N,N-Diethylethylenediamine for on-tissue derivatization: a novel strategy for in situ MALDI imaging of fatty acids

Fatty acids (FAs) are essential components of lipid metabolism and play crucial roles in biological systems. However, due to their low abundance and poor ionization efficiency, the detection of FAs in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) poses significant challenges, making the spatial distribution of these molecules in tissues difficult to analyze. We present a novel on-tissue derivatization strategy for FAs using N,N-diethylethylenediamine (DEEA) as the derivatization reagent, which converts the carboxyl group of FAs into a positively charged moiety via amidation, thereby improving their ionization efficiency in positive ion mode. By systematically optimizing experimental parameters including catalyst type, reaction time, and matrix concentration, combined with α-cyano-4-hydroxycinnamic acid (CHCA) matrix, in situ imaging of nine key FAs (palmitic acid, linoleic acid, eicosapentaenoic acid and docosahexaenoic acid, etc.) was successfully achieved in rat kidney tissues. The derivatization products were verified by ESI-Q-TOF-MS, confirming the reliability of the method. Furthermore, comparative analysis with the conventional derivatization reagent 2-picolylamine (PA) demonstrated that DEEA markedly enhanced the derivatization efficiency of FAs. This study employs DEEA as a derivatization reagent for MALDI imaging of FAs. This derivatization method effectively enhances the ionization efficiency of FAs in the positive ion mode of MALDI-TOF-MS, thereby providing a new and referable approach for the imaging of FAs in biological tissues.