Detectors olfactometry

Fig. 3. (a) Flame ionization detector (fid) response to an extract of commercially processed Valencia orange juice, (b) Gas chromatography—olfactometry (geo) chromatogram of the same extract. The abscissa in both chromatograms is a normal paraffin retention index scale ranging between hexane and octadecane (Kovats index). Dilution value in the geo is the -fold that the extract had to be diluted until odor was no longer detectable at each index. 

Figure G1.8.1 Diagram of the sniff port constructed from a laboratory filter (based on Acree et al., 1976 see Acree, 1997) showing the filter pump (with the check ball removed) attached to a humidifier, shut-off valve, and charcoal filter. The vacuum side of the pump is positioned over a flame ionization detector (FID) with the hydrogen gas turned off. The make-up gas helps lift the narrow (<0.2-mm-o.d.) gas chromatography (GC) effluent stream into the much larger olfactometry air stream without loss of resolution, and the 300 ml/min air combustion gas produced by the FID also prevents loss of resolution.

The method of choice for the analysis of odorants and subsequent identification is gas chromatography-olfactometry (GC—O), a method which combines the separation capability of volatile compounds by GC with the selective and sensitive odor detector human nose (Fuller, Steltenkamp and Tisserand, 1964). A scheme of the set-up can be seen in Figure 8.6. 

Odor-active components in cheese flavor, many of which are derived from milk lipids, can be detected using GC-olfactometry (GC-O). GC-0 is defined as a collection of techniques that combine olfactometry, or the use of the human nose, as a detector to assess odor activity in a defined air stream post-separation using a GC (Friedich and Acree, 1988). The data generated by GC-0 are evaluated primarily by aroma extract dilution analysis or Charm analysis. Both involve evaluating the odor activity of individual compounds by sniffing the GC outlet of a series of dilutions of the original aroma extract and therefore both methods are based on the odor detection threshold of compounds. The key odourants in dairy products and in various types of cheese have been reviewed by Friedich and Acree (1988) and Curioni and Bosset (2002). 

The standard measurement procedure for odor determination (VDI, 1994) is called olfactometry. It uses the human olfactory sense (Gostelow et al, 2001) for the determination of odor qualities. The human nose is an extremely sensitive odor detector and is used in subjective and objective sensory measurements. The latter expresses the strength of odor in terms of the number of dilutions of odor-free air required to reduce the sample odor to threshold concentration (Gostelow et al, 2001). The threshold concentration is reached when the human nose can just smell the odorous substance. Because every nose has different sensitivity, the standard test procedure involves four people at the same time. Prior to olfactory... 

Since all aroma molecules are more or less volatile, the technique that a priori is best suited to screen the odor active molecules from the rest of molecules is Gas Chromatography-Olfactometry (GC-O). This technique makes use of the human nose as detector for the compounds eluting out of the chromatographic column, typically a fused silica capillary column (Acree et al. 1984). There are several different approaches for GC-O differing in the way in which the olfactometric signals are... 

Ferreira, V, Pet ka, J., Aznar, M., and Cacho, J. (2003b). Quantitative gas chromatography-olfactometry. Analytical characteristics of a panel of judges using a simple quantitative scale as gas chromatography detector. J. Chromatogr. A., 1002, 169-178. 

Analysis of trace compounds. All fractions were checked by capillary gas chromatography (GC) with FID and sulfiir specific detection (flame photometric detector, FPD ThermoQuest CE, Egelsbach). Subsequently the different fractions were analyzed by capillary gas chromatography-mass spectrometry (GC-MS). Specific unknowns were enriched by preparative multidimensional gas chromatography (MDGC). For further structure elucidation complementary analyses using GC-MS and capillary gas chromatography-Fourier transform infrared spectroscopy (GC-FTIR) as well as H-NMR were applied. All new compounds have been synthesized and characterized by GC-olfactometry (GC-0). 

Aroma of muskmelon, sulfur volatile sensory evaluation by GC-olfactometry, 36-47 Aroma threshold, determination, 81 Aroma volatiles in meat, sulfur-containing, See Sulfur-containing aroma volatiles in meat Artifacts, sulfur compounds in foods, 3 Atomic emission detector comparison to flame photometric and sulfur chemiluminescence detectors, 17,21... 

The description of a gas chromatograph modi ed for the snif ng of its ef uent to determine volatile odor activity was rst published by Fuller et al. [63]. In general, GC-olfactometry (GC-0) is carried out on a standard GC that has been equipped with a snif ng port, also denominated olfactometry port or transfer line, in substitution of, or in addition to, the conventional detector. When a FID or a mass spectrometer is also used, the analytical column ef uent is split and transferred to the conventional detector and to the human nose. GC-0 was a breakthrough in analytical aroma research, enabling the differentiation of a multitude of volatiles, previously separated by GC, in odor active and nonodor active, related to their existing concentrations in the matrix under investi gation. Moreover, it is a unique analytical technique that associates the resolution power of capillary GC with the selectivity and sensitivity of the human nose. 

Fig. 5.9. Apparatus for the gas chromatography-olfactometry of static headspace samples. 1 Sample in ther-mostated glass vessel, 2 septum, 3 gastight syringe, 4 injector, 5 hydrophobed glass tube, 6 carrier gas, e. g, helium, 7 purge and trap system, 8 cold trap, 9 gas chromatograph with capillary column, 10 sniffing port, 11 flame ionization detector (according to Guth and Grosch, 1993)...

GC-Olfactometry (GC-O) or sniffing describes techniques that use the human nose to detect and evaluate volatile compounds eluting from a GC separation (Delahunty, 2006). Assessors sniff the eluate from a specifically designed odour port parallel to FID or MS detection. GC-O applications have become common not limited to the food and flavour industry to assign specific flavour characteristics to each of the volatile compounds identified. The human nose plays the role of the detector. However, the human nose is often more sensitive than any physical detector, and GC-O exhibits supplementary capabilities that can be applied to any fragrant product. 

Olfactometry techniques can be classified into two categories dilution methods, which are based on successive dilutions of an aroma extract until no odour is perceived at the sniffing port of the GC, and the intensity methods, in which the aroma extract is injected and the assessor records the odour intensity and perception as a function of time. The technical solution is straightforward with a split at the end of a chromatographic column and a heated transfer line to a GC external sniffer port. The eluting compounds are splitted, for example, 1 50 to an FID or MS detector and the sniffing port. The column effluent is combined at the sniffer port with a laminar stream of inert make-up gas, which is heated to a constant temperature and additionally humidified. 

Gas chromatography-olfactometry (GC-O) has now become a widely used method that associates the separation capability of gas chromatography to the specific sensitivity of the human nose as a detector. It was first mentioned in the literature in 1964 (1), after separation of volatiles in packed columns. However, its use in flavor and fragrance laboratories might well be older (2). At that time it was only of limited qualitative assistance to flavorists and perfumers in the evaluation of odor constituents. It is still used in this informal manner in many laboratories. 

Grosch et al. prepared beef samples in a similar way and analyzed samples using GC-MS equipped with an olfactometry detector (15). Grosch extracted the... 

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