The thermodynamic properties of condensed phases, i.e. the activities of components, can be determined from partial pressures measured by the Knudsen cell mass spectrometric method. Improvements in accuracy and yield of this method are obtained with the use of twin cells, an idea proposed in the 1960s. The multiple cell method was perfected in 1977 in our laboratory. Changes to molecular beam sampling and furnace assemblies were required to make the multiple Knudsen cell technique work properly. This paper summarizes these prerequisites, and presents a new device and the associated method of measurements, as well as the necessary tests performed with a silver sample in each cell.
A special reactor coupled to a high-temperature mass spectrometer was specifically designed for the study of vaporization and thermal cracking of organometallic precursors. This reactor has two kinds of settings. One is a single Knudsen effusion cell which enables the analysis of the composition of saturated vapors and the determination of the partial pressure of each gaseous molecule in equilibrium with its condensed phase. This cell is an evaporation/sublimation cell (operating from 243 to 473 K), which can be tightly closed--like a vacuum chamber--in order to protect organometallic compounds against moisture and atmospheric components. This cell can be independently weighed usefully to evaluate the equilibrium vapor pressures of the sample using the mass-loss method. During experiments, the effusion aperture is externally opened for direct mass spectrometric measurements. The other setting dedicated to the study of thermal decomposition of gaseous molecules consists of a set of tandem cells: the previously described Knudsen cell and a cracking cell (operating from 293 to 973 K).
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