How to sample a seizure plant: the role of the visualization spatial distribution analysis of Lophophora williamsii as an example

: Natural compounds in plants are often unevenly distributed, and determining the best sampling locations to obtain the most representative results is technically challenging. Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) can provide the basis for formulating sampling guideline. For a succulent plant sample, ensuring the authenticity and in situ nature of the spatial distribution analysis results during MSI analysis also needs to be thoroughly considered. In this study, we developed a well-established and reliable MALDI-MSI method based on preservation methods, slice conditions, auxiliary matrices, and MALDI parameters to detect and visualize the spatial distribution of mescaline in situ in Lophophora williamsii. The MALDI-MSI results were validated using liquid chromatography–tandem mass spectrometry. Low-temperature storage at −80°C and drying of “bookmarks” were the appropriate storage methods for succulent plant samples and their flower samples, and cutting into 40 μm thick sections at −20°C using gelatin as the embedding medium is the appropriate sectioning method. The use of DCTB (trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile) as an auxiliary matrix and a laser intensity of 45 are favourable MALDI parameter conditions for mescaline analysis. The region of interest semi-quantitative analysis revealed that mescaline is concentrated in the epidermal tissues of L. williamsii as well as in the meristematic tissues of the crown. The study findings not only help to provide a basis for determining the best sampling locations for mescaline in L. williamsii, but they also provide a reference for the optimization of storage and preparation conditions for raw plant organs before MALDI detection. KEY POINTS: An accurate in situ MSI method for fresh water-rich succulent plants was obtained based on multi-parameter comparative experiments. Spatial imaging analysis of mescaline in Lophophora williamsii was performed using the above method. Based on the above results and previous results, a sampling proposal for forensic medicine practice is tentatively proposed.