Understanding the mechanism of nitroaromatic vapour uptake in PDMS-based pre-concentrators using 4-nitrotoluene

Abstract

Pre-concentration is an important tool to improve the detection of low vapour pressure analytes. Quantification of the amount of analyte sorbed and released and how it relates to the pre-concentrator material and analyte of interest is important for analyte detection and pre-concentrator material development. In particular, establishing the diffusion mechanism enables correlations between the type of sorbing phase and the sorption mode to be identified, which may then provide strategies to improve pre-concentrator materials. We describe the development of a UV-vis absorption spectroscopy method to monitor the real time uptake of analyte vapours by pre-concentrator materials and apply it to characterising the uptake of the nitroaromatic 4-nitrotoluene (pNT) by three pre-concentrator materials under continuous flow conditions: poly(dimethylsiloxane) (PDMS), PDMS:1,4-divinylbenzene (DVB), and PDMS-co-DVB. The results show that PDMS and PDMS:DVB films up to ∼100 μm thick initially exhibit pNT uptake consistent with Case II diffusion as opposed to a Fickian process. In contrast, the PDMS-co-DVB films exhibited more complex behaviour, with mass uptake kinetics consistent with anomalous diffusion that varies with film thickness, which we attribute to the presence of different phases in the films. The pNT uptake for the thicker DVB-containing films under continuous flow was lower than that of PDMS although the rate of diffusion was faster, which was attributed to the presence of large pores in these materials. The results demonstrate that the mechanism of diffusion of analyte vapours into PDMS-based pre-concentrators is composition dependent and impacts the balance between the rate of analyte uptake and the total analyte uptake during continuous flow measurements.