5.3.4.   Influence of the Carrier Gas

In section 5.3.1, it was shown that the Makrolon matrix expands when exposed to analyte for a long time. This effect can also be used for adapting the sensitive layer to a specific analytical problem by modifying the composition of the carrier gas. In figure 26, the kinetics of R134a is shown if pure air and air mixed with R22 is used as carrier gas. It is visible that even the quite low amount of adding 1% R22 to air as carrier gas significantly reduces the time needed for the desorption of R134a. In figure 27 the autoscaled signal of the R134a kinetics with pure air as carrier gas minus the autoscaled signal of the R134a kinetics in an air-R22 mixture is plotted. The positive difference in the plot demonstrates that the faster kinetics caused by the modified carrier gas plays a role for the desorption of highly concentrated R134a. Consequently, the addition of bigger analytes to air as carrier gas can be used to modify the desorption kinetics and to accelerate measurements. At the moment the accelerated kinetics of sorption and desorption can be best explained by the expansion of the micropores during the occupation of the pores by rather big molecules of the carrier gas and its additives (see also section 5.3.1). During the following exposure to analyte, the analyte molecules can sorb faster into the expanded micropores replacing the molecules of the carrier gas. Yet, further research on the exact mechanisms of the expansion of the polymer matrix and on the influence of different carrier gas mixtures has to be done. Up to now, it is only certain that the concentrations and the sizes of the molecules of the additives to the carrier gas play a role.

figure 26:  Autoscaled signal of the sorption and desorption of R134a in pure air as carrier gas and in an air – R22 mixture as carrier gas.

figure 27:  The difference of the autoscaled signals of the sorption and desorption of R134a in pure air and in an air - R22 mixture is plotted versus the concentrations of R134a and the time (60 seconds of sorption and 290 seconds of desorption).