Thermogravimetric Analysis with a Heated Quartz Crystal Microbalance
Ward L. Johnson, Elisabeth Mansfield
Thermogravimetric analysis with an SC-cut quartz crystal microbalance (QCM) were demonstrated at temperatures in the range from 20 C to 450 C. A measurement system was built around a crystal sensor head that was mounted in a small tube furnace. Changes in third-overtone C-mode frequencies of the crystal were measured during thermally activated decomposition and combustion of a poly(methyl methacrylate) (PMMA) film with an initial mass of approximately 6 μg, and corresponding changes in mass m were estimated through the use of the Sauerbrey equation. Noise and drift in frequencies and associated m were determined through least-square fitting and compared with commercially available high-resolution conventional TGA systems. The microbalance-based TGA (μ-TGA) system is found to have two orders of magnitude lower noise than high-resolution TGA at temperatures below 200 C and to have at least an order of magnitude lower drift over the entire measured temperatures. However, increasing temperature dependence of crystal frequencies at elevated temperatures and noise in temperature measurements lead to noise in the determination of temperature dependent mass above 400 C that is comparable in magnitude to that of high-resolution TGA. Enhancements in performance of μ-TGA depend primarily on the implementation of reliable piezoelectric resonators with low temperature dependence over the entire measured range.