Dynamic headspace injection

There are basically three methods of liquid sampling in GC direct sampling, solid-phase extraction and liquid extraction. The traditional method of treating liquid samples prior to GC injection is liquid-liquid extraction (LLE), but several alternative methods, which reduce or eliminate the use of solvents, are preferred nowadays, such as static and dynamic headspace (DHS) for volatile compounds and supercritical fluid extraction (SFE) and solid-phase extraction (SPE) for semivolatiles. The method chosen depends on concentration and nature of the substances of interest that are present in the liquid. Direct sampling is used when the substances to be assayed are major components of the liquid. The other two extraction procedures are used when the pertinent solutes are present in very low concentration. Modem automated on-line SPE-GC-MS is configured either for at-column conditions or rapid large-volume injection (RLVI). 

Direct methods of analysis such as distillation [158,167,168], liquid-liquid extraction [ 159,169], headspace analysis [ 170-172], dynamic headspace analysis [157,173-178], and direct injection [179] have been used mainly for specific volatile components. 

Extractant phase None Direct aqueous injection (DAI) Static headspace with gas syringe (SHS) Dynamic headspace/ purge trap (P T)... 

Gas Chromatographic Methods. Gas chromatographic methods may be used for measuring volatile oxidation products. Static headspace, dynamic headspace, or direct injection methods may be employed. Specific aldehydes may be measured as indicators for oxidative stability of oils and fats. Thus, propanal is an and as indicator for stability of omega-3 fatty acids, whereas hexanal is best for following the oxidative stability of omega-6 fatty acids. 

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... 

Gas chromatographic (GC) methods have been used for determining volatile oxidation products. Static headspace, dynamic headspace or direct injection methods are the three commonly used approaches. These methods were compared in an analysis of volatile compounds in an oxidized soybean oil. It was found that each method produced significantly different GC profiles (Frankel 1985). The dynamic headspace and direct injection methods gave similar results, but the static headspace is more sensitive to low molecular weight compounds. Lee and co-workers (1995) developed a dynamic headspace procedure for isolating and analyzing the volatiles from oxidized soybean oil, and equations were derived from theoretical considerations that allowed the actual concentration of each flavor component to be calculated. 

I Dynamic Headspace Furaneol Isolation Using Excess Na2S04.. The most difficult 3 volatiles to isolate from aqueous foods include the sugar related compounds such as (2 Furaneol and Maltol. Until recently the accepted meAod to isolate these compoimds for analysis was to extract the blended, filtered product directly with ether (e.g. in a continuous liquid-liquid extractor). This often required another step to separate volatiles from non-volatiles before GC injection. 

Various gas chromatographic (GC) methods, such as direct injection, dynamic headspace, and static headspace, have been used for the analysis of volatile products, resulting from the oxidative deterioration of vegetable oils. Though advantages and disadvantages are apparent with each GC method, for routine analyses, static headspace is the method of choice because it is rapid and requires no cleaning between samples. ... 

In the dynamic headspace method, the sample is put in a thermal desorption unit in order to desorb the RS a continuous flow of a carrier gas pushes the RS into a trapping system which is refrigerated and where they are accumulated prior to analysis. Then the RS are rapidly desorbed by rapid heating and carried onto the column via the carrier gas. There are different ways to apply this technique. The arrangement when purge gas passes through the sample is often called the purge and trap technique (some other equipment uses the acronym DCI (desorption, concentration, injection)). This method is particularly useful for very low concentrations of RS as the total amount of a substance is extracted and can be applied directly to powders without need to dissolve them. The main drawback is that the dynamic headspace methods are not readily automated. ... 

Table 3.2 Headspace analysis of oxidized soya bean oil (peroxide value 9.5) by three methods direct injection, dynamic headspace gas chromatography (HSGC) and static HSGC. OlOOH = oleic acid hydroperoxide LoOOH = linoleic acid hydroperoxide LnOOH = linolenic acid hydroperoxide.

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.

Abbreviations SHS = static headspace, DHS = dynamic headspace, DI = direct injection, t = trans, t,c = trans,cis, t,t = trans,trans Calculated on the basis of l-octen-3-ol which has the lowest threshold value X 10-5... 

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