Preparation of Hard Palm Seeds for Matrix-Assisted Laser Desorption/Ionization-Imaging Mass Spectrometry Analysis

Matrix-assisted laser desorption/ionization-imaging mass spectrometry (MALDI-IMS) is applied to identify compounds in their native environments. Currently, MALDI-IMS is frequently used in clinical analysis. Still, an excellent perspective exists for better applying this technique to understand chemical compounds' physiological information in plant tissues. However, preparation may be challenging for specific samples from botanical materials, as MALDI-IMS requires thin slices (12-20 µm) for appropriate data acquisition and successful analysis. In this sense, previously, we developed a sample preparation protocol to obtain thin sections of Euterpe oleracea (açaí palm) hard seeds, enabling their molecular mapping by MALDI-IMS.

Here, we show that the developed protocol is suitable for preparing other seeds from the same genus. Briefly, the protocol was based on submerging the seeds in deionized water for 24 h, embedding samples with gelatin, and sectioning them in an acclimatized cryostat. Then, for matrix deposition, an xy motion platform was coupled to an electrospray ionization (ESI) needle spray using a 1:1 (v/v) 2,5-dihydroxybenzoic acid (DHB) and methanol solution with 0.1% trifluoroacetic acid at 30 mg/mL. E. precatoria and E. edulis seed data were processed using software to map their metabolite patterns.

Hexose oligomers were mapped within sample slices to prove the adequacy of the protocol for those samples, as it is known that those seeds contain large amounts of mannan, a polymer of the hexose mannose. As a result, peaks of hexose oligomers, represented by [M + K]⁺ adducts of (Δ = 162 Da), were identified. Thus, the sample preparation protocol, previously developed tailor-made for E. oleracea seeds, also enabled MALDI-IMS analysis of two other hard palm seeds. In short, the method could constitute a valuable tool for research in the morpho-anatomy and physiology of botanical materials, especially from cut-resistant samples.