FT-Raman spectroscopy for quantitative analysis of salt efflorescences

Salt crystallization in porous materials constitutes one of the major causes of decay of buildings and archeological sites. Depending on the environmental conditions, salts can crystallize inside the porous network (subflorescence or cryptoefflorescence) on the material surface (efflorescence or compact crust). Generally speaking, the salt deposits may comprise different inorganic ions (i.e. sodium or potassium sulphates, nitrates or chlorides), and they cause several damages, i.e. flaking, alveolar weathering, crumbling, loss of material or dissolution. In order to determine the nature and the concentration of salts, ionic chromatography (IC) is generally used. Although valuable, the IC analytical method presents a number of drawbacks, i.e. requires long sample preparation times, different sample dilutions, and the determined values are referred only to the ionic species (cations or anions). For a quick quantitative identification, the Raman spectroscopy appears more suitable, but a calibration of such analytical method is necessary. The present paper deals with the application of Fourier transform (FT)-Raman spectroscopy to the study of soluble salts constituting the most diffuse efflorescences in monuments and archeological sites. The FT-Raman calibration was set up by measuring the band integration area of each standard salt solution at the more intense and/or well-resolved band. Twelve control mixtures were tested, and the calibration curve fitting was evaluated. The obtained results were compared with IC determinations performed on the same salt mixtures. The FT-Raman spectroscopy demonstrated to be an easy approach to salt quantitative analyses, that in addition enables the direct identification of the nature of the analyzed samples. Copyright © 2012 John Wiley & Sons, Ltd.