Differentiation of 16-O and 18-O by LIBS measurement of aluminum-monoxide molecular emissions

Abstract

A possible approach to analyze water absorption in thin-film polymers is to measure the content of oxygen introduced in the form of water. For this purpose, a differentiation between initial oxygen content (e.g. from functional groups in polymers) and oxygen resulting from water absorption by isotopic labeling is necessary. Conventional techniques to analyze water contents require bulk samples or do not allow spatially resolved analysis on thin-films.Laser-Induced-Breakdown-Spectroscopy (LIBS) is a well-suited technique to perform spatially resolved analysis and measurements on molecular emission bands. A pronounced isotopic shift for AlO molecular emission bands in LIBS spectra is reported. So far, AlO bands have only been measured on samples, where aluminum and oxygen already are present in the form of a chemical compound, such as in Al2O3. This work focused on the proof of concept regarding the generation of a laser-induced aluminum plasma (LIP) and external insertion of H218O into the LIP. H218O works as a source of oxygen and allows to introduce oxygen directly into the aluminum plasma. This way, both separated components are brought together and a reaction between aluminum and oxygen is forced, resulting in AlO molecular emission.Thus differentiation of generated Al18O and Al16O bands, resulting from natural abundant 16O, can be realized. Conducted experiments included the generation of an isotopic shift in AlO emission bands and investigation of a possible correlation between introduced amounts of oxygen and signal intensities, thus investigating the possibilities for quantification. Within this work, the adjustment of necessary experiment parameters is shown. The experiments demonstrate the possibility of external H218O introduction direct into the LIP and thus differentiation of Al16O and Al18O emission bands via the isotopic shift. Varying amounts of introduced H218O produces different signal intensities in recorded Al18Obands, allowing the application of a simple calibration method. The discussed method requires optimization and further studies to achieve application for thin-film polymer analysis.