(Piper nigrum) and Ashanti pepper (Piper guineense) were assessed applying the aforecited correlation method . The odor profile of the essential oils of leaves and flowers of Hyptispectinata (L.) Poit. was also investigated by using the peak-to-odor impression correlation .
The choice of the GC-O method is of extreme importance for the correct characterization of a matrix, since the application of different methods to an identical real sample can distinctly select and rank the odor-active compounds according to their odor potency and/or intensity. Commonly, detection frequency and posterior intensity methods result in similar odor intensity/concentration relationships, while dilution analysis investigate and attribute odor potencies.
6.3.5 Gas Chromatographic Enantiomer Characterization of Essential Oils
Capillary GC is currently the method of choice for enantiomer analysis of essential oils and enantio-selective-GC (Es-GC) has become an essential tool for stereochemical analysis mainly after the introduction of cyclodextrin (CD) derivatives as chiral stationary phases (CSPs) in 1983 by Sybilska and Koscielski, at the University of Warsaw, for packed columns , and applied to capillary columns in the same decade [81,82]. Moreover, Nowotny et al. first proposed diluting CD derivatives in moderately polar polysiloxane (0V-1701) phases to provide them with good chromatographic properties and a wider range of operative temperatures .
The advantage on the application of Es-GC lies mainly in its high separation efficiency and sensitivity, simple detection, unusually high precision and reproducibility, as also the need for a small amount of sample. Moreover, its main use is related with the characterization of the enantiomeric composition and the determination of the enantiomeric excess (ee) and/or ratio (ER) of chiral research chemicals, intermediates, metabolites, flavors and fragrances, drugs, pesticides, fungicides, herbicides, pheromones, and so on. Information on ee or ER is of great importance to characterize natural flavor and fragrance materials, such as essential oils, since the obtained values are useful tools, or even "fingerprints," for the determination of their quality, applied extraction technique, geographic origin, biogenesis, and also authenticity .
A great number of essential oils have already been investigated by means of Es-GC using distinct CSPs; unfortunately, a universal chiral selector with widespread potential for enantiomer separation is not available, and thus effective optical separation of all chiral compounds present in a matrix may be unachievable on a single chiral column. In 1997, Bicchi et al.  reported the use of columns that addressed particular chiral separations, noting that certain CSPs preferentially resolved certain enantiomers. Thus, a 2,3-di-O-ethyl-6-O-tert-butyldimethylsilyl-b-CD on polymethylphe-nylsiloxane (PS086) phase allowed the characterization of lavender and citrus oils containing linalyl oxides, linalool, linalyl acetate, borneol, bornyl acetate, a-terpineol, and cis- and trans-nerolidol. On the other hand, peppermint oil was better analyzed by using a 2,3-di-O-methyl-6-O-tert-butyldimethylsilyl-b-CD on PS086 phase, and especially for a- and b -pinene, limonene, menthone, isomenthone, menthol, isomenthol, pulegone, and methyl acetate. Konig  performed an exhaustive investigation of the stereochemical correlations of terpenoids, concluding that when using a heptakis (6-O-methyl-2,3-di-O-penthyl)-b-CD and octakis (6-O-methyl-2,3-di-O-penthyl)-g-CD in polysiloxane, the presence of both enantiomers of a single compound is common for monoter-penes, less common for sesquiterpenes, and never observed for diterpenes.
Substantial improvements in chiral separations have been extensively published in the field of chromatography. At present, over 100 stationary phases with immobilized chiral selectors are available , presenting increased stability and extended lifetime. It can be affirmed that enantioselec-tive chromatography has now reached a high degree of sophistication. To better characterize an essential oil, it is advisable to perform Es-GC analysis on at least two, or better three, columns coated with different CD derivatives. This procedure enables the separation of more than 85% of the racemates that commonly occur in these matrices , while the reversal of enantiomer elution order can take place in several cases. The analyst must be aware of some practical aspects prior to an Es-GC analysis: as is well accepted, variations in linear velocity can affect the separation of enantiomeric pairs; resolution (RS) can be improved by optimizing the gas linear velocity, a factor of high importance in cases of difficult enantiomer separation. Satisfactory resolution requires RS > 1, and baseline resolution is obtained when RS > 1.5 . Resolution can be further improved by applying slow temperature ramp rates (1-2°C/min is frequently suggested). Moreover, according to the CSP used, the initial GC oven temperature can affect peak width; initial temperatures of 35-40°C are recommended for the most column types. Furthermore, attention should be devoted to the column sample capacity, which varies with different compounds; overloading results in broad tailing peaks and reduced enantiomeric resolution. The troublesome separation and identification of enantiomers due to the fact that each chiral molecule splits into two chromatographic signals, for each existing stereochemical center is also worthy of note. As a consequence, the increase in complexity of certain regions of the chromatogram may lead to imprecise ee and/or ER values. In terms of retention time repeatability, and also reproducibility, it can be affirmed that good results are being achieved with commercially available chiral columns.
The retention index calculation of optically active compounds can be considered as a troublesome issue due to complex inclusion complexation retention mechanisms on CD stationary phases; if a homologous series, such as the n-alkanes, is used, the hydrocarbons randomly occupy positions in the chiral cavities. As a consequence, n-alkanes can be considered as unsuitable for retention index determinations. Nevertheless, other reference series can be employed on CD stationary phases, such as linear chain FAMEs and FAEEs. However, retention indices are seldom reported for optically active compounds, and publications refer to retention times rather than indices.
The innovations in Es-GC analysis have not only concerned the development and applications of distinct CSPs, but also the development of distinct enantioselective analytical techniques, such as Es-GC-mass spectrometry (Es-GC/MS), Es-GC-O, enantioselective multidimensional gas chroma-tography (Es-MDGC), Es-MDGC/MS, Es-GC hyphenated to isotopic ratio mass spectrometry (Es-GC/IRMS), Es-MDGC/IRMS, and so on.
It is obvious that an enantioselective separation in combination with MS detection presents the additional advantage of qualitative information. Notwithstanding, a difficulty often encountered is that related to peak assignment, due to the similar fragmentation pattern of isomers. The reliability of Es-GC/MS results can be increased by using an effective tool, namely retention indices. It can be assumed that in the enantioselective recognition of optically active isomers in essential oils, mass spectra can be exploited to locate the two enantiomers in the chromatogram, and the LRI when possible, enables their identification . In addition, the well-known property of odor activity recognized for several isomers can be assessed by means of Es-GC/MS/O and can represent an outstanding tool for precise enantiomer characterization (Figure 6.4).
As demonstrated by Mosandl and his group , Es-GC-O is a valid tool for the simultaneous stereodifferentiation and olfactive evaluation of the volatile optically active components present in essential oils. It is worthwhile to point out that the preponderance of one of the enantiomers, defined by the enantiomeric excess, results in a characteristic aroma , and is of great importance for the olfactive characterization of the sample.
6.3.6 LC and Liquid Chromatography Hyphenated to MS in the Analysis of Essential Oils
Some natural complex matrices do not need sample preparation prior to GC analysis, for example, essential oils. The latter generally contain only volatile components, since their preparation is performed by SD. Citrus oils, extracted by cold-pressing machines, are an exception, containing more than 200 volatile and nonvolatile components. The volatile fraction represents 90-99% of the entire oil, and is represented by mono- and sesquiterpene hydrocarbons and their oxygenated derivatives, along with aliphatic aldehydes, alcohols, and esters; the nonvolatile fraction, constituting 1-10% of the oil, is represented mainly by hydrocarbons, fatty acids, sterols, carotenoids, waxes, and oxygen heterocyclic compounds (coumarins, psoralens, and polymethoxylated flavones—PMFs) .
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You have probably heard the term Aromatherapy and wondered what exactly that funny word, „aromatherapy‟ actually means. It is the use of plant oils in there most essential form to promote both mental and physical well being. The use of the word aroma implies the process of inhaling the scents from these oils into your lungs for therapeutic benefit.