ISO spectrum of W33A with subtraction of a local linear continuum between 4.48–4.35 µm (dotted line). The laboratory spectra of CH₃CN (gray), CH₃CN:H₂O (red) and CH₃CN:CO₂ (blue) at 15 K are also shown. The CN stretching of methyl cyanide ice in the laboratory spectrum, used for deriving upper limits for this species toward W33A, is indicated with an arrow. The inset in the bottom right shows the complete spectrum of ices toward W33A. The region analyzed for deriving CH3CN upper limits is indicated by the shadowed area.