Direct sampling / concentration of headspace vapours.

 

The Multi-purpose Direct Inlet Accessory (U-INLET) may be added to any manual UNITY thermal desorption platform to provide a simple and convenient mechanism for direct introduction / 'on-line' sampling of headspace vapours from a wide range of bulk sample containers. Vapours are either pumped or swept through an inert, heated sampling line directly into the electrically-cooled focusing trap of UNITY without first being collected on a sorbent tube. After sample collection, the focusing trap is thermally desorbed to initiate analysis. The entire sampling and analysis process may be carried out on- or at-line and facilitates analysis by real-time detectors (enose sensor arrays, process MS, etc.) as well as laboratory GC(-MS).

 

The Direct Inlet Accessory comprises a purge line (which may be used to direct carrier gas into the sample vessel) and a heated inert sampling line through which headspace vapours are pumped or purged straight into the focusing trap. Sampling times and sequences are all controlled using a dedicated mode of UNITY software. The accessory may be used to sample vapours from the same vessel semicontinuously in a user-defined sequence. This is either done to increase the total volume of vapour sampled and enhance sensitivity or to monitor changes in vapour composition over time. UNITY and the Direct Inlet Accessory may also be combined with some commercial headspace systems to automate the concentration and analysis of multiple samples.

Applications

Key application areas include:

  • characterisation of VOC profiles from natural products and manufactured goods - food, flavour, fragrance analysis
  • measurement of residual volatiles in materials
  • monitoring emissions from living organisms - plants, microbes, fungi, insects, etc.
  • monitoring trace VOC pollutants in environmental water and soil samples
  • off-odour / shelf-life testing
  • monitoring organic vapour products from gas-phase reactions.

Enhanced sensitivity

The UNITY-Direct Inlet system significantly improves the sensitivity of conventional headspace methods because hundreds of millilitres of vapour can be sampled and multiple sample volumes can be introduced to the focusing trap before the trapped vapours are finally desorbed and analysed. Exhaustive extraction of volatiles is possible if this stepwise sampling process is carried to completion. Another advantage is that headspace vapours do not have to reach equilibrium before analysis. This saves time and allows the analysis of compounds over an extended volatility range.

Selective elimination of water and solvents

Selective elimination of unwanted interferents such as water and ethanol simplifies the chromatographic analysis of complex vapour profiles. Selective focusing of key olfactory compounds - esters, ketones and aldehydes - for example, allows detailed flavour profiling of potable spirits while water and ethanol are swept, unretained from the system. This process also facilitates flavour profiling by enose sensors. Without the selective concentration of UNITY and a Direct Inlet Accessory, the concentration of key olfactory compounds would be too low for satisfactory detection and sensor response would be swamped by ethanol and water.

Versatility

The UNITY-Direct Inlet system is compatible with a wide range of sample vessels. It may be used for purging of headspace vapours from smaller, sealed containers (such as reaction vessels or headspace vials) or for pumping vapours from open or compressible containers such as bell jars or Tedlar bags.

 

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