Cumulative column density ratios normalized to ¹²CO and the isotopic ratios (see Eq. (7)) shown as a function of height for a vertical cut through the disk at a radius of 150 AU. In the top panel we show a 10^-3M_⊙ disk around a T Tauri star with f = 10^-2 (only small grains), while in the bottom panel the settling parameter is f_large = 0.99. The dotted red lines indicate where the temperature reaches 20 K, below which CO starts to freeze out.