Cold trapping headspace

Small solid seuaples can be analyzed directly by dynamic headspace sampling using a platinum coil and quartz crucible pyrolyzer and cold trap coupled to an open tubular column (341,369,379). This method has been used primarily for the analysis of mineral samples and of additives, catalysts and byproducts in finished polymers which yield unreliable results using conventional headspace techniques owing to the slow release of the volatiles to the headspace. At the higher temperatures (450-1000 C) available with the pyrolyzer the volatiles are more readily and completely removed from the sample providing for quantitative analysis. 

Collect sample on headspace cold trap system... 

Another configuration of MAP gas-phase extraction relates to dynamic headspace sampling, often referred to as purge and trap sampling. The container can be fitted with an aperture enclosing a trap, or a sorbent, cooled by some common means. This allows the application of a prolonged, low-power irradiation, or of a multi-pulse irradiation of the sample, thus providing a means to extract all of the volatile analytes from the matrix. The contents of the trap can then be transferred (by elution for a chemical or sorbent trap, or by thermal desorption for a cold trap) to an analytical instrument, such as a... 

The headspace is the air above or around a fragrant substance that contains the volatile compounds. This can be collected for analysis when extraction of the volatiles from the material is not viable. This technique has been extensively developed for the collection and analysis of flower volatiles since many flowers do not yield an extract that reflects the odour of the fresh flower, while others are simply too rare to be available in sufficient quantity for extraction. Many different techniques have been applied to the collection of volatiles from the air above flowers including the use of cold traps, solvent traps, adsorbent materials... 

A third sample preparation method is purge and trap, which aims to extract as close to all of the analyte as possible from the solid or liquid sample and is a deviation from headspace sampling. It works by bubbling a purge gas such as helium through the heated sample vial. The gas carries analyte up into an adsorption tube packed with selective stationary phase. After all the analyte has been trapped in the tube, the gas flow is reversed through the tube to remove any residual solvents. The tube is then directed to the injector port and, heated to desorb the analytes, which are then cold-trapped onto the head of the GC column. From there, the concentrated sample is heated for GC separation. 

When analytes are under the limit of detection (LOD) of the technique is necessary to use enrichment techniques. In headspace analysis, for this purpose the target analytes must be separated from the headspace gas either by absorption into a liquid or by adsorption onto a solid adsorbent and also by condensation in a cold trap. (Kolb, 1999). Solvent free techniques are particularly desirable in case of trace analysis to avoid problems with solvent impurities. Consequently, cryogenic trapping is the preferred choice to improved detection limits in static headspace analysis... 

Volatile and aromatic components Cocoa volatile and aromatic components are analyzed by capillary GC, with samples derived from headspace enrichment or cold-trapping techniques. Analysis is similar to that of coffee volatiles FIDs are used and identification is by the use of reference compovmds. 

Fig. 1 Comparison of dynamic, static, and SPME headspace sampling, (a) Dynamic headspace sampling uses a sorbent or cold trap to concentrate volatile analytes before analysis by the GC. (b) Static headspace sampling uses direct transfer of a volume of gas from the headspace above the heated sample vial directly to the GC for analysis. Injection designs are illustrated in Fig. 2. (c) SPME headspace sampling uses a fiber support with solid-phase coating. The fiber is placed in the headspace and reaches equilibrium with the headspace volatile analytes. The SPME fiber is transferred by means of a syringe and thermally desorbed in the injector of the GC for analysis.

Fig. 5.9. Apparatus for the gas chromatography-olfactometry of static headspace samples. 1 Sample in ther-mostated glass vessel, 2 septum, 3 gastight syringe, 4 injector, 5 hydrophobed glass tube, 6 carrier gas, e. g, helium, 7 purge and trap system, 8 cold trap, 9 gas chromatograph with capillary column, 10 sniffing port, 11 flame ionization detector (according to Guth and Grosch, 1993)...

Raisanen [990] has described a device consisting of a cylindrical sample bottle holder, a dry carrier gas flow system, and a moisture transducer as a non-toxic replacement for Karl Fischer analysis of plastics. The sample of test material contained in a 20 mL septum bottle is heated to a preset temperature, usually just below the softening point of the resin. Evolved volatiles in the bottle headspace are passed through a cold trap filter to an analysis cell where the moisture content of the flowing gas is measured. High boilers are filtered out. A sophisticated algorithm, which makes use of the fact that as the sample approaches total dryness the rate of evolution of water is proportional to the water remaining in the sample, allows accurate determination of the... 

P. Werkhoff and W. Bretschneider, Dynamic headspace gas chromatography concentration of volatile components after thermal desorption by intermediate cryofo-cusing in a cold trap. II. Effect of sampling and desorption parameters on recovery, J. Chromatogr. 488 (1987). 

In the SPME extraction method, the fiber was exposed to the headspace above the truffle sample for 30 min at either 80°C or at room temperature. In the HS Tenax adsorption procedure, thin strips of sample material were placed between plugs of glass wool in an empty stainless steel tube, which was then mounted in a desorber oven. The oven was heated to 60°C, and approximately 20 mL of He gas was used to flush the tube to the Tenax-filled cold (-40°C) trap over a 4-min period. The trapped volatiles were desorbed by rapid electrical heating to 250°C for 45 seconds and transferred to a GC column. 

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