Design and characterization of a high-power laser-induced acoustic desorption probe coupled with a fourier transform ion cyclotron resonance mass spectrometer.

We report here the construction and characterization of a high-power laser-induced acoustic desorption (LIAD) probe designed for Fourier transform ion cyclotron resonance mass spectrometers to facilitate analysis of nonvolatile, thermally labile compounds. This "next generation" LIAD probe offers significant improvements in sensitivity and desorption efficiency for analytes with larger molecular weights via the use of higher laser irradiances. Unlike the previous probes which utilized a power-limiting optical fiber to transmit the laser pulses through the probe, this probe employs a set of mirrors and a focusing lens. At the end of the probe, the energy from the laser pulses propagates through a thin metal foil as an acoustic wave, resulting in desorption of neutral molecules from the opposite side of the foil. Following desorption, the molecules can be ionized by electron impact or chemical ionization. Almost an order of magnitude greater power density (up to 5.0x10(9) W/cm2) is achievable on the backside of the foil with the high-power LIAD probe compared to the earlier LIAD probes (maximum power density approximately 9.0x10(8) W/cm2). The use of higher laser irradiances is demonstrated not to cause fragmentation of the analyte. The use of higher laser irradiances increases sensitivity since it results in the evaporation of a greater number of molecules per laser pulse. Measurement of the average velocities of LIAD-evaporated molecules demonstrates that higher laser irradiances do not correlate with higher velocities of the gaseous analyte molecules.