Author contributions
S.W., L.G. & Y.P. conceived and designed the study; D.J. C.Z. & H.W. conducted sample preparation; D.J. conducted laser capture microdissection, GC-MS and LC-MS analysis; K.Q. conducted DESI/PI-MSI measurements; D.J., Z.L., L.Z., X.L., L.Z., Y.Z., S.W., L.G. & Y.P. analysed data; L.H. made some constructive suggestions for the manuscript. Z.L., D.J., K.Q., S.W., L.G. & Y.P. wrote the manuscript with input from all authors.