Laser-induced molecular desorption and particle ejection from organic films

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Abstract

We present a comparative study on UV matrix-assisted laser desorption (MALD), laser-induced thermal desorption (LITD), IR laser ablation, and desorption via electrical substrate heating. In order to determine to what extent the material is desorbed as individual molecules or ejected as particles, respectively, the ablated material was intercepted by a trapping plate and the particle size distribution was analyzed by atomic force microscopy (AFM). The ratio of ejected particles to free molecules is strongly dependent on the laser volatilization technique used. High energy densities in the sample, for example in UV MALD, favor molecular desorption leading to very smooth films on the trapping plate. In contrast, IR polymer ablation and LITD of micrometer thick organic films is dominated by ejection of clusters and particulates. Volatization by electrically heated flash filaments is not significantly different from substrate mediated thermal laser desorption (LD). It may allow to miniaturize and simplify mass spectrometric instrumentation for applications requiring desorption–ionization techniques.

Introduction

A basic requirement for mass analysis by desorption–ionization methods is that the desorption process should release intact analyte molecules from the sample. In this respect, laser desorption (LD) is by far the most successful of many desorption techniques available for high molecular weight, polar, non-volatile, and thermally labile sample materials 1, 2, 3, 4, 5, 6. The numerous laser volatilization techniques which are commonly applied in mass spectrometric analysis differ significantly in the way how energy is deposited into the sample. In laser-induced thermal desorption (LITD) the volatilization of absorbate layers is induced indirectly by laser heating of the substrate whereas in resonant laser desorption the molecules which are to be volatized are directly excited by the laser radiation.

In matrix-assisted laser desorption/ionization (MALDI), the analyte molecules are incorporated into a solid matrix. Resonant excitation of the matrix by UV laser pulses (UV MALDI) or IR laser pulses (IR MALDI) leads to co-desorption of neutral as well as ionized matrix and analyte molecules. A severe drawback of the MALDI technique is that a reproducible and homogenous sample preparation is difficult, so that quantitative results can often not be obtained. A better understanding of the desorption mechanism in MALDI would certainly help to solve some of these problems. Dreisewerd et al. [7]and Schürenberg et al. [8]introduced a quasi-thermal desorption model for MALDI that involves a pressure driven disintegration of the matrix material into microscopic particles from which thermal desorption leads to molecular gas phase species of matrix and analyte. Molecular dynamics simulations of laser ablation and desorption of organic solids by Zhigilei et al. 9, 10, 11also support the picture of an ablation plume consisting of a mixture of clusters and molecular species. These authors report that a threshold fluence separates two mechanisms for the ejection of molecules: surface vaporization of single molecules at low fluences and collective ejection and ablation at high fluences. The calculations yield that the amount of ejected material rises sharply at the threshold fluence and that at fluences exceeding the threshold large molecular clusters constitute a significant part of the plume. Even higher laser fluences induce an explosive homogeneous phase transition from solid to gas phase, leading to a strongly forward peaked plume and high maximum velocities of the ejected material. Although many experimental observations 8, 12, 13, 14, 15, 16, 17can be explained by these models, no direct evidence of clusters and particulates formed under MALDI conditions has been obtained yet. In IR MALDI, photons in the wavelength range from 1 to 10.6 μm are used for resonant vibrational excitation of the matrix molecules. The penetration depth of the laser energy is much higher than for UV photons and therefore the energy density is rather low, often not even sufficient for complete sublimation of the ablated sample volume. Therefore, a model based on spallation of the matrix material due to mechanical stress is commonly accepted 18, 19.

Matrix molecules are not only restricted to relatively small organic molecules. Haefliger and Zenobi [20]have proposed to embed analyte molecules in a polymeric membrane (polyvinyl chloride; PVC) to reduce the sample heterogeneity. In their experiment a thin 50-μm film was completely vaporized by a single IR laser shot and in a second step the analyte molecules entrained in the expanding plume were ionized by pulsed UV radiation. The progress that has been realized by this sample preparation technique is the high reproducibility of the relative peak intensities at different locations on the membrane and a high dynamic range. However, the drawback of the technique is that it is very sensitive to fluctuations of the desorption laser power and that the noise level is much worse compared to desorption of pure analyte films from silica.

Laser-induced thermal volatilization via substrate heating has also been investigated by various groups 21, 22, 23, 24, 25, 26. In the case of (sub-) monolayers only individual molecules in thermal equilibrium with the surface are desorbed 24, 25, 26, whereas in the case of multilayer surface coverages ejection of particles is reported 21, 22.

In this study we present a direct investigation of the composition of material which is ejected in UV matrix-assisted laser desorption (MALD), IR laser ablation, LITD, and volatilization from electrically heated filaments. The ejected material was collected on a trapping plate and the particle size distribution was determined by atomic force microscopy (AFM). Direct observation of the laser deposited material is a new approach to compare essential characteristics of the different volatization techniques used in mass spectrometry.

Section snippets

Experiment

The transfer experiments were performed in a vacuum chamber which was pumped down to 10−6 Torr. AFM imaging of the deposited material was then performed ex situ.

The samples for LITD and MALDI experiments were prepared by the dried droplet technique which is commonly used in MALDI applications [27]. Poly(ethylene glycol) PEG 3000 and 2,5-dihydroxybenzoic acid (DHB) were dissolved in methanol at a molar ratio of 1:100 and then dosed onto a clean silica support. In Fig. 1 the AFM image of a

Results and discussion

For a correct interpretation of the AFM images one has to consider that the deposited material can change in morphology and composition due to diffusion and desorption processes on the trapping plate. Another complication of the interpretation of the data stems from the fact that identical sticking probabilities on the trapping plate cannot be assumed for particles of different energy and size. We therefore complemented our measurements with experiments where annealing of the deposited material

Conclusion

We systematically investigated various laser volatilization techniques applied in mass spectrometric analysis. UV MALD, LITD, IR polymer ablation, and electrical substrate heating techniques were compared with respect to volatilization of individual molecules and particles. The liberated sample material was deposited on a trapping plate and analyzed by AFM to determine the particle size distribution. Our experiments show that the ratio of ejected particles and free molecules is strongly

Acknowledgements

Our work is financially supported by the Swiss National Science Foundation (grant: 52422.97).

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