Resonance-Enhanced Multiphoton Ionization Jet Spectroscopy and Mass Spectrometry of Tyrosine-Containing Dipeptides Using a Pulsed Laser Desorption/Volatilization Method

Pulsed laser desorption is used as a means of volatilizing thermally labile tyrosine-containing dipeptides for entrainment into a supersonic jet expansion. The jet expansion provides ultracold molecules whose spectral features are probed by resonant two-photon ionization (R2PI) spectroscopy in a time-of-flight mass spectrometer. In the dipeptides studied, spectral contours ∼1.5 nm FWHM are observed. Although no sharp spectral features were obtained, the shape of the contour is quite distinctive, and isomeric dipeptides can be distinguished. In addition, the ions produced are monitored in a time-of-flight mass spectrometer. It is shown that soft ionization can be obtained for each of these dipeptides by optimizing the jet cooling and exciting at the maximum intensity of the specific origin transition. By an increase in the laser energy, fragmentation is produced which can be used for structural identification of each of these dipeptides. Other factors that may affect these results, such as thermal decomposition, are also investigated.