Discrimination of Shellac and Nitrocellulose Varnishes Using Ambient Mass Spectrometry and Partial Least Squares Regression

Historical surface coatings on Czechoslovak furniture from the interwar period (1918–1945) frequently combine shellac and nitrocellulose lacquers, yet their compositional characterisation in aged specimens remains analytically challenging. We report the first application of atmospheric solids analysis probe mass spectrometry (ASAP-MS) combined with partial least squares regression for semiquantitative estimation of shellac/nitrocellulose ratios in historical varnish systems. ATR-FTIR spectroscopy combined with principal component analysis represents a standard tool in conservation practice for varnish characterisation; however, our comparative evaluation demonstrated that this approach is limited to qualitative discrimination and does not support reliable semiquantitative composition estimates. The ASAP-MS workflow was applied to microsamples from an authenticated historical furniture object of Czech origin (ca. 1939), yielding direct compositional estimates that can guide the selection of appropriate restoration treatments and reduce the risk of chemically incompatible interventions.