Effects of packing density, flow and humidity on the performance of needle trap devices

• The densest packing is not possible with needle trap devices.
• Sampling flow has to be restricted to <10–15 mL min−1.
• NTDs can reach detection limits in the pptv range with 50 mL samples.
• Water saturated samples do not restrict the use of NTDs.

Needle trap devices (NTDs) have become a promising alternative to solid-phase microextraction (SPME) due to their robustness and exhaustive sampling while maintaining all the advantages of SPME. This study investigates the compromise required in packing NTDs starting from the hypothesis that their diameter makes perfect packing impractical. The most limiting parameter of NTDs is the small amount of sorbent that can be fitted in the trap. On evaluating packing density, it is found that the densest packing cannot practically be achieved with NTDs. This poor packing leads to oscillations in the fluid flow profiles and so sampling flows up to 10–15 mL min−1 are recommended for this methodology. The limited amount of sorbent materials inside the needles makes breakthrough another limiting aspect of NTDs. However, one of the most significant advantages of these devices is that they have a large preconcentration factor, which results in method detection limits in the pptv range with sample volumes <100 mL. This methodology gives promising results in the analysis of water saturated samples as the limited amount of sorbents reduces water retention. Moreover, it is desirable for a small amount of water to be retained with NTDs as this improves the desorption of the retained compounds in the GC injector and allows sharper injection band-widths to be obtained.