Multivariate treatment of LIBS data of prehistoric paintings

Abstract

Dois fragmentos de pinturas rupestres pertencentes a dois sítios arqueológicos brasileiros, sendo um no estado do Piauí (Sítio Toca da Extrema II) e outro no estado do Rio Grande do Norte (Sítio arqueológico Xique-Xique), foram analisados por LIBS (espectroscopia de emissão em plasma induzido por laser). Um instrumento construído no laboratório (laser Nd:YAG, 1064 nm, 5,2 ns), contendo um policromador echelle, foi empregado para a aquisição dos espectros. Estudos preliminares envolvendo uma pintura realizada sobre uma tela de madeira contendo diferentes camadas de material foram realizados para determinar a melhor energia do pulso de laser. Para cada fragmento de pintura, foram realizados 8 pulsos sucessivos em 16 locações diferentes, sendo 8 locações sobre a pintura e 8 sobre o substrato. Neste trabalho, foi demonstrado que a utilização de técnicas multivariadas é indispensável devido à grande complexidade dos espectros obtidos durante estudo de profundidade de amostras arqueológicas.Laser-induced breakdown spectroscopy (LIBS) was employed in the study of two fragments of prehistoric rock wall paintings found at two Brazilian sites: in Piauí State (Toca Extrema II) and in Rio Grande do Norte State (Xique-Xique). A time-resolved echelle based LIBS system assembled in the laboratory (laser Nd:YAG, 1064 nm, 5.2 ns) was employed for spectral acquisition. Preliminary depth profile studies were carried out in surrogate samples of a multilayer painting on wood to determine the best energy for the laser pulse. For each fragment of wall painting, the depth profile composition was investigated by firing 8 successive laser shots at each of 16 locations (8 in the area of pigment and 8 in the area of substrate). It is demonstrated in this work that the use of multivariate techniques is compulsory due to the high complexity of the spectral data obtained from depth profile studies of the archaeological samples. Keywords: prehistoric paintings, principal component analysis, LIBS, laser-induced breakdown spectroscopy, depth profile IntroductionLaser-induced breakdown spectroscopy (LIBS) is an attractive technique for archaeological analysis because it is nearly non-destructive and does not require sample preparation. It also provides rapid multi-element analysis, even for the light elements, and the spatial resolution is almost microscopic. Depth profile analysis is possible and the use of portable instrument allows for in situ analysis of large and unmovable samples.1-3 LIBS utilizes a pulsed laser focused on a small spot on the surface of the sample. The high irradiance (energy per area) produces vaporization Borba et al. 959 Vol. 23, No. 5, 2012 of a tiny quantity of the sample constituents and promotes the formation of a plasma of high temperature capable of atomize and/or ionize most of the chemical elements present in the sample, which are excited to higher electronic energy levels. After relaxation (and an initial intense continuous emission), atomic and ionic emissions at specific wavelengths for each element occu...

Review on Experimental and Theoretical Investigations of Ultra-Short Pulsed Laser Ablation of Metals with Burst Pulses

“…In 1998, the first LIBS measurements were reported using ultra-short pulsed lasers [ 214 ], and it was found that the ablation process itself and the atom excitation is more reproducible for fs than for ns pulses [ 215 ]. Hence, less energy is needed for excitation, which is crucial for the characterization of specific samples in order to keep the destruction of those limited, e.g., when examining paintings [ 216 , 217 , 218 ] or explosives [ 219 , 220 , 221 ]. Additionally, the sensitivity is higher when using fs lasers [ 215 ].…”

Laser Induced-Breakdown Spectroscopy (Libs): Histórico, Fundamentos, Aplicações E Potencialidades

“…68 A utilização dessas ferramentas são indispensáveis quando o objetivo é usar as informações espectrais para proposição de modelos de classificação. As ferramentas referidas foram empregadas com sucesso na proposição de modelos de classificação para amostras de ligas metálicas, 43 polímeros, 44 couros, 86 cosméticos, 87 pinturas pré-histórica, 88 forense, 89 rochas vulcânicas 90 entre outros. [91][92][93] Os métodos não supervisionados de reconhecimento de padrões, como a principal component analysis (PCA) e hierarchical cluster analysis (HCA) também são empregados.…”

“…As aplicações nesse campo de atuação estão entre as mais desafiadoras e concentram-se nas análises de plantas, fertilizantes e solos. 88,96 Esses desafios se devem basicamente à ampla variedade de amostras e sua complexidade, que refletirá na extensão da interação entre laser-amostra, bem como às condições adversas de trabalho. 26,170 A seguir, apresentaremos uma revisão sobre os avanços da LIBS para análise de plantas, fertilizantes e solos.…”

A Simple Device for Lens-to-Sample Distance Adjustment in Laser-Induced Breakdown Spectroscopy (LIBS)

“…The problem becomes more serious when priceless, rare and unique samples, such as archeological or historic artifacts, need to be probed by a LIBS system. 9–14…”

“…The problem becomes more serious when priceless, rare and unique samples, such as archeological or historic artifacts, need to be probed by a LIBS system. [9][10][11][12][13][14] Auto-focus systems have been described in the literature for improving the accuracy in positioning samples at correct DLS. 15,16 Although very efficient, these systems are usually based on motorized stages and need a CCD camera to locate the exact DLS of the sample.…”