Detectability of deuterated species vs. temperature. The top and middle panels show the signal to noise needed for a 10σ detection of CH₃D and HDO, respectively, assuming a spectrum from 3 to 5 μm with R ∼ 2700. The hatched regions show the S/N predicted per spectral element in the region of the spectrum with CH₃D or HDO features in a 2.5 hr observation with JWST. CH₃D is detectable at high significance for objects cooler than 800 K; HDO is never detectable at high significance in 2.5 hr. The bottom panel shows a simulated brown dwarf population from Saumon & Marley (2008); it uses a power-law initial mass function (IMF) index α = 1, masses between 0.006 and 0.1 M_Sun, and uniform age distribution between 0 and 10 Gyr.