Fading of colorants by atmospheric pollutants: mass spectrometry studies

Chemical ionization mass spectra have been recorded for a number of artists' colorants that fade rapidly when exposed to atmospheric levels of ozone and other photochemical air pollutants. These colorants include the natural colorant curcumin, several indigos and a number of alizarin derivatives. Also included are isatins, which are produced in the fading of indigo colorants by atmospheric pollutants. The protonated molecular ion MH (M = compound mol. wt.) was the most abundant peak in the methane chemical ionization spectra of most of the artists' colorants studied. Other peaks that are of important diagnostic value are discussed for each category of colorant. Examples of applications are included and illustrate the usefulness of chemical ionization mass spectrometry in assessing air pollutant-induced damage to paintings and other colorant-containing objects of artistic and historical value.