting which molecular ion undergoes deuterium exchange in collisions with HD. CH$_4^+$ turns out to be a problem case, where prediction fails. The method of laser induced reaction (LIR) is used to determine the population ratio of the lowest ortho-to-para states of H$_2$D$^+$ relaxed in collisions with H$_2$. Preliminary results indicate that the ortho-to-para ratio of H$_2$D$^+$ is substantially reduced in para-H$_2$. This points at the important role of nuclear spin in deuterium fractionation, in particular at the destruction of ortho-H$_2$D$^+$ in collisions with ortho-H$_2$. More systematic LIR experiments are needed for a chemical model of deuterium fractionation including state-to-state modifications of the species involved.