Mass Spectrometric Techniques for Coatings Characterization
Authors: Simonsick WJ 
Published: March 2009

Low detection limits, molecular weight and structural information, and complex mixture analysis capabilities are attributes which make mass spectrometry a powerful tool for paint analysis. This presentation will describe how mass spectrometry has been used to characterize binder systems, stabilization packages, and solvent formulations. These components of coatings contain a variety of chemical moieties encompassing a range of molecular weights from low-weight solvents to high-weight polymers and therefore offer quite a challenge to the analytical chemist.

Solvent formulations are easily identified and quantified by a capillary gas Chromatograph coupled to a mass spectrometer (GC/MS). The paint is loaded into a heated injector. The solvent components are volatilized and carried onto a cooled GC column. The column is temperature programmed, and the individual solvent constituents are separated by a highefficiency capillary column. As the compounds elute, they are ionized by an electron beam. The ions are mass filtered, and a reproducible mass spectrum is recorded. Identification is based upon library comparison.

Stabilization packages (antioxidants, UV stabilizers) can also be analyzed in a similar fashion. These packages are added to paints to prolong the usable lifetime of the coating. Unfortunately, stabilizers are much less volatile than solvents and therefore more difficult to analyze by GC/ MS. Aluminum-clad columns have extended the molecular weight range amenable to GC/MS. However, many stabilizers are not volatile enough for GC/MS analysis. Circumventing the volatility constraints imposed by GC, stabilizers can be introduced directly into the mass spectrometer. Chemical ionization MS can be used to selectively ionize the stabilizer in the presence of a complex binder matrix.

Pyrolysis (Py) or thermal degradation is the most common method for analyzing binder systems by mass spectrometry. The polymers are thermally degraded into lower-molecular-weight fragments which can be loaded onto a cooled GC column, chromatographed, detected, and identified by MS. Typically, low-molecular-weight fragments or monomers are identified. The analyst tries to elucidate the original binder structure from the low-weight thermal fragments. The GC prohibits the analysis of higher weight fragments. Alternatively, the pyrolyzates can be introduced directly into the ion source, ionized, and subsequently identified (Py-MS). Unfortunately, the mass spectra are often too complicated for easy interpretation. Other ionization strategies affording simpler spectra must be used. Py-GC/MS and direct Py-MS methods are reproducible and therefore the spectra can be archived for later library comparison in quality control/assurance or in competitive product analysis.

A shortcoming of pyrolytic methods is that the molecular weight integrity of the sample is lost. Copolymers may often not be distinguished from blends. In recent years the advent of desorption/ionization techniques have allowed the analysis of intact molecules exceeding 10 000 Daltons. This work has been primarily focused in the biochemical arena; however, there has been success on synthetic polymers. Furthermore, as the solvent content of paints is lowered to comply with federal regulations, lower-molecular-weight, more reactive oligomers must be used. Examples of desorption/ionization techniques for the molecular weight and structural characterization of polyester oligomers, targeted reaction products, and melamine crosslinkers will be shown.