Isothermal effusion cells

The non-isothermal Knudsen eflusion technique was used to study vapor pressures of tars. The experimental details have been described previously [Oja and Suuberg 1997, 199S]. About 10 mg of dry tar was placed into a hermetic effusion cell with a small orifice, from which the saturated vapor effuses out into the vacuum outside the cell. To overcome effects caused by changes in tar con iosition during effusion, the experiment involved first a continuous cool-down followed by a continuous heat-up of the sample. From the mass loss data (by talcing into account both cool-down and heat-up as a whole cycle) the vapor pressure was calculated using the Knudsen equation. 

A major aspect of aU multiple-cell measurements is the relative difference in the effusate distribution due to variations in cell orifice shape, represented by the GFR. This needs to be measured for each set of effusion cells in a complementary run with a standard substance in each cell. Typically these runs are done between activity measurement runs. This is a good opportunity to systematically check the isothermal nature of the effusion cells, as discussed later. With suitable field and source apertures, the source of the molecular beam Ues fuUy within the cross section of the effusion orifice. In this case the flux distribu-... 

ChatiUon, C., Alhbert, M., Moracchioli, R., Rattoret, A. (1976) High-temperature studies by mass spectrometry use of heat pipe devices to maintain isothermal conditions in effusion cells. Journal of Applied Physics, 47, 1690-1693. 

Currently, high temperature mass spectrometry is one of the most powerful methods in high temperature chemistry. A particular feature of this method is a high temperature molecular beEim source, namely a Knudsen (or effusion) cell. A Knudsen cell is an isothermal enclosure with a small orifice of precisely... 

The furnace must provide an isothermal hot zone large enough to contain the effusion ceU(s) thermal gradients within the Knudsen cell cannot be tolerated [44]. In most laboratory applications, long tube furnaces... 

---