a, Images of positive ions (sample 2) that are characteristic of (from left to right) peptides/proteins (C4H8N+, m/z 70), polyaromatic hydrocarbons (C9H7+, m/z 115; C10H8+, m/z 128; C11H7+, m/z 139; C11H9+, m/z 141; C12H8+, m/z 152; C13H9+, m/z 165; C14H10+, m/z 178; C15H9+, m/z 189; and C16H10+, m/z 202) and the sedimentary matrix (Al+, m/z 27; Si+, m/z 28; and K+, m/z 39), along with a three-colour overlay image of these ions in which green represents proteinaceous matter, red represents polyaromatic hydrocarbons and blue represents sediment. b, Positive ion spectra from selected regions of interest indicated in the three-colour overlay image in a (green demarcations highlight areas dominated by proteinaceous matter; red lines frame regions rich in polyaromatic hydrocarbons). Characteristic aliphatic and polyaromatic hydrocarbon peaks are indicated by triangles and circles, respectively, in the top spectrum (red regions of interest), whereas typical protein fragment ions are denoted by stars in the bottom spectrum (green regions of interest; see Supplementary Information). c, Negative ion images of sample 13a (see Fig. 5m) representing (top row, from left to right) sulfur-containing materials (SO3−, m/z 80 and C4HS−, m/z 81), phosphate (PO2−, m/z 63 and PO3−, m/z 79) and the sedimentary matrix (SiO3−, m/z 76; SiHO3−, m/z 77; and AlSiO4−, m/z 119), together with a three-colour overlay image of these ions in red, green and blue, respectively. The bottom row shows the spatial distribution of ions representing eumelanin (C3N−, m/z 50; C3NO−, m/z 66; and C5N−, m/z 74) and epoxy (C2H3O2−, m/z 59 and C3H3O2–, m/z 71), along with a five-colour overlay image featuring sulfur-containing ions (red), phosphate (green), eumelanin (yellow), epoxy (cyan) and sediment (blue); the total ion image is shown on the far right.