Infrared spectra of three residues at 300 K, a) from H₂O:CH₃OH:NH₃ ice mixture photolysed at 25 K and warmed until 300 K, b) from H₂CO:NH₃:HCOOH ice mixture only warmed until 330 K, and c) from CH₂NH:HCOOH ice mixture only warmed until 330 K, compared with d) pure HMT in the same experimental conditions at 260 K, and e) pure PMI in the same experimental conditions at 500 K. κ sign to remaining HCOO⁻ in the ice mixture.