Analytical mass spectrometry of artists’ acrylic emulsion paints by direct temperature resolved mass spectrometry and laser desorption ionisation mass spectrometry

Abstract

Direct temperature resolved mass spectrometry (MS) is a microanalytical technique to analyse modern paints by thermal separation and ionisation of organic pigments and polymeric fractions from a platinum/rhodium filament inside the ionisation chamber of the mass spectrometer. Most organic pigments in modern paints are desorbed at lower temperatures. Ethyl acrylate/methylmethacrylate or butyl acrylate/methylmethacrylate copolymers used in acrylic emulsion paints produce mono and oligomeric subunits released from the copolymers at high temperature by pyrolysis. Characteristic low voltage electron ionisation and ammonia chemical ionisation (CI) mass spectra of these copolymers facilitate their identification. DTMS of three different commercial acrylic emulsion paints showed low and high temperature events that could be related to the presence of organic pigments and the acrylic copolymers. Polyethylene glycols with molecular weight up to 2000 Da were identified as additives under ammonia CI conditions. The azo pigments PY3, PY73 and PY74, and the phthalocyanine pigment PG7 reported show molecular ions and a few characteristic fragment ions under direct temperature resolved mass spectrometry (DTMS) analytical conditions. Yellow azo pigments were identified under DTMSMS conditions by their high energy collisionally induced fragment patterns of their parent ions. Laser desorption ionisation mass spectrometry (LDIMS) using a nitrogen laser (337 nm) of an acrylic emulsion paint with four different organic pigments produced radical cations, protonated or sodiated ions of the pigments. The amide bond in the azo pigment PY3 was photolytically cleaved and produced a specific fragment ion. The trace additive polyethylene glycol was observed preferentially while the acrylic copolymers were transparent.