RESUMEN
This work explores the performance of laser ablation-inductively coupled plasma mass spectrometry using different types of single-collector devices (sector field and time-of-flight instrumentation) for lead isotopic analysis of bronze coins, minted in the ancient city of Bilbilis. The aim of the study was achieving sufficient discrimination power to reveal similarities and differences for coins originating from different historical periods, and to obtain information on the possible source of the lead ores used in their production, while restricting the damage inflicted to the samples such that it is not visible to the naked eye. It was found that satisfactory results (RSD in the 0.15-0.30% range for (207)Pb/(206)Pb and (208)Pb/(206)Pb ratios) could be finally obtained, despite the noisy nature of signals generated upon ablation of the highly inhomogeneous coins, by means of a methodology based on: (a) selection of the line profiling ablation mode; (b) use of a dual pass spray chamber that permits the simultaneous introduction of a solution (containing thallium of known isotopic composition), thus resulting in a wet plasma that showed an increased robustness towards matrix effects and (c) detection using a TOF-ICPMS unit, which proved to be much better suited to deal with the transient signals obtained, while being also sufficiently sensitive to obtain good counting statistics, owing to the high lead level (average around 5%) present in the samples. Moreover, under these conditions, the simultaneous aspiration of the thallium spike permitted accurate correction for mass discrimination, such that it was not necessary to use external matrix-matched standards for calibration.
RESUMEN
Three different cases of deterioration were identified in a single natural shelter containing prehistoric paintings. The microbial communities induced by run-off water, a black crust covering a portion of the shelter, and a cryptoendolithic microbial community, were studied. Molecular analyses based both on DNA and RNA were performed to identify the major components of these microbial communities present and metabolically active, respectively, at the studied location. While similar microbial communities were expected due to the proximity of the studied sites, clearly different communities were detected suggesting that specific microorganisms adapt to different micro-environments. Phototrophic microorganisms represented the major portion of total RNA and DNA in the studied microbial communities. Run-off water induced the presence of a large proportion of putative photosynthetic bacteria belonging to Chloroflexi. Different groups of bacteria were associated to phototrophs and were represented by Alpha, Beta, Delta, and Gammaproteobacteria, Acidobacteria, Actinobacteria, Bacteroidetes, and Planctomycetes. The present study confirms a need for careful monitoring of microbial communities associated to distinct cases of deterioration even within a single location, underlining the interest of phototrophic microorganisms as indicators of colonization on cultural heritage at natural shelters.