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50 75 100 125 150 175 200 m/z

FIGURE 6.1 Representation of the similarity between mass spectra of monoterpenes: sabinene (a) and ß-phellandrene (b); and sesquiterpenes: bicyclogermacrene (c) and germacrene B (d).

with optimized run conditions to provide 3-10 times faster analysis times [46-48]. The technique can be accomplished by manipulating a number of analysis parameters, such as column length, column I.D., stationary phase, film thickness, carrier gas, linear velocity, oven temperature, and ramp rate. Fast GC is typically performed using short, 0.10 or 0.18 mm I.D. capillary columns with hydrogen carrier gas and rapid oven temperature ramp rates. In general, capillary gas chromato-graphic analysis may be divided into three groups, based solely on column internal diameter types; namely, as conventional GC when 0.25 mm I.D. columns are applied, fast GC using 0.10-0.18 mm I.D. columns, and ultrafast GC for columns with an I.D. of 0.05 mm or less. In addition, GC analyses times between 3 and 12 min can be defined as "fast," between 1 and 3 min as "very fast," and below 1 min as "ultrafast." Fast GC requires instrumentation provided with high split ratio injection

40 50 60 70 80 90 100 110 120 130 m/z

40 50 60 70 80 90 100 110 120 130 m/z

50 75 100 125 150 175 200 m/z

50 75 100 125 150 175 200 m/z

FIGURE 6.1 Representation of the similarity between mass spectra of monoterpenes: sabinene (a) and ß-phellandrene (b); and sesquiterpenes: bicyclogermacrene (c) and germacrene B (d).

systems because of low sample column capacities, increased inlet pressures, rapid oven heating rates, and fast electronics for detection and data collection [49].

The application of two methods, conventional (30 m x 0.25 mm I.D., 0.25 mm rff column) and fast (10 m x 0.10 mm I.D., 0.10 mm rff column), on five different citrus essential oils (bergamot, mandarin, lemon, bitter oranges, and sweet oranges) has been reported [49]. The fast method allowed the separation of almost the same compounds as the conventional analysis, while quantitative data showed good reproducibility. The effectiveness of the fast GC method, through the use of narrowbore columns, was demonstrated. An ultrafast GC lime essential oil analysis was also performed on a 5 m x 50 mm capillary column with 0.05 mm stationary phase film thickness [50]. The total analysis time of this volatile essential oil was less than 90 s; a chromatogram is presented in Figure 6.2.

Another technique, ultrafast module-GC (UFM-GC) with direct resistively heated narrow-bore columns, has been applied to the routine analysis of four essential oils of differing complexities; chamomile, peppermint, rosemary, and sage [51]. All essential oils were analyzed by conventional GC with columns of different lengths; namely, 5 and 25 m, with a 0.25 mm I.D., and by fast GC and UFM-GC with narrow-bore columns (5 m x 0.1 mm I.D.). Column performances were evaluated and compared through the Grob test, separation numbers, and peak capacities. UFM-GC was successful in the qualitative and quantitative analysis of essential oils of different compositions with analysis times between 40 s and 2 min versus 20-60 min required by conventional GC. UFM-GC allows to drastically reduce the analysis time, although the very high column heating rates may lead to changes in selectivity compared to conventional GC, and that are more marked than those of classical fast GC. In a further work the same researchers [52] stated that in UFM-GC experiments the appropriate flow choice can compensate, in part, the loss of separation capability due to the heating rate increase.

Besides the numerous fast GC application on citrus essential oils, other oils have also been subjected to analysis, such as rose oil by means of ultrafast GC [53] and very fast GC [54], both using narrow-bore columns. Rosemary and chamomile oils have been investigated by means of fast GC on two short conventional columns of distinct polarity (5 m x 0.25 mm I.D.) [55]. The latter

Seconds

FIGURE 6.2 Fast GC analysis of a lime essential oil on a 5 m x 5 mm (0.05 mm film thickness) capillary column, applying fast temperature programming. The peak widths of three components are marked to provide an illustration of the high efficiency of the column, even under extreme operating conditions (for peak identification see on Ref. [50]). (From Mondello, L. et al., 2004. J. Sep. Sci, 27: 699-702. With permission.)

Seconds

FIGURE 6.2 Fast GC analysis of a lime essential oil on a 5 m x 5 mm (0.05 mm film thickness) capillary column, applying fast temperature programming. The peak widths of three components are marked to provide an illustration of the high efficiency of the column, even under extreme operating conditions (for peak identification see on Ref. [50]). (From Mondello, L. et al., 2004. J. Sep. Sci, 27: 699-702. With permission.)

oil has also been analyzed through fast HS-SPME-GC on a narrow-bore column [56]. Fast and very fast GC analyses on narrow-bore columns have also been carried out on patchouli and peppermint oils [57].

6.3.4 Gas Chromatography-Olfactometry for the Assessment of Odor-Active Components of Essential Oils

The discriminatory capacity of the mammalian olfactory system is such that thousands of volatile chemicals are perceived as having distinct odors. It is accepted that the sensation of odor is triggered by highly complex mixtures of volatile molecules, mostly hydrophobic, and usually occurring in trace-level concentrations (ppm or ppb). These volatiles interact with odorant receptors of the olfactive epithelium located in the nasal cavity. Once the receptor is activated, a cascade of events is triggered to transform the chemical-structural information contained in the odorous stimulus into a membrane potential [58,59], which is projected to the olfactory bulb, and then transported to higher regions of the brain [60] where the translation occurs.

It is known that only a small portion of the large number of volatiles occurring in a fragrant matrix contributes to its overall perceived odor [61,62]. Further, these molecules do not contribute equally to the overall flavor profile of a sample; hence, a large GC peak area, generated by a chemical detector does not necessarily correspond to high odor intensities, due to differences in intensity/ concentration relationships.

The description of a gas chromatograph modified for the sniffing of its effluent to determine volatile odor activity was first published in 1964 by Fuller et al. [63]. In general, gas chromatograhy-olfactometry (GC-O) is carried out on a standard GC that has been equipped with a sniffing port, also denominated olfactometry port or transfer line, in substitution of, or in addition to, the conventional detector. When a flame FID or a mass spectrometer is also used, the analytical column effluent is split and transferred to the conventional detector and to the human nose. GC-O was a breakthrough in analytical aroma research, enabling the differentiation of a multitude of volatiles, previously separated by GC, in odor-active and non-odor-active, related to their existing concentrations in the matrix under investigation. Moreover, it is a unique analytical technique that associates the resolution power of capillary GC with the selectivity and sensitivity of the human nose.

GC-O systems are often used in addition to either a FID or a mass spectrometer. With regard to detectors, splitting column flow between the olfactory port and a mass spectral detector provides simultaneous identification of odor-active compounds. Another variation is to use an in-line, nondestructive detector such as a TCD [64] or a photoionization detector (PID) [65]. Especially when working with GC-O systems equipped with detectors that do not provide structural information, retention indexes are commonly associated to odor description supporting peak assignment.

Over the last decades, GC-O has been extensively used in essential oil analysis in combination with sophisticated olfactometric methods; the latter were developed to collect and process GC-O data, and hence, to estimate the sensory contribution of a single odor-active compound. The odor-active compounds of essential oils extracted from citrus fruits (Citrus sp.), such as orange, lime, and lemon, were among the first character impact compounds identified by flavor chemists [66].

GC-O methods are commonly classified in four categories: dilution, time-intensity, detection frequency, and posterior intensity methods. Dilution analysis, the most applied method, is based on successive dilutions of an aroma extract until no odor is perceived by the panelists. This procedure, usually performed by a reduced number of assessors is mainly represented by CHARM (combined hedonic aroma response method) [67], developed by Acree and coworkers, and AEDA (aroma extraction dilution analysis), first presented by Ullrich and Grosch [68]. The former method has been applied to the investigation of two sweet orange oils from different varieties, one Florida Valencia and the other Brazilian Pera [69]. The intensities and qualities of their odor-active components were assessed. CHARM results indicated for both the oils that the most odor-active compounds are associated with the polar fraction compounds: straight chain aldehydes (C8-C14), b-sinensal, and linalool presented the major CHARM responses. On the other hand, AEDA has been used to investigate the odor-active compounds responsible for the characteristic odors of juzu oil (Citrus junos Sieb. ex Tanaka) [70] and dadai (Citrus aurantium L. var. cyathifera Y. Tanaka) [71] cold-pressed essential oils.

Time-intensity methods, such as OSME (Greek word for odor), are based on the immediate recording of the intensity as a function of time by moving the cursor of a variable resistor [72]. An interesting application of the time-intensity approach was demonstrated for cold-pressed grapefruit oil [73], in which 38 odor-active compounds were detected and, among these, 22 were considered as aroma impact compounds. A comparison between the grapefruit oil GC chromatogram and the corresponding time-intensity aromagram for that sample is shown in Figure 6.3.

A further approach, the detection frequency method [74,75], uses the number of evaluators detecting an odor-active compound in the GC effluent as a measure of its intensity. This GC-O method is performed with a panel composed of numerous and untrained evaluators; 8-10 assessors are a good agreement between low variation of the results and analysis time. It must be added that the results attained are not based on real intensities and are limited by the scale of measurement. An application of the detection frequency method was reported for the evaluation of leaf- and wood-derived essential oils of Brazilian rosewood (Aniba rosaeodora Ducke) essential oils by means of enantioselective-GC-olfactometry (Es-GC-O) analyses [76].

Another GC-O technique, the posterior intensity method [77], proposes the measurement of a compound odor intensity, and its posterior scoring on a previously determined scale. This posterior registration of the perceived intensity may cause a considerable variance between assessors. The attained results may generally be well correlated with detection frequency method results, and to a lesser extent, with dilution methods. In the above-mentioned research performed on the essential oils of Brazilian rosewood, this method was also used to give complementary information on the intensity of the linalool enantiomers [76].

Other GC-O applications are also reported in literature using the so-called peak-to-odor impression correlation, the method in which the olfactive quality of an odor-active compound perceived by a panelist is described. The odor-active compounds of the essential oils of black pepper

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Aromatherapy Ambiance

Aromatherapy Ambiance

Aromatherapy, a word often associated with calm, sweet smelling and relaxing surroundings. Made famous for its mostly relaxing indulgent  feature, using aromatherapy has also been known to be related to have medicinal qualities.

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