Laboratory NH₄SH:H₂O data scaled with the same factor as that used to fit the 6.85 µm band in Figure 9 plotted with the global continuum-subtracted spectra of four protostars, zoomed in on the 4.8–5.6 µm region where a weak and broad absorption is observed. The profile of the observed absorption matches well the NH₄⁺ ν₄ + SH⁻ libration combination mode in the laboratory spectra. Plots of the global continua applied to the observed spectra can be found in Appendix E.1.