Headspace Volatile Compounds

Much interest is devoted to the analysis of volatiles released from food materials be it fresh or during a processing function such as cooking for example. The introduction of a gaseous material onto a gas chromatographic column is a most simple step to achieve and, as the name implies, is a most suitable way to achieve a separation of the components of the same gas. There are situations, however, where the separation of the volatiles is difficult as a result of the wide range of compounds that can be produced in the headspace or by simple virtue of their relatively low concentration. Hence it can be necessary 

Our laboratory has also introduced a related apphcation for the detection of food volatiles by applying the Microwave-Assisted Process (MAP ) to fresh citrus fruits [34-40]. These examples demonstrate well the capacity GC has to monitor a wide range of chemical functionahties with a single analytical protocol. 

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Chaintreau, A., Grade, A., and Munoz-Box, R. Detertrrinationof partition coefficients and quantification of headspace volatile compounds, Anal. Chem., 67(18) 3300-3304, 1995. 

Headspace volatile compounds were collected by using a SPME (Supelco Co., Bellefonte, PA) fiber coated with Carboxen-poly(dimethylsiloxane) (75 pm thickness, 10 mm length). Samples were ground with a scalpel and 2 g were weighed into 5-mL (actual volume) screw-capped vials. Vials were tightly stoppered with hole-caps with a Teflon-rubber disk. For extraction, vials were hold at 30 C in a water bath, and the fiber was inserted into the vials and exposed to headspace for 30 min. Sample order was randomized. 

Rizzolo, A. Gerh, R Piinzivalli, C. Buratti, S. Torreggiani, D. Headspace volatile compounds during osmotic dehydration of strawberries (cv Camarosa) Influence of osmotic solution composition and processing time. LWT—Food Science and Technology, 2007,40(3), 529-535. 

Figure 11. Effects of initial peroxide contents on the headspace volatile compounds of soybean oils during storage at 55°C.

Means of headspace oxygen contents expressed in % after 1, 2, 3, 4, 5 and 6 days storage Means with the same letter are not significantly different at p=0.05 Means of headspace volatile compounds expressed in the electronic counts of gaschromatographic peak after 1, 2, 3, 4, 5 and 6 days of storage. 

A Chaintreau, A Grade, R Munoz-Box. Determination of partition coefficients and quantitation of headspace volatile compounds. Anal Chem 67 3300-3304, 1995. 

Unless otherwise noted, PDMS/DVB fiber was used for sampling headspace volatile compounds. 

F. Isolation of Headspace Volatile Compounds from Samples... 

The SPME technique has been developed by Arthur and Pawliszyn [8] and now is well known as a simple, rapid, and sensitive sampling method for liquid or gaseous volatile samples. However, the analytes adsorbed on the SPME fiber generally depend on their polarities and the SPME fiber affinity. Because coffee aroma consists of compounds having many kinds of functional groups, proper selection of the type of fiber to use is important to obtaining accurate and reproducible results [15]. In 2000, in a comparison of three fibers (PDMS, PDMS/DVB, and Carboxen/PDMS) for brewed coffee headspace volatile compounds, it was reported that PDMS/DVB gave the overall best sensitivity, especially for phenols such as 2-methoxyphenol, 4-ethyl-2-methoxyphenol, 4-ethenyl-2-methoxy-phenol, and polar compounds such as 4-hydroxy-2,5-dimethyl-3(2//)-... 

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). 

In static headspace sampling [301,302] the polymer is heated in a septum-capped vial for a time sufficient for the solid and vapour phases to reach equilibrium (typically 2 hours). The headspace is then sampled (either manually or automatically) for GC analysis, often followed by FID or NPD detection. Headspace sampling is a very effective method for maintaining a clean chromatographic system. Changing equilibrium temperature and time, and the volumes present in the headspace vial can influence the sensitivity of the static headspace system. SHS-GC-MS is capable of analysing volatile compounds in full scan with ppb level... 

Headspace solid phase microextraction (HS-SPME). With this extraction technique, it is possible to concentrate volatile compounds thus allowing their detection even at trace levels, as in the case of volatile and semi-volatile terpenes in archaeological findings [7,31]. Chapter 10 outlines how resinous materials are investigated using HS-SPME-GC/MS. 

SPME can be used to extract organic compounds from a solid matrix as long as target compounds can be released from the matrix into the headspace. For volatile compounds, the release of analytes into the headspace is relatively easy because analytes tend to vaporise once they are dissociated from their matrix. For semi-volatile compounds, the... 

These results demonstrate clearly that headspace SPME/GC-MS is well adapted to the detection of volatile or semi-volatile terpenes from resins or gum resins. The method is rapid and simple. A moderate heating (80°C) of the sample allows the extraction of less volatile compounds such as particular diterpenes or diterpenoids which are more specific. 

E. E. Stashenko, J. R. Martinez, Sampling volatile compounds from natural products with headspace/solid phase micro extraction, J. Biochem. Biophys. Methods, 70, 235 242 (2007). 

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. 

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. 

More recently, solid phase microextraction (SPME) [22] has been applied to the analysis of bug pheromones, using two techniques. In the first, headspace volatiles are trapped on the SPME fiber, analogous to trapping on SuperQ [e.g., 23]. Alternatively, if the source of the pheromone is known, the SPME fiber can be wiped on the cuticle to directly adsorb the compounds [24]. In either case, the fiber is then thermally desorbed directly into a GC or GC-MS. Whereas this method is excellent for analysis, with good recoveries, it does not provide a sample that can be used for bioassays or for isolation of an active compound. 

Various sample enrichment techniques are used to isolate volatile organic compounds from mammalian secretions and excretions. The dynamic headspace stripping of volatiles from collected material with purified inert gas and trapping of the volatile compounds on a porous polymer as described by Novotny [3], have been adapted by other workers to concentrate volatiles from various mammalian secretions [4-6]. It is risky to use activated charcoal as an adsorbent in the traps that are used in these methods because of the selective adsorption of compounds with different polarities and molecular sizes on different types of activated charcoal. Due to the high catalytic activity of activated charcoal, thermal conversion can occur if thermal desorption is used to recover the trapped material from such a trap. 

In a more comprehensive study, 46 constituents were identified in the interdigital secretion of the white-tailed deer, 0. virginianus [129]. Only relatively volatile compounds up to methyl salicylate were identified in the secretion, because samples for GC-MS analysis were enriched from the headspace gas of the secretion collected on cotton swabs. Some variations in the relative concentrations of the compounds between the secretions from dominant and subordinate animals were observed, but it was not possible to conclude definitely whether these differences were related to age or dominance. 

Several sampling procedures are applicable to volatile compounds but method application often depends on the compound(s) to be sampled (Dean, 2003). Part of the issue of sampling volatile compounds arises because some volatile substances sublime rather than boil, whereas other volatile substances emit significant quantities of vapor well below their boiling point. For sampling volatile hydrocarbons in the field, two procedures are generally recommended zero headspace and solvent extraction. However, these two procedures do not necessarily give equivalent results. 

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