Headspace sampling techniques advantages

The coupling of a pervaporator to a gas chromatograph is one of the most promising uses of pervaporation and is worth a more detailed discussion, because of the advantages that pervaporation presents as compared with both static and dynamic headspace sampling techniques. In the static approach, the sample is placed in a closed chamber and heated until the volatile compoimds in the headspace reach the equilibrium with the sample. Then, part of... 

Of these methods, dynamic headspace sampling is probably the least well known, yet it has a number of advantages over other techniques in use. 

Solid-phase microextraction, first reported by Belardi and Pawliszyn in 1989, is an alternative sampling technique. The method has the advantages of convenience and simplicity, and it does not release environmentally polluting organic solvents into the atmosphere. The method is based on the extraction of analytes directly from liquid samples or from headspace of the samples onto a polymer- or adsorbent-coated fused silica fiber. After equilibration, the fiber is then removed and injected onto the gas chromatograph. ... 

Headspace sampling is another technique that is widely used in the plastics industry for the analysis of volatile components in plastic resins. A major advantage of headspace GC is the relative cleanliness of the sample entering... 

The SPME device not only combines extraction and concentration but also directly transfers the absorbed compounds into a GC injector. These features of HS-SPME provide major advantages over previous headspace techniques. Coupling to GC, GC-MS (including ion-trap), split/splitless and on-column injection or desorption of the analytes in an SPME-HPLC interface have been described. A significant difference in sensitivity between direct and headspace sampling can occur only for very volatile analytes. HS-SPME introduces some selectivity into the extraction technique as only analytes with sufficient vapour pressure at room temperature are detected. An obvious drawback of HS-SPME is that semi- and non-volatiles will not be present in detectable amounts in the headspace. In combination with GC this is actually advantageous and enables faster equilibration than sampling from liquid [992]. 

SPME is a rapid, solventless extraction/concentration technique that affords significantly lower detection levels for higher molecular weight/higher boiling point compounds than static headspace. Its many advantages over other sample preparation techniques for flavor, fragrance, and odor analysis have been pointed out in numerous chapters in this book. 

Corwen [58] used this method for the analysis of ketones and aldehydes in seawater. Halocarbons were similarly separated from environmental samples by Kaiser and Oliver [59]. There have been many other applications of the technique [60-69]. The major advantage of the headspace method is simplicity in handling the materials. At most, only one chemical, the salt used in the salting-out procedure, needs to be added and in most cases the headspace gas can be injected directly into a gas chromatograph or carbon analyser. On the other hand the concentration of organic materials present is limited by the volume of seawater in the sample bottle. This is very much a batch process. 

The advantage of headspace mode is that only volatile components that will not contaminate the GC are injected. InvolatUes do not partition into the headspace and so never enter the injector. Effectively, the analyte is decoupled from the influence of the drug (but see the discussion on validation below). However, many analytes that are amenable to GC by direct injection are not sufficiently volatile to give a high-enough vapour pressure to be detected by conventional headspace injection. These semi-volatile components can sometimes be successfully analysed using a variant of the headspace technique known as total vaporisation headspace injection. In this instance, a few microlitres of the sample solution are injected into the headspace vial, which is then incubated at a temperature that vaporises the solvent completely into the headspace. 

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