Between sampling and analysis of air samples storage delays can occur that sometimes are considerable for remote sampling locations. In several independent storage tests alterations of trace gas mixing ratios via diffusive loss during storage have been observed for CO2 and N2O in our flasks with PFA O-ring seals.
The permeation flux depends on the dimension of the O-ring, the partial pressure difference and the permeation coefficient. This latter exhibits a temperature dependence that may account for some of the variations between the different storage tests.
Flask storage tests resulted in the following correction term applied for PFA-flasks
Flasks sealed with a PCTFE valve have been developed (http://www.laboratory-glassware.com/). This polymer has a smaller permeation coefficient for most gases. Storage tests have not indicated loss of CO2 or N2O since more than one year.
However, it appears that PCTFE sealing is inferior for CO (further tests being performed).