Chromatography

Trace Level VOC Analysis in Different Sample Matrices  

Apr 20 2015 Read 3796 Times

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In the routine analysis for VOC’s (volatile organic compounds), the required system structure is composed of a sampler with special sample techniques followed by GC-MS analysis. The determination of the concentration of low- and medium-volatile organic compounds in drinking and surface water is especially important. 

Easy-to-use and long time reliable handling systems are necessary and irreplaceable. Consequently, a unique ‘in-vial’ purge-and-trap system is presented, which is suitable for solid and liquid samples. The VSP4000 (Versatile Sample Preparator) purge and trap system can handle up to 80 samples in 20 ml standard vials. The sophisticated system is used in the Purge-and-Trap mode and also in the Thermal Desorption mode up to 280°C. The typical detection limit is in the lower ppt range, even for critical substances.
Figure 1. The VSP4000 (Versatile Sample Preparator)

The purge-and-trap technique is particularly suitable to get low detection limits and best performance. In principle there are two purge-and-trap techniques on the market: the standard method and the in-vial purging method. In the standard purge-and-trap method, the sample is pumped from the sample vial into a separate u-shaped purge vessel. To restrict contamination, the purge vessel and the tubing must be cleaned after each cycle. As a result of the u-shaped vessel principle, contaminations can not be avoided especially in the field of trace analysis. The in-vial-purging method uses the sample vial with a sample volume of 10 ml instead of an external purging vessel. 
Functional Principle of the in-vial Purging of the VSP4000
The in-vial purging technique purges directly in the sample vial, where a long and a short needle is inserted into the septum. The long needle extends to the bottom of the sample vial through which the sample is sparged by the carrier gas and the volatile substances are completely blown out and enriched on the analytical trap. The purged water is retained on a Peltier water trap, and thus does not enter the analytical trap. All purged analytes are trapped on the analytical trap at a temperature of -35°C. After the purging process the analytical trap is desorbed thermally very fast and all concentrated analytes are transferred to the capillary column of the GC.

In contrast to the headspace technique, all light and medium volatile analytes are completely purged from the sample as the carrier gas purges the sample completely. With the headspace technique less than 100% of the analytes are available for the GC-MS analysis. 
The in-vial purging technique of theVSP4000 works in principle splitless and allows the cryo-focussing of 100% of the purged analyts on the analytical trap. The classic purge vessel technique requires a split and therefore this principle has a lower performance. 

Figure 2. The schematic principle of the in-vial-purging

Trap Enrichment at -35°C
During the purging process all purged analytes are frozen on a thermoelectrically cooled trap at a temperature of -35°C. This temperature is necessary for trapping all very volatile components e.g. dichlorodifluoromethane with a boiling point of -29.8°C. The trap is packed with Tenax, extremely miniaturised and is used for all analytes from the very volatile components up to the semi volatile components like hexachlorobutadiene. The durable trap allows up to 3000 analyses and is suitable for more than 100 different analytes with one method. 
The small diameter of the trap ensures the lowest possible thermal capacity and therefore rapid desorption. All enriched analytes are abruptly transferred to the directly coupled column of the gas chromatograph resulting in narrow and high peaks. 
Figure 3. Chromatogram showing the difference between in-vial purging and the classic purge vessel technique

Figure 4. Chromatogram showing the performance for vinyl chloride

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