ABSTRACT
The use of permeation tubes as primary standards for trace gas analysis was first documented by O’Keeffe and Ortman in 1966 (1). The principle of this device is based on diffusion of gas or vapor through a plastic membrane at very slow rates. A liquefied gas or volatile liquid sealed in a section of Teflon FEP tubing placed in a metered air stream can be used as a dynamic calibration standard. The diffusion rate is a nonlinear function of temperature; therefore, constant temperature conditions must be maintained for the permeation tube during gravimetric standardization and use as a calibration source. Also, the diffusion rate is influenced by the molecular weight of the carrier gas employed for dilution. The difference between air and nitrogen is negligible; but if a lighter gas such as helium or a heavier gas such as argon is used, corrections or recalibration would be required.
