TPD simulation showing that ammonia can desorb in more than one temperature range depending on its chemical nature and chemical environment. If ammonia is locked in ammonium salts, it desorbs at higher temperature with respect to the water snow line. The heating rate of this simulation is β = 1 K/century, a typical estimation for high-mass stars (Viti & Williams 1999; Viti et al. 2004). For low-mass stars, the heating rate would be slower and the effect of delay even more pronounced.