VSI EV-101 Vial Evacuator. The instrument prepares vials for whole air (or soil water) sampling by drawing a vacuum, filling them with helium (or other gas) and drawing a vacuum again. This is done by piercing the vial septa with needles. The turret holds 12 vials (standard 20 ml size) for convenient batch conditioning. The instrument can be factory adapted to hold vials of other dimensions as required. The evacuated vials / vacuum tubes can be used with a range of commercially available whole air sampling systems including the VSI Automatic Air Sampler AS-100 for lab and field use, and the UAV-based AS-110 Whole Air Sampler. Evacuated vials work with Rhizon soil water samplers.
In brief, the VSI EV-101 Vial Evacuator in its standard configuration uses 20 ml crimp-top glass vials as sample vials, sealed with 5 mm thick PTFE-lined grey butyl rubber stoppers and aluminium crimp caps. The Vial Conditioner can be loaded with up to 12x 20 ml glass vials and conditions the vials by rinsing and evacuating. After vacuum is applied, crimp-sealed vials can be purged with synthetic air, helium (or similar) for 1 min at 200 mL min-1 using two G26 side port needles. The number of flush - evacuation cycles can be adjusted. Finally, the rinsed vials are evacuated using a rotary vane pump through a single G26 side port needle to a final pressure of approximately 0.5 Pa. Our system has been laboratory tested to produce well evacuated vials with no contaminant gases after flushing. The EV-101 can be factory adapted to suit other vial sizes and types (e.g. 10 ml vials, other dimension etc.) as required.
The vial conditioner VSI EV-101 has e.g. recently been demonstrated to successfully prepare sample vials for whole air sampling in a UAV-based sampling campaign, using VSIs automatic UAV air sampling device. See Leitner, S., et al. (2023). "UAV-based sampling systems to analyse greenhouse gases and volatile organic compounds encompassing compound-specific stable isotope analysis." for details.
Images for illustration purposes only, design subject to change without notice
Conditioning of whole air sampling vials typically involves the removal of any contaminants that might interfere with the analysis of the collected air sample. This is usually done by flushing the collection vessel with clean air or a specific gas prior to evacuating vials by a vacuum pump (Cambaliza et al. 2009). Multiple cycles of flushing and evacuation may improve the conditioning process. The best method used to condition vial depends on the type of sample being collected and the analysis being performed.
Some common methods are:
The interaction of the vial surface with the sampled air may become more important for some (e.g. 18O) analysis and for humid samples. Therefore, drying the sample during sampling, potentially by flushing the sampled air through magnesium perchlorate, and keeping the vials dry prior to sampling may minimise the potential effects of water and vial surface exchange. In general, Borosilicate- or Na-glass vials are frequently considered more suitable for gas sample tasks than e.g. polystyrole syringes (Rochette & Bertrand 2003). However, these authors and Knohl et al. (2004) suggested that the septa, rather than the (glass) vial itself, may be a major source of contamination. A possible mechanism could be a immediate contamination of vial contents by "slow closing" septa, diffusion of e.g. CO2 through the septa, or absorption or release of CO2 and other substances by the septa material. Thus the insertion of a more inert disc between (e.g. butyl rubber) septa and the gas sample may minimise the effect of contamination and increase the storage time of whole air samples. Potential materials for such disks are Polychlorotrifluoroethylene (PCTFE) or Polytetraflourethylene (PTFE). The pressure to which a vial should be subsequently evacuated for air sampling depends on the specific application and the analytical method being used. In general, the vial should be evacuated to a pressure low enough to ensure that the air sample is representative of the ambient air, but not so low as to compromise the vial. For most whole air sampling applications, the vial is typically evacuated to a pressure lower than 10 Pa (0.1 mbar), depending on the specific requirements of the analysis. The EV-101 is able to reach a low final pressure of approximate 0.5 Pa using a rotary vane pump.
The advantages and disadvantages of specific conditioning methods depend on the nature of the air sample to be collected and the analytical procedure. In any case, purging with clean, synthetic air or another inert gas before and in-between evacuation cycles is a simple and effective method that can be used for most whole air sampling applications. In any case, it is advisable to apply external referencing (external standards), e.g. vials with air at ambient CO2 mixing ratio and known isotopic composition co-positioned into the autosampler, to minimize potential sampled-based error sources and determine the precission of subsequent analysis. In sum, it is important to use appropriate equipment and techniques to ensure that most suitable vial / septa types for whole air sampling are properly conditioned. The VSI EV-101 has been shown to fulfill all requirements of a reliable vial conditioning device, giving the flexibility to be adapted to various vial geometries as required.