Not understanding the temp diff you speak of. For example N2 & O2 have mass ratio of 7/8 so both at same temp have the N2 moving faster (on average) by root(8/7). That would be true in a 80% N2, 20% O2 mix like air of the reverse (80% O2, 20% N2 mix) I.e. the percent concentration has NOTHING TO DO WITH THE QUESTION. It is true that as you go up (miles), in the air the relative concentration of heavier O2 would decrease even if there were no chemistry going on. You can best understand this if you consider very, very low density mix. I.e. one with mean free a foot or so. After a collision both free fall under gravity until the next collision but the fall time is less for the faster molecule, so on the average it loses less altitude free falling before it randomly scatters again. Re-state your question, it you still have one, after understanding what I have just explained.