Photoionization of dipeptides with femtosecond laser pulses

Imaging time-of-flight secondary ion mass spectrometry is a very important technique for characterizing organic and biological systems. However, the signal is very low due to small amounts of desorbed molecules, During the ion bombardment process, neutral species as well as positive and negative ions are desorbed, with neutral molecules being the main species, Therefore, ionization of the neutral species should improve sensitivity. Here, we report the use of femtosecond laser photoionization of ion beam desorbed dipeptides to enhance the detection sensitivity. Ionization using 200 nm. laser pulses is found to break the C-C bond in the C-C=O group or the C-C bond between the functional group and the backbone, which agrees with the alpha -cleavage mechanism proposed previously. Photoionization produces 5-10 fold hip,her dipeptide ion yields than the secondary ion yields achieved with the use of the incident ion beam only.