Design and Characterization of a High-Power Laser-Induced Acoustic Desorption Probe Coupled with a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer

Abstract

We report here the construction and characterization of a high-power laser-induced acoustic desorption (LIAD) probe designed for Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometers to facilitate analysis of non-volatile, 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 5.0 × 10 9 W/cm 2 ) is achievable on the backside of the foil with the high-power LIAD probe compared to the earlier LIAD probes (maximum power density ~9.0 × 10 8 W/cm 2 ). 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.

Laser-Induced Acoustic Desorption/Atmospheric Pressure Chemical Ionization Mass Spectrometry

“…The LIAD probe employed in this study has been previously described [19]. The only difference between the probe used here (10” length) and the one described in the literature is that the literature probe is longer.…”

“…This makes it possible to decouple the desorption and ionization processes, thus allowing the use of a variety of methods to ionize analytes, such as electron bombardment [13,14] and chemical ionization (CI) [8,15-17]. LIAD has been implemented with a Fourier-transform ion cyclotron resonance mass spectrometer [18,19], a quadrupole ion trap [20], a linear quadrupole ion trap (LQIT) [21], and a quadrupole/time-of-flight mass spectrometer [22]. In these studies, the analyte was deposited onto either a thin titanium or aluminum foil or silicon wafer.…”

Laboratory Studies of Ion Chemistry in the Interstellar Medium

“…Our studies of the chemistry of PAH ions are limited by the low volatility of the parent molecules. We have therefore implemented a new approach, Laser-Induced Acoustic Desorption, which has been extensively developed by Hilkka Kenttämaa and her research group (Shea et al 2007). With this method, PAHs are uniformly deposited onto a thin titanium foil and mounted in the ion source region of the instrument.…”

Recent advances in experimental techniques to probe fast excited-state dynamics in biological molecules in the gas phase: dynamics in nucleotides, amino acids and beyond

“…A thicker foil will withstand a higher intensity laser pulse but will cause greater attenuation of the acoustic wave. Generally, optimal foil thicknesses have been found to be tens of micrometres [87,90]. …”