Purging, trace analysis

Gas Chromatography. Gas chromatography is a technique utili2ed for separating volatile substances (or those that can be made volatile) between two phases, one of which is a gas. Purge-and-trap methods are frequently used for trace analysis. Various detectors have been employed in trace analysis, the most commonly used being flame ioni2ation and electron capture detectors. 

Hg from melted ice was measured by reduction to Hg(0), which was purged from solution by bubbling Ar gas. Hg(g) was trapped by metallic Au coated on sand. (Mercury is soluble in gold.) For analysis, the trap was heated to liberate Hg, which passed into a cuvet. The cuvet was irradiated with a mercury lamp, and fluorescence from Hg vapor was observed. The detection limit was 0.04 ng. Blanks prepared by performing all steps with pure water in place of melted glacier had 0.66 0.25 ng Hg/L, which was subtracted from glacier readings. All steps in trace analysis are carried out in a scrupulously clean environment. 

Gas chromatography (GC) instruments may be equipped with various detectors to accomplish different analytical tasks. Flame ionization and thermal conductivity detectors are the most widely used detectors for routine analyses, nitrogen-phosphorus detectors are used for the trace analysis of nitrogen-containing compounds, and electron-capture detectors are used for halogen-containing compounds. GCs may also be equipped with peripheral accessories such as autosamplers, purge and trap systems, headspace samplers, or pyrolyzer probes for special needs in sample introduction. 

Simmonds PG. 1984. Analysis of trace halocarbons in natural waters by simplified purge and cytotrap method. J Chromatogr289 117-127. 

For the analysis of trace quantities of analytes, or where an exhaustive extraction of the analytes is required, purge and trap, or dynamic headspace extraction, is preferred over static headspace extraction. Like static head-space sampling, purge and trap relies on the volatility of the analytes to achieve extraction from the matrix. However, the volatile analytes do not equilibrate between the gas phase and matrix. Instead, they are removed from the sample continuously by a flowing gas. This provides a concentration gradient, which aids in the exhaustive extraction of the analytes. 

Determination of volatiles at the trace level is also possible by pre-concentrating the headspace volatiles on a suitable adsorbent. The trapped compounds are subsequently recovered by thermal desorption in front of a cooled trap connected to the capillary column or by solvent elution followed by splitless or on-column injection. These methods, called dynamic headspace enrichment or purge-and-trap , have been applied to trace level analysis of volatiles, using conventional electrically heated systems [ 31, 32 ], a Curie-point Pyrolyser... 

For the analysis of traces of volatile materials, purge-and-trap is a common procedure. 

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