Profilometry of plant surfaces to facilitate consistent, in vivo laser ablation electrospray ionization of specialized metabolites

A laser ablation electrospray ionization (LAESI) source was custom-built and integrated with a profilometer from commercially available parts. This integration allowed reliable infrared laser ablation on samples with a three-dimensional surface morphology by means of topographically guided repositioning of the sample to maintain laser focus. Surface features with height variations in the range of millimeters have been successfully ablated in a reproducible manner. The smallest ablation mark diameter achieved was 30 micrometer. The mass spectrometry imaging capabilities of the custom-built system were developed, tested and evaluated on cotton (Gossypium hirsutum) leaf and stem samples. Furthermore, the distribution of characteristic glucosinolates in thale cress (Arabidopsis thaliana) leaves was analyzed and compared between wounded leaves and an unharmed control group. The quality of the imaging results was low due to an unexpected variability in measured signal intensities. After exclusion of common error sources, three iterations on the ion source geometry were conceived, assembled, and tested, along with a commercial solution to investigate sensitivity and repeatability of LAESI as an ionization technique. The evaluated experimental parameters include the mass spectrometer’s stability of response, limit of detection for 13C6-Phenylalanine, and stability of the electrospray. It was found that the custom-built and commercial ion sources operate at a comparable level of stability, which renders the respective ion source capable of resolving a 1-fold change in concentration.



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