Photoacoustic Analysis of Matrix-Assisted Laser Desorption/Ionization Processes with Pulsed Infrared Lasers

Abstract

A fast piezoelectric detection system was utilized to record time-resolved photoacoustic signals reflecting the thermal expansion and the rate of material ablation in infrared matrix-assisted laser desorption/ionization (IR-MALDI). Glycerol was employed as a liquid and 2,5-dihydroxybenzoic acid (2,5-DHB) as a crystalline matrix. An Er:YAG laser (λ = 2.94 μm; τL ∼ 100 ns) and a wavelength-tunable optical parametric oscillator (OPO) laser system (λ = 1.4−4.0 μm; τL = 6 ns) were used for desorption/ionization. Material ejection and MALDI ion signals were recorded as a function of laser fluence, pulse duration, and wavelength. For glycerol as matrix, material ejection was found to be temporally confined to about the laser pulse duration when the OPO was employed. For excitation with the Er:YAG laser, a prolonged material ejection with an approximately exponential decay was observed with a characteristic time constant of ∼1 μs. For both lasers, material ejection was observed already at fluences substantially below the MALDI ion threshold. For excitation with the OPO, a correlation between a change in the desorption/ablation mechanism and the onset of ion generation was found. Crystalline 2,5-DHB preparations were investigated with the OPO only. Material ejection from this matrix was found to exhibit a significantly different dependence of the overall ejected material on laser fluence. The article also provides an introduction to the underlying photoacoustic theory adapted to the MALDI process.