Worldwide Solutions - February 2008 - (Page 3) GERSTEL Solutions Worldwide News Comparing automated extraction techniques Dynamic Headspace (DHS) provides highest performance raditional headspace gas chromatography (HS-GC) is a technique that is widely used to determine volatile organic compounds (VOCs) in liquid or solid samples. HS-GC is a rugged and simple-to-perform technique that is easily automated. Unfortunately, HS-GC does not provide the level of sensitivity achievable by Headspace-Solid Phase MicroExtraction (HS-SPME)-GC or by Dynamic Headspace (DHS)-GC. GERSTEL set out to compare the performance of these three techniques based on a range of analytes and the following sample matrices: ground coffee, shower gel and cheese. The GERSTEL MultiPurpose Sampler (MPS) ensured that all three techniques were reliably automated. „Whether for the extraction and concentration of analytes from shower gel, coffee or cheese, the DHS technique won out in all cases while providing quality of results in terms of repeatability equal to the other techniques. DHS is simply more sensitive, T The DHS process from extraction to sample introduction DHS Background and System Overview The DHS station provides sample thermostating and agitation combined with purging of the sample headspace with a controlled flow of inert gas. The result is fast, efficient and reproducible extraction of analytes from liquid or solid samples. Extracted compounds are trapped and concentrated on a replaceable adsorbent-filled trap, which is subsequently thermally desorbed in the integrated GERSTEL Thermal Desorption Unit (TDU) followed by determination of the analytes using GC/MS. While in the GERSTEL MPS autosampler, samples are stored in standard headspace vials at ambient temperature. Optionally, samples can be stored at controlled temperatures between 4 °C to 200 °C. Lower sample temperatures can help reduce decomposition of heat sensitive samples such as food and biological materials. Higher temperatures can be used to simulate sample behavior under “stressed” conditions. During extraction, samples can be agitated to enhance and speed up the extraction process. The temperature of the adsorbent tube during the DHS process can be independently controlled from 20 °C to 70 °C for optimal trapping of the analytes of interest. The adsorbent tube can be dry purged for water removal to ensure the best possible chromatography and MS stability. A new adsorbent tube can be used for every sample, eliminating the risk of cross contamination or the same tube can be used for multiple samples as in standard Purge and Trap instruments. standard disposable 20 mL headspace vials and for each sample, a separate adsorbent tube can be used. This means that carry over from sample to sample can be completely eliminated or at least greatly reduced. For the concentration step, a number of standard adsorbents can be used, such as carbon-based adsorbents, Tenax TA or even PDMS foam sorbent. A selection of e pre-packed adsorbent tubes is available. p Using a tube with two or more adsorbents U for the analysis enables the system to cover fo a wider range of polarities or a wider boiling-point range. in There are no valves or transfer lines in the Thermal Desorption Unit (TDU) used th to desorb the DHS tubes. This means that For more information: AppNote 1/2007: „Automated Dynamic Headspace Sampling using Replaceable Sorbent Traps“, http://www.gerstel.de/p-gc-an-2007-01.pdf Coffee: The DHS technique won out in all cases providing lower detection limits”, says Eike Kleine-Benne, Ph.D., R&D project manager for GERSTEL. In DHS, equilibrium between the phases is deliberately avoided as analytes are purged away from the sample headspace and trapped on a suitable adsorbent. This means that analytes are more efficiently removed from the liquid, viscous or solid sample and transferred to the analysis system providing a marked improvement in sensitivity and detection limits compared with classical Headspace GC. DHS is a simple and reliable analytical tool used to concentrate and determine small amounts of analytes from liquid or solid samples. The GERSTEL MPS performs all DHS steps in a reliable and repeatable manner. Samples are placed in GERSTEL Solutions Worldwide – February 2008 loss of analytes is dramatically reduced. An inert gas transfers analytes efficiently from the sample to the adsorbent tube and later from the tube to the directly attached cool trap and on to the GC/MS system. Compared with SPME, the DHS adsorbent trap provides a much better phase ratio enabling significantly lower detection limits. All steps in the DHS process are selected by mouse-click in the MAESTRO software (cf. Page 18) and are performed reliably by the MPS. The steps in the DHS process are intelligently overlapped using the PrepAhead function. This means that the DHS process for a sample is performed during the GC run of the preceding sample for maximum throughput and system utilization. Since only the headspace is purged, there is no risk of foaming and system contamination and the associated instrument downtime for cleaning. 3 http://www.gerstel.de/p-gc-an-2007-01.pdf
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