Infrared matrix-assisted laser desorption and ionization by using a tunable mid-infrared free-electron laser

Initial results of infrared matrix-assisted laser desorption/ionization (IR-MALDI) mass spectrometry of proteins by using the Vanderbilt free-electron laser as the source of selective vibrational excitation are reported. The ability of this laser to initiate desorption and ionization by excitation of specific vibrational modes is demonstrated. For the first time it is shown that IR-MALDI mass spectrometry at wavelengths other than those available from conventional fixed-frequency IR lasers, that is, 2.79 (Er:YSGG), 2.94 (Er:YAG), and 9.3-10.6 μm (CO₂), is feasible and exhibits similar performance. IR-MALDI mass spectra were taken in the wavelength ranges 2.8-4 and 5.5-6.5 μm, covering the absorption bands of the O-H and C=O stretch vibrations typical of many organic compounds such as succinic acid, fumaric acid, or nicotinic acid, which were used as matrices in these studies. A comparison between these results and Er:YAG/YSGG MALDI data are given. The potential of IR-MALDI at wavelengths near the C=O stretch vibration and the possibilities for studies of the IR-MALDI mechanisms by using this kind of tunable source are discussed.