Atmospheric Pressure Mass Spectrometry Imaging Using Electrospray-Assisted Laser Desorption/Ionization with Gas Transportation through a Heated Tube and Minimal Sample Preparation.

Mass spectrometry (MS) is a valuable tool that enables label-free analysis and the ability to measure multiple molecules. The atmospheric pressure MS imaging (MSI) method usually requires tedious sample preparation. A simple ionization method with minimal sample preparation is needed for high-throughput analysis. We have developed an ion source that does not require sample preparation such as thinning, curing, planarization, or addition of matrix by the electrospray-assisted laser desorption/ionization with gas transportation (ELDI-GT). The sample is transported with nitrogen gas through a heated tube to the electrospray. The ion signal of protonated caffeine was measured under different transport conditions. The ion signal intensity was found to increase 11-fold by changing the flow rate and tube temperature from 2.8 cm/s and 473 K to 25 cm/s and 673 K. ELDI-GT was able to visualize the localization of caffeine crystals at a pixel size of 50 µm using MSI because of the effective GT using the heated tube. The dependence of the ion signal intensity was discussed on the amount of heat applied to the sample in the heated tube. ELDI-GT allowed accurate localization of caffeine at a pixel size of 50 µm without the need to apply thinning and matrix to a sample.