Desorption, aroma compounds, volatile

Activated charcoal High retention of nonpolar volatiles Water absorption. Artifact formation. Some difficulties with desorption Less used for aroma compounds... 

Trap desorption. The choice of the thermal desorption apparatus is critical in order to avoid contamination and to be able to work with aroma compounds in a wide range of retention indices. In all systems, problems can be encountered due to reactive compounds or cold spots within the analyzer. It is recommended that all transfer lines, valves, or surfaces in contact with the volatile compounds be made of an inert material such as fused-silica or deactivated glass-lined stainless steel. Even more ideal are systems that do not have long... 

For volatile trapping, preparation of the traps and conditioning requires -1 hr. Trapping of the aroma compounds requires 15 min. Thermal desorption of the traps followed by GC analysis requires -1 hr. 

If the analytes of interest are volatile or semivolatile, solvent extraction is not always necessary, and head-space techniques (HS) can be applied for the analysis, typically utilizing GC as the final analytical step. HS analysis can be defined as a vapor-phase extraction, involving ftrst the partitioning of analytes between a non-volatile liquid or solid phase and the vapor phase above the liquid or solid. The vapor phase is then transferred further and either analysed as vapor or (ad)sorbed to an (ad)sorbent. The head-space techniques have been widely utilized in the analysis of volatiles, such as fi agrances and aroma compounds, in various food and agricultural samples (81-84). The dynamic head-space (DHS), or purge-and-trap technique, is easily coupled on-line with GC. In an on-line system, desorption of trapped analytes for subsequent analysis is usually performed using on-line automated thermal desorption (ATD) devices. 

Distillation, as defined in this chapter, also includes direct thermal analysis techniques. These techniques involve the heating of a food sample in an in-line (i.e., in the carrier gas flow of the (jC) desorber. (jenerally, aroma compounds are thermally desorbed from the food and then cryofocused to enhance chromatographic resolution. This technique has been used for a number of years for the analysis of lipids and was later modified to include aqueous samples [32,33]. Aqueous samples were accommodated by including a water trap after the desorption cell. This general approach has been incorporated into the short path thermal desorption apparatus discussed by Hartman et al. [34] and Grimm et al. [35]. A schematic of this apparatus is shown in Figure 3.5. In the schematic shown, a sample of food is placed in the desorption tube and quickly heated. The volatiles are distilled into the gas flow that carries them into the cooled injection system where they are cryofocused prior to injection into the analytical colunm. 

The aroma compounds of dry cured Parma ham have been analyzed by thermal desorption GC-MS after extraction by means of the DHS technique [7], Using GC MS, 122 substances, including hydrocarbons, aldehydes, alcohols and esters, were identified in the volatile fraction. The same research group used DHS and SDE techniques to isolate aroma components from Parmesan cheese [8], By means of GC MS, more than 100 substances were characterized in the extracts. This application is discussed later in this chapter in section 3. 

The volatile compounds in the atmosphere of cold stored Black Perigord Truffles (Tuber Melanosporum) were adsorbed onto a Tenax trap by means of a vacuum pump. The efficiency of the sampling method was sensorially validated. The volatiles eluted from the trap by heat desorption were analysed by capillary gas chromatography - mass spectrometry. A total of 26 compounds was identified. Their contribution to the final aroma impression was discussed. 

In a similar vein, Parliment (33) used reverse phase C18 adsorbents to concentrate and fractionate low levels of volatile organic compounds from dilute aqueous streams. The aqueous phase must be particulate free to prevent fouling of the adsorbent, but soluble solids are not necessarily a problem. For example, a commercial cola beverage containing caramel color, caffeine, phosphoric acid and sweetener was passed over a reverse phase column. Desorption with acetone produced an aroma concentrate which could be analyzed by gas chromatography. 

Identified by Merritt and Robertson (1966) in their coffee aroma (see Section 5.B), by Ho etal. (1993) in the volatile compounds of a roasted Colombian coffee (headspace trapping with short-path thermal desorption GC/MS), the concentration given is 0.1 ppm. 

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