Nonresonant Femtosecond Laser Vaporization with Electrospray Postionization for <italic>ex vivo</italic> Plant Tissue Typing Using Compressive Linear Classification

T detection and identification of molecules within complex biological matrixes (i.e., plant tissue) requires homogenization, filtration, and liquid extraction of the sample to prepare for analysis using techniques such as gas chromatography/mass spectrometry (GC/MS), liquid chromatography-mass spectrometry (LC-MS), capillary electrophoresis-mass spectrometry (CE-MS), and LC-NMR. Direct analysis of the plant tissue would be advantageous to reveal spatial information about the phytochemicals. Two common techniques for ex vivo tissue analysis are matrix-assisted laser desorption ionization (MALDI) and secondary ion mass spectrometry (SIMS). However, both techniques require high vacuum resulting in dehydration of the tissue sample. Mass analysis techniques such as desorption electrospray ionization (DESI) and atmospheric pressure MALDI allow the natural state of the tissue to be probed ex vivo, preventing dehydration. DESI impinges an electrospray plume onto a sample resulting in the desorption of molecules through the droplet pickup mechanism prior to mass analysis. The spatial resolution for DESI is dictated by the electrospray plume diameter (∼200 μm) and mixing which occurs when the spray interacts with the surface-bound molecules. Laser-based methods provide higher spatial resolution in the lateral direction, in principle λ/2, and allow for depth profiling in the z-direction. One laser-based method, electrospray-assisted laser desorption ionization (ELDI), uses an ultraviolet nanosecond laser pulse to induce desorption of the sample followed by electrospray ionization mass spectrometry (ESI-MS) to postionize the molecules for mass analysis. Another laser-based method, laser-ablation electrospray ionization (LAESI), uses an infrared nanosecond laser to resonantly couple into the O-H stretch of water that is inherently present in biological samples. This allows for the desorption of molecules followed by postionization using ESIMS. These nanosecond laser-based techniques have achieved spatial resolution of 100-150 and 200-300 μm for ELDI and LAESI, respectively, and 50 μm in the z-direction (LAESI). Atmospheric pressure MALDI (AP-MALDI) can provide high spatial resolution as well, but the use of an electrospray to postionize the desorbed neutrals has been shown to increase the ion abundance and sensitivity of the analysis. An intense, nonresonant femtosecond laser pulse can also be used to vaporize a sample without the addition of a matrix or the need for a resonant transition due to the high intensity of the pulse, 10 W cm. Femtosecond laser vaporization of solid material results in very clean vaporization profiles in comparison to nanosecond ablation where thermal effects dominate and poor etch profiles are obtained. Laser electrospray mass spectrometry (LEMS) employs a nonresonant femtosecond laser pulse to vaporize material and hence does not require the addition of a matrix (or water rich sample) and therefore can be used for direct