A liquid beam of aqueous solutions of arginine (AH), its hydrochloric acid salt (AH(2)Cl) and its sodium salt (ANa) was irradiated with a pulsed IR laser at 3509 cm(-1) (2.85 mu m). Positive and negative ions ejected to the gas phase were mass analyzed. From the AH(2)Cl and the ANa aqueous solutions, the pre-existing ions in the solution were ejected directly to the gas phase. From the AH solution, on the other hand, both a protonated arginine ion, AH(2)(+) and an argininate ion, A(-) were observed in the gas phase. Production of these ions is accounted for the four-photon excitation of water molecules in the solution. (c) 2005 Elsevier B.V. All rights reserved.