Compositional analysis of traditional liquid gold with separation of compounds containing heavy atoms in ion mobility-mass spectrometry.

"Liquid gold" has been traditionally used for over a century to decorate ceramicware, but its chemical composition has not been thoroughly investigated. One of the keys to successfully characterizing liquid gold, which is a complex mixture, is to distinguish Au-containing products from other chemicals. In this paper, we propose a separation based on the difference in collision cross section, of which chemicals with heavy atoms are relatively smaller than those without in ion mobility-mass spectrometry (IM-MS). Chemicals containing a single Au atom (and Pt atom) were successfully separated from other species in the two-dimensional distribution map for IM-MS. By a detailed analysis of the spectra obtained by IM-MS/MS with collision-induced dissociation before and after IM separation, we found that liquid gold (gold resinate) was a mixture of a series of (1) Au reacted with α-pinene-related units and (2) Au reacted with abietic acid units. α-Pinene and abietic acid are the main components of turpentine and rosin, the raw materials of liquid gold as reported previously (Anal. Sci. 2024, 40, 133-139). All Au-containing species contain sulfur atoms. Species of Au reacted with α-pinene-related units with different degrees of unsaturation and oxidation have also been identified. Liquid gold, a complex mixture of chemicals containing Au, has been successfully analyzed compositionally.