The adduct was purified by flash chromatography and then hydrolyzed to afford zingiberene in good yield (99% of purity). Quality assessment of flavors and fragrances by HPGC has been widely used (Mosandl, 1992).
Enantiomers also can be separated by capillary columns coated with P-cyclodextrin derivatives. Thus, Takeoka et al. (1990) separated sesquiterpene hydrocarbons using a permeth-ylated P-cyclodextrin (PM-P-CD) as GC stationary phase. Sesquiterpenes include ar-curcumene, a- and P-bisabolenes, 8- and P-elemenes, a-copaene, 8-cadinene, cis- and trans-calamenenes, and bicyclogermacrene. Four years later, Koenig et al. (1994) separated the two enantiomers of ar-curcumene and P-bisabolene in ginger oil, using a fused capillary column coated with heptakis (2,3-O-methyl-6-O-t-butyldimethylsilyl-P-cyclodextrin in polysiloxane OV-1701 (50% w/w). The column temperature was 115° C and carrier gas:hydrogen (0.5 bar). The chromatogram shows that ar-curcumene exists in the ( + ) form, whereas P-bisabolene contains the two enantiomers ( + ) and (—) in equal quantity. Enantiomeric separation of the characteristic aromatic compounds in fresh rhizomes of Japanese ginger was carried out using the off-line multidimensional GC (MDGC) system and confirmation of the odor character of each enantiomer by GC/olfactometry (Nishimura, 2001). GC has been widely used for preparative purposes as well. Packed columns giving poor separation were replaced by megabore columns that are wide-bore capillary columns (WBCCs) of 0.5 to 0.8 mm i.d. coated with thick films of 3 to 5 ^m of stationary phases. Van Beek (1991) reported the quantitative separation of some sesquiterpene hydrocarbons (SQHCs) using a 30 m/3 ^m DB-1 megabore column. From a SQHC fraction containing a-zingiberene (69%), P-bisabolene (19%), P-sesquiphellandrene (9%), and minor compounds (3%), and obtained by preparative HPLC and then injected several times in the WBCC, P-bisabolene could be obtained sufficiently pure for 1H- and 13C-NMR analyses. P-Sesquiphellandrene was purified as well. This type of column is also useful in the sniffing method (at the end of the column).
Other GC Methods: Dynamic Headspace
The thermal desorption cold trap injector (TCT) was used as a part of the off-line multidimensional GC (MDGC) system. It was shown that the TCT can be used not only for headspace analysis, but also as a part of an MDGC system. Direct vaporization of a pulverized sample of ginger can be subjected to heating at 250° C for 1 minute in a vaporizer directly connected to the GC on a GC/MS apparatus. Out of 54 constituents, 25 were identified (Chen et al. 1987). They found little decomposition regardless of the heating time and temperatures and a similar composition with the essential oil obtained from the same sample. GC of headspace vapor from dry ice—cooled trap of low-boiling compounds from steam-distilled ginger was reported by Kami et al. (1972). The identification of peaks was carried out by comparing the retention time with authentic samples. The identification was supported by a chemical reaction including 2,4-dinitrophenylhydrazones (for carbonyl compounds), 3,5-dinitrobenzoates (for aliphatic alcohols), a mercuric complex (for sulfide derivatives), and hydroxamic acid (for monoterpenes). They were analyzed directly or after regeneration by TLC, GC, and combined GC/MS.
The GC dynamic headspace is a very suitable technique and useful to analyse liquid or solid aromatic materials and has been widely used in flavors and fragrances. It uses an inert gas (helium or nitrogen) to flush a small flask containing the aromatic product for 10 to 15 minutes at room temperature. Volatile compounds are trapped on Tenax GC and then thermally desorbed at 250° C for a few seconds in the capillary column, under the usual conditions. De Pooter et al. (1985) used this technique for ginger powder, and the GC pattern was quantitatively similar to that of the corresponding essential oil prepared by hydrodistillation.
It is an excellent and powerful method for the comparison of different samples of ginger powders from different origins. The method can be indirectly used with a separated flask connected to a Tenax GC trap (in glass or stainless steel). Volatiles are extracted by solvent extraction (ethyl ether). The extract after evaporation of the solvent is then injected in the GC column and GC/MS apparatus.
Under gas chromatographic conditions, gingerols are decomposed into zingerone, aldehydes, and shogaols according to Scheme 3.1.
Hexanal and minor amounts of the other aldehydes and zingerone were also formed on treatment with hot alkali of an extract of gingerols obtained by dry column chro-matography. Connell (1970) used two packed columns (3 ft X 1/8 inch): 1.6% SE 30 at 188° C and Apiezon M at 200° C on Embacel, respectively.
Chen et al. (1987) used HPGC on Carbowax 20 M and OV-1 columns to study the thermal degradation products of gingerols in steam-distilled oil from ginger. Significant higher concentrations of aliphatic aldehydes (C6 to C12) and 2-alkanones in the steam-distilled sample confirmed that thermal degradation of nonvolatile gingerols occurred during steam distillation. On the other hand, during steam distillation or hydrodistillation
Scheme 3.1 Thermal degradation of gingerols during gas chromatography. (adapted from Connell
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