Femtosecond Pumping Rate Dependence of Fragmentation Mechanisms in Matrix-Assisted Laser Desorption Ionization

The benzyltriphenylphosphonium (BTP) thermometer ion is utilized to characterize the fragmentation mechanisms of matrix-assisted laser desorption/ionization (MALDI) for femtosecond ultraviolet laser pulses. We demonstrate that the survival yield of BTP approaches unity under these conditions, which suggests that a minimal amount of fragmentation is occurring. It is also shown that the survival yield of BTP is insensitive to the laser fluence. However, the magnitude of fragmentation for the matrix increased notably for the same fluence range. These results indicate that the amount of energy transferred from the matrix ions to the BTP thermometer ions is minimal because the femtosecond desorption applied here occur within the stress-confinement regime. This observation is in agreement with recent molecular dynamics simulations which predict that it should be possible to separate both desorption and ionization processes in the regime of stress-confined desorption. Our results indicate that angiotensin is the largest biomolecule which could be routinely measured with these pulses. A mass upper-limit supports the hypothesis that ionization is hindered by the increased thermal gradients imposed in the lattice and associated velocity distribution within the ablation process from the much higher lattice heating rate with femtosecond pulses. This effect results in the temporal overlap between the neutral molecules and the matrix ions being too small to result in sufficient proton exchange for ionization.