| AVS 58th Annual International Symposium and Exhibition | |
| Vacuum Technology Division | Monday Sessions |
| Session VT-MoA |
| Session: | Optical and Mass Spectroscopy for Gas Analysis and Pump Modeling |
| Presenter: | Robert Ellefson, REVac Consulting |
| Authors: | R. Ellefson, REVac Consulting D. McClelland, Mound Technical Solutions, Inc. |
| Correspondent: | Click to Email |
Gas sampling through long, small-bore capillary tubing has long been used as a method to reduce the atmospheric pressure (or higher pressure) process gas to a low pressure for analysis by a mass spectrometer (MS). With care of sampling system design and operation, the species integrity of the process gas can be preserved in the sampling which enables accurate compositional analysis. The gas dynamics of the gas stream within the capillary tube equilibrating with the capillary wall and measured at the MS leads to a stable composition at the MS when equilibrium is achieved. A model that includes the wall material interaction is presented with prediction of stabilization time for various gas species. Data from four different capillary materials or surfaces are given to show the interaction process. The capillaries tested are: 304SS, 304 Sulfinert® SS, PEEK (polymer) and fused silica tubing. All capillaries have 0.25 mm i.d. and a 2 m length for direct comparisons. Composition profiles versus time are measured for a dry nitrogen sample followed by room air (50% RH) which shows the gas dynamics of the equilibration of adsorbed gases (e.g. H2O and CO2)) with the various interior surfaces of each capillary. The effect of capillary length and i.d (defining the surface area to be equilibrated) is included in our model and measurements. Equilibration times of 50 sec for H2O are seen at room temperature for a 2 m capillary with 10 sccm flow rate. Longer times are needed to reach the low H2O concentration in the nitrogen (drying the capillary surface). Raising the temperature of the capillary reduces equilibration time as expected.
The exit end of the capillary flows into the low pressure region created by the sampling forevacuum with a port to the MS for analysis. The effect on equilibration time of a Silcotek® surface treatment of interior surfaces of the inlet to the MS is measured and compared with equilibration time for the regular 304SS surface of the machined inlet.