Volatile compounds/components

Once a mass spectrum from an eluting component has been acquired, the next step is to try to identify the component either through the skill of the mass spectroscopist or by resorting to a library search. Most modem GC/MS systems with an attached data station include a large library of spectra from known compounds (e.g., the NIST library). There may be as many as 50,000 to 60,000 stored spectra covering most of the known simple volatile compounds likely to be met in analytical work. Using special search routines under the control of the computer, one can examine... 

Evaporation. Evaporation can be used to separate volatile compounds from nonvolatile components and often is used to remove residual moisture or solvents from soHds or semisoHds. Thin-film evaporators and dryers are examples of evaporation equipment used for this type of appHcation. Some evaporators are also appropriate for aqueous solutions. 

When the sample solvent evaporates at the front end of the liquid, volatile compounds co-evaporate with the solvent and start moving through the main column. In this way, volatile components can be lost through the early vapour exit or, if venting is delayed, the most volatile compounds reach the detector even before the end of... 

GC using chiral columns coated with derivatized cyclodextrin is the analytical technique most frequently employed for the determination of the enantiomeric ratio of volatile compounds. Food products, as well as flavours and fragrances, are usually very complex matrices, so direct GC analysis of the enantiomeric ratio of certain components is usually difficult. Often, the components of interest are present in trace amounts and problems of peak overlap may occur. The literature reports many examples of the use of multidimensional gas chromatography with a combination of a non-chiral pre-column and a chiral analytical column for this type of analysis. 

One of the distinguishing features of liverworts is their capacity to produce a variety of volatile, often very fragrant, oils that occur in oil bodies distributed over much of the surface of the organism. Several of these volatile compounds figure prominently in defining geographically different chemotypes in several countries. Toyota et al. (1997) examined 280 specimens of C. conicum collected at Kamikatsu-cho and Katsuura-cho, both Katsuura-gun, Tokushima. Three chemotypes were identified based upon their major components. Chemotype-I-accumulated (-)-sabinene [420], chemotype-II-accumulated (H-)-bomyl acetate [421], and chemotype III was characterized by methyl cinnamate [422] (see Pig. 5.4 for structures). In addition to a number of compounds known from previous studies of C. conicum, three... 

The content of volatile compounds in tea ranges from 0.01-0.03% w/w. More than 630 volatile compounds have been reported in tea (Yamanishi, 1996). Aroma is one of the critical parameters of tea quality despite the very low levels of volatile components which produce the typical tea aroma. 

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). 

The compounds given off during the roasting of coffee are not necessarily found in the finally roasted bean, and so only a few such compounds are included. In a list of volatile components in foods which is regularly brought up to date4 more than 800 volatile compounds are listed for coffee when it is roasted, and of these 60 to 80 contribute to coffee aroma.5 Comparison of the 14 most potent odorants from roasted Arabica and Robusta coffees, revealed significant differences,6 (see Table 2). 

Steam extraction has been used for gasoline and diesel fuel. High-molecular-weight components of the diesel fuel cannot be removed easily, although a total removal of up to 91% is possible. When used to remove low-volatility compounds in a soil with a high percentage of clay, performance is expected to be ca. 85%. The mobile in situ steam extraction system can reduce VOCs in soils by more than 50% of their initial level. Based on pilot studies, the stationary steam extraction system is expected to have a 90% removal efficiency.54... 

The second, more theoretical approach is based on the chemical analysis of the volatile compounds which are emitted. The scope is to find and to identify one or more volatile compounds which are specific for the current status of the cooking process. A possible disadvantage of this approach is that after the identification of volatile components it cannot be assured that suitable gas sensors are available or can be developed. 

The relative volatility, a, is a constant that under equilibrium conditions can be used to express the distribution of a volatile compound between a gas phase made of A and water vapor and a water phase containing A. This constant is for a component A defined as follows ... 

Selecting an approach Off-flavors are typically due to volatile compounds present at extremely low levels. (Flavor is sensed more by the olfactory system than the tongue, which senses only 5 flavors, sweet, sour, bitter, salty, and umami). GC is ideal for detecting low levels of volatile components. In this case, headspace GC will allow you to treat the plastic directly. Since the off-flavor is suspected to be derived from the polypropylene packaging material, you decide to compare different samples ( good vs. bad ) of the material using headspace GC with both a flame ionization detector (FID) and a sniff port. These chromatograms are shown in Fig. 21.9. 

Flavor is one of the major characteristics that restricts the use of legume flours and proteins in foods. Processing of soybeans, peas and other legumes often results in a wide variety of volatile compounds that contribute flavor notes, such as grassy, beany and rancid flavors. Many of the objectionable flavors come from oxidative deterioration of the unsaturated lipids. The lipoxygenase-catalyzed conversion of unsaturated fatty acids to hydroperoxides, followed by their degradation to volatile and non-volatile compounds, has been identified as one of the important sources of flavor and aroma components of fruits and vegetables. An enzyme-active system, such as raw pea flour, may have most of the necessary enzymes to produce short chain carbonyl compounds. 

This is an alternative technique to headspace analysis for the identification and determination of volatile organic compounds in water. The sample is purged with an inert gas for a fixed period of time. Volatile compounds are sparged from the sample and collected on a solid sorbent trap—usually activated carbon. The trap is then rapidly heated and the compounds collected and transferred as a plug under a reversed flow of inert gas to an external gas chromatograph. Chromatographic techniques are then used to quantify and identify sample components. 

Mass chromatography of mlz 146 and 148 and mlz 180 and 182 is shown to be highly selective for di- and trichlorobenzenes. These components are only present in relatively minor amounts. A mass chromatogram at mlz 88 showed the presence of the rather volatile compound dioxane. This sediment sample obviously is heavily polluted with non-biodegraded mineral oil fractions and a number of other components (i.e. stearic acid, chlorinated benzenes), which point to spills of numerous bulk chemicals. 

Based on the quantitative determination of pentadecane it was calculated that—with a sample load of about 200pg—alkanes are detected by this screening method if their concentration is 5ppm or more. It is obvious that highly volatile compounds when present as such (e.g. dioxane) cannot be measured quantitatively because considerable losses of such components occur during the evaporation of the suspension liquid from the pyrolysis wire when it is prepared. Quantities measured for such compounds must therefore be considered as minimum values. 

In addition to MMS, the natural seeds preferred by ants contain other volatile compounds. Ants respond to some of these, and some are present in three or four different seeds. This may mean that the seeds emit several chemical signals, or that the necessary signals have multiple components. However, MMS may also hold other meanings for the ants. There is a trace of MMS in male Cam-... 

The composition of low-volatility two-component compounds in the Ga-Se system as controlled using chemical transport reactions has been studied (Zavrazhnov et al. 2003). For long-term heat treatment of a low-volatility phase in the presence of a transport agent, the annealed phase composition was found to be determined only by the temperature and the nature of the sample. In the case of a two-temperature anneal of gallium selenides with added iodine, the sample composition ranges are presented as a function of the cold and hot zones temperatures. A scheme of their experimental arrangement is shown in Fig. 6.14. 

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