Raman Spectroscopy Against Harmful Nitrogen-Based Compounds in Cultural Heritage Materials

Abstract

Nitrogen-based compounds are widespread in the environment due to various sources of natural and anthropogenic origin that introduce them from the most reducing form (the acidic ammonium cation) to the most oxidized (the nitrate anion). In addition, some environmental conditions, such as pH and redox potential, favor secondary reactions of nitrogenous compounds. An example of this is the harmful effect of nitrates on cultural heritage (CH), which poses a threat to its preservation. This is due to their high solubility and mobility, which allow them to penetrate the structure of the materials. As a result of their crystallization/dissolution and hydration/dehydration cycles, the precipitation of nitrate salts in the pores causes internal fractures, leading to the subsequent deterioration and loss of the material. The detection of these salts is a straightforward process in analytical chemistry, but it is imperative to use nondestructive and noninvasive analytical techniques, such as Raman spectroscopy, because of the need to preserve CH. In this work, we have compiled the sources and pathways that contribute to the formation of nitrogen-based compounds, especially nitrate salts in various CH components. Finally, the Raman spectrum characteristic of the nitrate family has also been explained, including the most damaging nitrates found in CH, such as niter, nitratine, nitrocalcite, nitromagnesite, nitrobarite, and nitrammite, and has been compiled. In addition, less common nitrates, some nitrites, and other ammonium compounds have been included in this database.