Mass spectrometric methodology for the analysis of highly oxidized diterpenoid acids in Old Master paintings

Diterpenoid resins from larch and pine trees and the corresponding fractions in a >100-year-old wax-resin adhesive and varnish and a 200-year-old resin/oil paint sample were analysed with by gas chromatography/mass spectrometry (GC/MS) using several off-line and on-line derivatization methods. The main resin compounds were highly oxidized abietic acids. Important products found are hydroxydehydroabietic acids (OH-DHAs), 7-oxoDHA, di-OH-DHAs and 15-OH-7-oxoDHA. The last two compounds have not been reported to occur in artworks before. Larixyl acetate, an important marker from larch resins, was found to be still present in high amounts in the adhesive. A large number of mass spectra of the different oxidation products and larixol and larixyl acetate are presented and their fragmentation behaviour under electron impact conditions is discussed. An index for the degree of oxidation (IDOX) of the abietic acids is presented as an indicator of the degree of oxidation of the matrix in which the resin is present. The IDOX was 0.10, 0.67, 0.81 and 0.76 for the fresh resins, the dark-aged adhesive, the aged varnish and the resin/oil paint, respectively (measured with pyrolysis (Py)-tetramethylammonium hydroxide (TMAH)-GC/MS). Py-TMAH-GC/MS and direct temperature-resolved mass spectrometry are reliable, valuable and fast techniques for the assessment of the presence and degree of oxidation of diterpenoid resins.