Peak line-to-continuum ratio of the a)–c) R(6) 10⁰0–00⁰0 line at 3 μm, d)–f) Q(6) 01¹0–00⁰0 line at 14 μm, and g)–i) between 13.7 and 14.1 μm. The non-LTE models with a constant abundance and a gas-to-dust (G/D) ratio of 100, 1000 and 100 000 are shown (Fig. 6), convolved to a spectral resolving power R = λ/ Δλ between 100 and 10⁵. Horizontal red dashed lines show the required signal-to-noise (S/N) in the continuum to detect a line at a 5σ level.