Tryptophan (Trp) was studied by two-colour Photoionization Efficiency (PIE) and Mass Analysed Threshold Ionization (MATI) spectroscopy using a laser desorption apparatus. Conformer A of Trp was excited into the S-1 state (34,878 cm (1)) and the second laser was scanned around the D-0 cation ground and the D-1 excited state. No ionization signal into the D-0 state could be found, but a clear threshold was observed for the D-1 state with an ionization energy of 66,704 +/- 3 cm (1) (8.27 eV). This observation is explained in terms of the electronic configurations of the S-1 and cationic states. (C) 2016 Elsevier B. V. All rights reserved.