Secondary negative ion images of B16 melanoma cells from a mouse lung colony. A & D: CN- ions (m = 26) – B & E: P- ions (m = 31) – C & F: I- ions (m = 127). D-F are enlargements of the top left part of A-C. In the bottom row, the use of specific colours for each type of ion (red for CN, green for P and blue for I) allows the superimposition of the various images leading to colour variations resulting from the co-localization of the observed ions. Thus, yellow coloration in CN+P corresponds to the co-localisation of CN- and P- ions while a co-localization of the iodine with very intense spots of the CN image (purple coloration in CN+I) can be seen. This iodine signal corresponds to the melanin, which appears in red on the CN+P image and in blue on the P+I image, since melanin does not contain any phosphorus. The strongest signals in the phosphorus image are attributed to cell nucleus, namely to active chromatin and nucleoli. Observed fields A-C: 20 μm × 20 μm, D-F and superimpositions: 10 μm × 10 μm.