Mass spectrometry imaging(MSI)has been widely used to simultaneously observe multiple biological molecules on tissue sections with high mass accuracy and high spatial resolution.Currently,matrix-assisted laser desorption/ionization(MALDI)is the major technique for MSI due to its good salt tolerance,high spatial resolution(approximately 5 μm)and wide mass range for protein imaging1.When coupling with a high mass resolution mass spectrometer such as Fourier transform ion cyclotron resonance mass spectrometer(FTICR MS),the high resolving power will provide high reliable ion images of multiple peaks in a single-scan experiment2,but this strategy is limited by the inherent low throughput or high time consumption of FTICR instrument3.In addition,MALDI imaging also suffers the challenges of complex sample preparation prior to analysis and the interferences of matrix molecules and/or matrix adducts for the detection of small molecules.In this study,we integrated liquid extraction and electrosonic spray ionization(LE-ESSI)technology to develop an ionization device to image tissue molecules under ambient environment.Equipped with a FTICR MS,this LE-ESSI device is applicable for in situ ionizing most molecular species such as metabolites,lipids,drugs,peptides,and intact proteins with high sensitivity.For MSI,ion images with high mass resolution(200,000 at m/z 400)or high spatial resolution(< 20 μm)could be obtained within 3 hours with excellent experimental reproducibility.The device was further used to investigate the tissue sections derived from a serial of the brain sections of traumatic brain injury model and normal control,and it was found that docosahexaenoic acid which involved in the recovery of brain injury was over-expressed in brain injury area during the recovery time.These findings indicate that the LE-ESSI coupled with FTICR MS may be a powerful MSI platform for investigating biochemical and pathological processes in tissues.