Abstract
Numerous applications of ambient Mass Spectrometry (MS) have been demonstrated over the past decade. They promoted the emergence of various micro-sampling techniques such as Laser Ablation/Droplet Capture (LADC). LADC consists in the ablation of analytes from a surface and their subsequent capture in a solvent droplet which can then be analyzed by MS. LADC is thus generally performed in the UV or IR range, using a wavelength at which analytes or the matrix absorb. In this work, we explore the potential of visible range LADC (532 nm) as a micro-sampling technology for large-scale proteomics analyses. We demonstrate that biomolecule analyses using 532 nm LADC are possible, despite the low absorbance of biomolecules at this wavelength. This is due to the preponderance of an indirect substrate-mediated ablation mechanism at low laser energy which contrasts with the conventional direct ablation driven by sample absorption. Using our custom LADC system and taking advantage of this substrate-mediated ablation mechanism, we were able to perform large-scale proteomic analyses of micro-sampled tissue sections and demonstrated the possible identification of proteins with relevant biological functions. Consequently, the 532 nm LADC technique offers a new tool for biological and clinical applications.
Highlights
Techniques generally require the use of a wavelength that can efficiently excite the molecules present in the system
We have demonstrated that the Laser Ablation and Droplet Capture (LADC) system at 532 nm followed by MS is a powerful technique for tissue micro-sampling towards the analysis of large scale biomolecules such as Shotgun proteomics
We have identified an ablation mechanism mediated by the substrate which is preponderant over conventional laser ablation mechanisms by direct absorption for low-absorbing samples studied in the low laser energy range
Summary
Techniques generally require the use of a wavelength that can efficiently excite the molecules present in the system (matrix or analytes). Some studies have tested the use of visible laser light tuned mostly at 532 nm, whereby a continuous wave laser in the transmission mode was used to desorb and inject molecules into the MS using ESI ion source[20,21] This was only performed with low absorbing molecules. They showed that the presence of a matrix did not improve desorption efficiency and that the mixture ion signal originated from the sample exposed to the near-IR laser They suspected the importance of the substrate in the desorption/ionization process because of possible heating of the plate material during irradiation. We explore the potential of a 532 nm LADC system, with conventional reflection geometry, as a means for tissue micro-sampling and subsequent large-scale biomolecule analysis using substrates of different optical properties. We were able to determine the efficiency of our system for spatially resolved micro-proteomics with relative quantification
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