Use of laser ablation in quantitative analysis of the elemental composition of art pigments

Abstract

535.33The ability to use laser ablation for preparation of art pigment samples in quantitative analysis of their elemental composition by atomic emission spectroscopy of inductively coupled plasma is shown. The proposed technique enables one to eliminate errors associated with both the influence of strong acids and the stoichiometric disruption in a sample. Introduction.A study of the technical features underlying art works (stratigraphic structure, color palette, materials, ratio of materials in each structural element, etc.) can provide the information required to define the period and site of creation of a work and its authenticity [1] and can identify the optimal method for minimizing losses during restoration. Despite the development of highly sensitive physicochemical analytical methods, the identification of art materials remains one of the most complicated steps in the technical study of art works owing to the uniqueness of each specimen. Laser spectral microanalysis is currently used more and more often to identify inorganic pigments because it does not require preliminary sample preparation and enables the elemental composition in each structural fragment of the work to be determined. The art pigments used by the master cannot always be unambiguously identified from the observed elemental composition because several elements occur in different pigments. Not only the elements present but also their quantitative contents must be determined. Atomic emission spectroscopy of inductively coupled plasma (ICP-AES) is currently one of the most accurate methods for quantitative determination of the elemental composition of various substances [2][3][4].The most important step in the quantitative analysis of the elemental composition of a substance using ICP-AES is sample preparation. The principal goal is to dissolve the determined elements and to avoid losses of components because the analyzed samples are introduced as solutions. Classical methods for chemical dissolution (destruction and mineralization) do not always dissolve completely the samples even if closed autoclave and microwave oven systems are used. Incomplete dissolution of the sample (formation of a precipitate) leads to a loss of part of the sample during its preparation. This is largely responsible for an increase in the total uncertainty of the resulting quantitative data.Mineral pigments differ in their ability to dissolve in acids. This is explained by their chemical structure from the viewpoint of materials science. Mineral pigments can be transition-metal oxides, hydroxides, and salts. The chromophores in these pigments are fragments of a crystal lattice consisting of cations with variable valence (e.g., Fe, Mn, Cu, Ni, Cr) that are bonded to OH groups, O atoms, and H 2 O molecules [1]. Hydroxides and basic or amphoteric oxides dissolve readily in acids. Therefore, lead and zinc white pigments; azurite; malachite; red-lead; verdigris; Paris green; zinc, barite, and strontium yellows; massicot; and others are capable of dissolving. The reaction...