Analysis of plant gums and saccharide materials in paint samples: comparison of GC-MS analytical procedures and databases

Abstract

Background: Saccharide materials have been used for centuries as binding media, to paint, write and illuminate manuscripts and to apply metallic leaf decorations. Although the technical literature often reports on the use of plant gums as binders, actually several other saccharide materials can be encountered in paint samples, not only as major binders, but also as additives. In the literature, there are a variety of analytical procedures that utilize GC-MS to characterize saccharide materials in paint samples, however the chromatographic profiles are often extremely different and it is impossible to compare them and reliably identify the paint binder.

Results: This paper presents a comparison between two different analytical procedures based on GC-MS for the analysis of saccharide materials in works-of-art. The research presented here evaluates the influence of the analytical procedure used, and how it impacts the sugar profiles obtained from the analysis of paint samples that contain saccharide materials. The procedures have been developed, optimised and systematically used to characterise plant gums at the Getty Conservation Institute in Los Angeles, USA (GCI) and the Department of Chemistry and Industrial Chemistry of the University of Pisa, Italy (DCCI). The main steps of the analytical procedures and their optimisation are discussed.

Conclusions: The results presented highlight that the two methods give comparable sugar profiles, whether the samples analysed are simple raw materials, pigmented and unpigmented paint replicas, or paint samples collected from hundreds of centuries old polychrome art objects. A common database of sugar profiles of reference materials commonly found in paint samples was thus compiled. The database presents data also from those materials that only contain a minor saccharide fraction. This database highlights how many sources of saccharides can be found in a paint sample, representing an important step forward in the problem of identifying polysaccharide binders in paint samples.

Figure 1

A comparative scheme of the two analytical procedures.

Figure 2

Content of the sugars freed from the hydrolysis of arabic gum, expressed in ppm as a function of the hydrolysis time.

Figure 3

GCI- chromatograms of a standard solution of aldoses and ketoses; DCCI – chromatogram of a standard solution of aldoses and uronic acids.

Figure 4

PCA score plots obtained using a) both DCCI and GCI profiles as database; b) DCCI profiles as database and GCI as samples; GCI profiles as database and DCCI as samples.