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The pigment lead white ((PbCO3)2·Pb(OH)2) was analyzed by LDI-MS/MS and LDI-Orbitrap-MS/MS for the first time. The elucidation of formulae from a suite of isotopically complex cluster ions was obtained from low- and high-resolution full- and tandem-mass spectra in concert with both deductive and inductive reasoning. The unique isotopic pattern of lead was used both to delineate the full-scan spectrum into superclusters and to identify the monoisotopic mass of each cluster, from which a lead-limited, accurate formula could be derived. Chemical formulae for the clusters of lead white followed the form PbxOyHz. Also, the in situ detection of lead white by LDI-MS was achieved for cross sections of painted works of art.

The peculiar ionization of anthraquinones, including the artistic dyes alizarin and purpurin, is examined. When alizarin (MW = 240) is ionized by either LDI or MALDI it exhibits a dominant ion of m / z 242 [M + 2H] + with a far greater abundance than expected from the 13 C isotopic contribution from the [M + H] + ion at m / z 241. For the first time, MS/MS analysis of these anomalous [M + 2H] + ions is presented, which indicates that they arise from a laser-induced photoreduction of one of the anthraquinone’s carbonyl groups. MS/MS daughter ions produced from neutral losses of either water or ammonia from the [M + 2H] + ions exhibit different relative abundances, depending upon their respective functional groups’ proximity to the reduced carbonyl. LDI-MS/MS was used for the in situ detection of alizarin in a painting cross section and a swatch of dyed silk.