Using LA-ICP-MS for the determination of deuterium in an effort to analyze water absorption by thin polymer films

Abstract

Polymers are a class of substances employed in a variety of fields, considering that their structural and mechanical properties can be fine-tuned for the desired application. When mechanical, thermal and chemical stability are required, high-performance polymers are employed, which in their application setting can be subject to moisture that degrades the polymers. Hence, water absorption by polymers – in particular thin polymer films – is of high technological interest, specifically for the semiconductor and microelectronics industry. For material characterization, water absorption is mostly analyzed gravimetrically or by titration, but the employed methods are either indirect or require large sample quantities. The presented work aims to realize the analysis of water absorption by means of Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) as a semi-non-destructive technique for the direct, elemental analysis of hydrogen. Deuterium analysis via ICP-MS was recently achieved for mixtures of regular water and deuterium oxide by detecting the polyatomic ion 40Ar2D+, which is formed in the ICP plasma through collision. However, analyzing solids via LA-ICP-MS based on the 40Ar2D+ ion is a novelty. A major advantage of Laser Ablation as sample introduction method compared to liquid ICP-MS is the enhanced rate of sample introduction, reducing wash-out times by orders of magnitude. Additionally, the sample does not have to be transformed into a liquid; hence, sample consumption and contamination are decreased, whilst spatial information is obtainable too. Transfer to Laser Ablation is accomplished by spiking polymer matrices with benzoic acid-2,3,4,5,6-d5 so as to prepare standards with defined amounts of added deuterium. Two types of solid standards are prepared: pellets and polymer films, where the pellets are mainly used for method development. Both ultimately allow for the quantification of deuterium, but only the film standards are investi¬gated more thoroughly as they match the matrix of the desired application example to a greater extent. For instance, ablation in helium and argon atmosphere are compared and internal standardi-zation approaches discussed. The optimized LA-ICP-MS parameters allow for calibration of deuterium in a region of 0-10 mg⸱g-1, where four standards with 5 mg⸱g-1 exhibit a relative standard deviation of 7-10 % (based on nine repeat measurements each). In principle, the calibration of the film standards facilitates the analysis of water absorption within 0-50 mg per gramme of polymer, when respecting the stoichiometric factor of deuterium in deuterium oxide. Water absorption of three industrial polyimide types is analyzed by subjecting samples to deuterium oxide in liquid form. First, samples were frozen to -18 °C in order to trap the water in the polymer. Then, the samples are ablated in a cooling stage, considering that water desorbs from the surface due to the dry ablation gas, creating a comparatively large background at the mass corresponding to the 40Ar2D+ ion. The in-house produced cooling stage is operated at -12 °C, thus reducing the background and enabling the detection of differences in water uptake between the polyimide types. The results of this thesis provide a foundation for further research on water absorption by polymers via LA-ICP-MS. The ability of Laser Ablation to obtain spatial resolution, i.e., the possibility to perform depth profiling/imaging, is a key benefit that makes the presented method a promising candidate to analyze local differences in polymer degradation caused by water absorbed by the matrix.