Visible wavelength MALDI using Coumarin laser dyes

Experiments are described that assess the viability of visible-matrix assisted laser desorption ionization (MALDI), and provide insight into its mechanism. Organic laser dyes Coumarin 510 and 519 were used as matrixes for the first time to record visible-MALDI mass spectra in both positive and negative ion modes. Coherent radiation between 355 nm and 520 nm was generated using an optical parametric oscillator (OPO), and the resultant MALDI ions were mass recorded in a triple quadrupole mass spectrometer. The Coumarin matrixes exhibit less fragmention when excited with visible light compared to ultraviolet (UV) radiation resulting in less background noise across the mass range of the spectrometer. A detailed examination of the effects of laser fluence and wavelength on C510 was carried out. The slopes of the log–log plots of signal versus fluence at low fluences are ∼9 and decrease significantly at higher fluence levels. This behavior is similar to what is observed using both infrared and ultraviolet sources, and suggest that the ionization process is enabled by the presence of ionizing particles activated by thermal energy deposited into the matrix.