The nature of dopant species present in the melamine crystal is identified using high-performance liquid chromatography (HPLC) combined with mass spectrometry (MS). Figure shows the mass spectra for negative ions generated from the control mobile phase (m.p.), melamine in water (Mel in aq.), TCNQ in THF (TQ in THF), TCNQ in aueous THF (TQ in aq. THF), TCNQ and melamine (1:1 molar ratio) in aqueous THF (TQ + Mel in aq. THF) by the electrospray ionization (ESI) method. The mass spectra were measured on the output solution of the HPLC for 15 minutes. The negative ion mass spectrum of TCNQ in THF exhibits the intense peak of TCNQ -at m/z = ca. 204, the peak of [TCNQ-HCN] - at m/z = 177, and the peak of contaminants (e.g. at m/z = 183) present in the mobile phase. The mass spectrum of TCNQ in a mixture of THF and water (1:1 volume ratio) exhibits no peak of TCNQ -and [TCNQ-HCN] -, but the peak at m/z = 194 that can be attributed to DCTC -. The pink colour of melamine crystals precipitated upon slow vaporization of the reaction solution is therefore attributed to DCTC -molecules encapsulated as impurities in melamine crystals. See the SI, section S1 for the positive ion mass spectra that exhibit the peaks associated with melamine.