Promises of a new versatile field-deployable sorbent tube thermodesorber by application to BTEX analysis in CH4

Abstract

Characterizing trace compounds in gaseous matrices such as natural gas is challenging owing to the low concentrations and the intricate interactions between compounds and matrices present. In contrast to whole gas sampling methods, direct in situ gas sampling with preconcentration of trace compounds on adsorbent tubes followed by thermal desorption and gas chromatography coupled to mass spectrometry (TD-GC–MS) is a powerful method enabling to screen unknown and targeted trace compounds with low detection limits and where moreover sample transfer and associated loss and contamination risks are avoided. Here, a new versatile, field-deployable thermodesorber prototype (‘nCx-TD’, nCx Instrumentation, Garlin, France) fitted with self-assembled purpose-built Tenax TA adsorbent tubes is presented. Its desorption and GC-injection performances are investigated using a 10 ppmv BTEX in CH4 synthetic gas and by contrasting the results to those obtained by solid phase microextraction (SPME) and direct injection of the synthetic gas. Unlike most commercial thermodesorbers, the nCx-TD is characterized by a fast “plug” injection without re-focusing trap, leading to high chromatographic peak resolutions. Between the closely eluting ethylbenzene and m- and p-xylene isomers, the Gaussian resolution obtained at a concentration of 10 ppmv with the nCx-TD was 2.9 while that obtained by SPME and direct injection was respectively 1.7 and 1.9. The nCx-TD-obtained peak resolutions increased significantly with the concentration (1 to 10 ppmv) while the SPME- and direct injection resolutions remained at a low constant level across concentrations tested. A real natural gas sample was sampled through the Tenax TA adsorbent tubes and analyzed via TD-GC–MS using the nCx-TD. More than 50 distinct trace compounds were detected, opening exciting perspectives of adsorbent tube multibed configurations and direct in situ field sampling on adsorbent tubes with in situ analysis through the nCx-TD and field-portable GC's.