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 variation in the range of millimeters have been successfully ablated in a reproducible manner, with the smallest achievable ablation mark diameter being 30 m. The 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 imaging results exhibited an unexpected level of noise that implicated the ion generation efficiency to being subject to an inexplicable randomness, lowering the quality of the obtained data. After exclusion of common error sources, three iterations on the ion source geometry were conceived, assembled, and tested, along with a commercial solution to compare sensitivity and stability in signal intensity obtained when performing the same measurement multiple times. The evaluated experimental parameters included the mass spectrometer's stability of response, limit of detection for 13C6-Phenylalanine, and the electrospray's stability by the proxy of electrospray current. 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 distinguishing a 1-fold change in concentration.

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