Characterization of laser-induced acoustic desorption coupled with a Fourier transform ion cyclotron resonance mass spectrometer.

Several experimental factors have been investigated that influence the efficiency of desorption and subsequent chemical ionization of nonvolatile, thermally labile molecules during laser-induced acoustic desorption/Fourier transform ion cyclotron resonance mass spectrometry (LIAD/FT-ICR) experiments. The experiments were performed by using two specially designed LIAD probes of different outer diameters (1/2 and 7/8 in.) and designs. Several improvements to the design of the "first generation" (1/2 in.) LIAD probe are presented. The larger diameter (7/8 in.) probe provides a larger surface area for desorption than the smaller diameter probe. Further, it was designed to desorb molecules on-axis with the magnetic field of the instrument. This is in contrast to the smaller probe for which desorption occurs 1.3 mm off-axis. This improved alignment, which provides better overlap between the desorbed molecules and trapped reagent ions, results in a substantial increase in the sensitivity of LIAD analyses. The thickness of the sample layer deposited on the irradiated metal foil and the number of laser shots fired on the backside of the foil were found to have a significant effect on the overall signal and the relative abundances of the ions formed in the experiment. Evaporation of a tetrapeptide, Val-Ala-Ala-Phe (VAAF), from Ag, Al, Au, Cu, Fe, and Ti foils, followed by protonation by protonated pyridine, revealed that the titanium foil provides the greatest signal. The importance of the laser power density was examined by desorbing a low MW polymer, polyisobutenyl succinic anhydride, at power densities ranging from 5.40 x 10(8) to 9.00 x 10(8) W/cm(2) at the backside of the foil. Higher laser power densities resulted in greater signals and an improved distribution for the higher molecular weight oligomers.