Cold plasma of CH₄, N₂, and CH₃CN observed in frequency switching mode with a frequency throw of 37 MHz. The data have been folded. Upper panel: observed spectrum around HC₃N J = 12 → 11 (produced in the plasma) without removing the baseline. Middle panel: resulting spectrum when a polynomial baseline is removed. The bottom spectrum is magnified in frequency and intensity to show the weak lines that were detected from HC₃N ν₅ (E_up ≃ 1000 K) and the in-plane torsion of CH₃CH₂CN (produced in the plasma, E_up ≃ 320 K). The integration time for this observation is 100 s.