Assuming that both δt A and δt EOF are independent random variables, the variance of their combined effects will just be the sum of the variances. The expected total uncertainty is δµ µ A+EOF = δµ µ In Figure we plot this combined effect, as a function of t A and t M . We take t EOF = 15 min, and consider uncertainties of 1 s and 2 s. From the influence of t A , the lesson is that the secondary marker should be chosen at the center between t = 0 and t EOF . However, if uncertainty in the determination of the EOF's time is larger (this can happen if the EOF marker is a relatively wide or non-Gaussian peak), compounds with low mobilities -close to the EOF-will be highly affected by a choice of t A with high mobility, further away from the EOF. Overall, the recommendation is to choose the secondary marker as close as possible to the midpoint towards the EOF, erring on the side of slightly lower mobilities only if necessary.