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; τ L ∼ 100 ns) and a wavelength-tunable optical parametric oscillator (OPO) laser system (λ ) 1.4-4.0 µm; τ L ) 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.

Desorption/Ionization Fluence Thresholds and Improved Mass Spectral Consistency Measured Using a Flattop Laser Profile in the Bioaerosol Mass Spectrometry of Single Bacillus Endospores

“…The collection and processing of data from the other samples was virtually identical, so only the results and a few important differences will be mentioned. It should be noted that MALDI experiments indicate that there may be a fluence threshold for the desorption of neutral material below the threshold for ionization 28,29 . Neutral material cannot be detected in the present instrument, so the thresholds reported here entail both desorption and ionization.…”

Periodic unmixing of a binary metallic vapor

“…7 Recently, photomechanical perturbations have been investigated in great detail for nanosecond ablation. 9 It was shown that the laser-induced stress can be split into at least two components, both unipolar ͑transversal͒ ablation-induced stress from the recoil momentum of the removed material, and a ͑transversal͒ thermoelastic excitation identified by a compression or tension wave that is bipolar with respect to the unperturbed flat surface.…”

“…When high photon fluxes are applied to surfaces, metastable phases of matter can emerge on short time scales. 7 In this transient regime, high pressures in the range of several gigapascal can be expected, from which strong acoustic perturbations 8 as well as thermoelastic and ablation-induced stress 9 can emerge.…”

Resolution of infrared matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry using glycerol; enhancement with a disperse laser beam

“…5,6 An interesting observation made in this work in this regard was that the threshold for an analyte ion, the protonated molecule of the analyte, was essentially the same as that for the matrix ions. It is known 6,18 that neutral molecules are ablated at fluences well below the threshold for ion formation. This means that the threshold fluence for the analyte ion is determined by that of the matrix ions.…”

“…The photoacoustic analysis of the material ablation induced by IR lasers reported by the same group showed that the amount of neutral ejection at the MALDI ion threshold was larger with glycerol than with solid matrices. 11,18 Hence, a larger number of collisions due to higher plume pressure in glycerol IR-MALDI seems to be …”