Use of Infrared Spectroscopy in Permeation Tests

The use of infrared (IR) analysis for permeation work has both advantages and disadvantages. Most advantageous is its ability to monitor a permeation test constantly without taking samples, as is necessary for most gas chromatography (GC) systems. Infrared detection also requires little attendance, and breakthrough time is very easy to discern.

Disadvantages include a longer delay in detected breakthrough time (because of the volume of the system) than with GC systems. However, the relative error introduced is quite small. Another problem is the necessity to purge the IR gas cell during calibration or before adding a different sample. This makes simultaneous testing of several cells difficult if not impossible. Last, a high static pressure [12.5 to 25 cm (5 to 10 in.) of H₂O] on the polymer membrane in the ASTM cell is created by the usual high flow rate (5 to 10 L/min) in commonly used IR systems. Although this pressure does not seem to affect permeation, these high flow rates are not necessary, and indeed, lower flow rates will reduce the error in detection of breakthrough time. The calculations and data to support these statements are reported.