Influence of the transfer segment between thermal desorber and gas chromatograph applied to essential oil samples

Given the persistent contamination of the injector in gas chromatography (GC) under direct liquid injection (DLI) mode, particularly when analysing complex matrices such as essential oils (EOs), thermal desorption (TD) has emerged as an interesting sample introduction technique to mitigate these limitations, despite the necessity of additional heating. However, the transfer segment (TS), bridging the TD and GC, represents an underestimated part of the equipment. In this study, the influence of various transfer modes on the chromatographic performance, applied to four EOs, was investigated. These modes included a regular transfer line (TL) at a constant high temperature, which was compared with a modulated setup where the GC column was connected directly with the cold trap outlet split of the TD. In this configuration, fixed and programmable TS temperatures were examined. Considerable shifts in retention times and changes in peak areas were noticed, with earlier and later eluting peaks behaving differently. The best results were obtained with a programmable TS temperature, rather than a constant one. Although the peak symmetry was not always optimal, a profound investigation revealed that this was due to early-stage separation of coeluting compounds, thus contributing to improved selectivity.