MALDI-MS imaging of lipids in corn using a flexible ultrasonic spraying device as matrix deposition method

Matrix application is a critical step in matrix-assisted laser desorption/ionisation mass spectrometry imaging (MALDI-MSI) analysis. Here, we describe a low cost, small size, and portable ultrasonic spraying device for various matrix deposition that is ideal for applying to obtain micron-sized (about 1–15 μm) cocrystallisation. The ultrasonic spray method was systematically compared with mainstream matrix application approaches based on optimization experiments. In the approach, ethanol was used as the solvent, and the spray direction was perpendicular to the MALDI sample slide. Seven different matrices were compared and we found that 2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene] malononitrile (DCTB) is a more suitable matrix for the detection of triacylglycerol (TAGs) and phosphatidylcholines (PCs) than other matrices. The DCTB matrix solution with concentrations of 5 mg mL−1 was applied on the surfaces of corn sections through ultrasonic spraying. The corn sections were then analysed through MALDI-MS in positive ion mode. MALDI-MSI allowed the simultaneous detection of multiple lipids including 16 TAGs and PCs in corn. The results demonstrated that the ultrasonic spraying device enables the deposition of MALDI matrix solution and provides the advantages of homogeneous micron-sized matrix crystallisation and satisfactory reproducibility.