Enhancement of lipid signals with ammonium fluoride in negative mode Nano-DESI mass spectrometry imaging

Lipids play an important role in biology. Their structural diversity presents challenges for untargeted mass spectrometry imaging (MSI) and analysis techniques. Enhancing the lipid coverage of MSI experiments is an important step to understanding their role in biological processes. Herein, we use nanospray desorption electrospray ionization mass spectrometry imaging (nano-DESI MSI) to explore the effect of ammonium fluoride (NH4F) as a solvent additive for enhanced detection of lipids in biological tissues. Although fatty acids and phospholipids are readily observed in nano-DESI MSI, other lipid classes are difficult to detect due to their low abundance and low ionization efficiency. We have evaluated enhancement factors of lipids in negative ionization mode in the presence of NH4F in the nano-DESI solvent. We observed a 10–110-fold enhancement in lipid signals detected as [M − H]- ions using 500 μM NH4F in 9:1 methanol:water in comparison with 9:1 methanol:water, which is typically used in nano-DESI MSI experiments. The observed signal enhancement is attributed to the efficient proton abstraction from lipids by F−. The enhancement factors observed for fatty acids are correlated to their interfacial pKa values. The optimized solvent composition enables sensitive imaging of both high- and low-abundance lipids without any adverse effect on the noise level. Overall, our results demonstrate a substantial improvement in sensitivity of nano-DESI MSI of lipids in biological tissues when NH4F is added to the nano-DESI solvent.