Headspace collection

The headspace is the air above or around a fragrant substance that contains the volatile compounds. This can be collected for analysis when extraction of the volatiles from the material is not viable. This technique has been extensively developed for the collection and analysis of flower volatiles since many flowers do not yield an extract that reflects the odour of the fresh flower, while others are simply too rare to be available in sufficient quantity for extraction. Many different techniques have been applied to the collection of volatiles from the air above flowers including the use of cold traps, solvent traps, adsorbent materials 

Having collected the sample on the adsorbent traps it is important to keep them cool and dark, as the glass and absorbent surfaces are sufficiently reactive to cause degradation of some materials if exposed to light or heat for long periods. However, if adequately protected, they can be stored or transported over long distances. 

Samples collected in this way can be desorbed using a solvent which provides a solution for conventional injection on any GC or GC-MS. Thermal desorption of specially designed traps directly onto a GC with the appropriate injection system is more sensitive, and very volatile materials are not obscured by large solvent peaks. However, there is the possibility of thermal degradation of the sample and the entire sample is used at once. 

The great advantage of headspace sampling is that it can be carried out relatively easily in the field with simple, portable equipment this has opened up many exciting possibilities for the natural product chemist who can now analyse the scent of just a single flower. This 

With the array of extremely sensitive instruments now available, the analytical chemist can identify materials of odour interest that are present at very low levels in natural materials. However, we still rely on traditional techniques and laboratory skills to fractionate samples and isolate new materials for identification. 

Acree and J. Barnard, Gas chromatography-olfactometry and charm analysis, Dev. Food Sci., 1994, 35, 211-220. 

Bicchi and D. Joulain, Review Headspace-gas chromatographic analysis of medicinal and aromatic plants and flowers, Flav. Frag. J., 1990, 5, 131-145. 

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Fig. 5.3. Odor changes that track floral color changes. Gas chromatography-mass spectrometry total ion chromatograms of floral headspace collected from young (upper trace) and old (lower trace) flowers of Lantana montevidense. Peaks 1,8,10, and 12 are metabolites of linalool (peak 3), all of which decrease dramatically with floral age and color change. Sesquiterpene hydrocarbons (peaks 4, 5, 9, and 11) show comparable decreases over time. Peaks 2, 6, 7, 13, and 14 are oxygenated aromatics and are present only in newly opened, rewarding flowers. Insert mass spectra highlight loss of phenylacetaldehyde ( peak 7) taken from young (a) and old (b) flowers. (M. R. Weiss and R. A. Raguso, unpublished data.)...

For the headspace analysis, approximately lOOg of dried mushrooms were placed in a headspace sampling vessel (Figure 4) and the headspace collected onto pairs of Tenax TA traps (Supelco, Inc., Bellefonte, PA) using a vacuum pump (Fischer Scientiflc, Co., Springfield, NJ) with a flow rate of 25mL/min. The headspace was sampled over 4 hour and 24 hour periods, respectively. The traps were thermally desorbed onto GC-FID and GC-MS systems. 

One alternative to dynamic headspace is the closed loop headspace collection device, developed by Brunke et al. (9), shown in Fig. 4. This device places a flower within a collection vessel, and the volatiles are collected on an appropriate trapping material. The difference between this method and conventional dynamic headspace is that the closed loop technique constantly circulates the air within the device using an in-line pump. Results obtained with the closed loop system are similar to those obtained with conventional dynamic head-space. 

Figure 4 Diagram showing a closed-loop headspace collection device. 1, Conical flask 2, flower 3, battery-powered suction pressure pump 4, headspace collection trap 5, activated charcoal tube (purification of the air stream) 6, direction of airflow.

Figure 9 Gas chromatograms (60 m X 0.25 mm I.D. SPB-1 MS column) representing the headspace collected with a Zenith trap at different times. (A) 2 min. adsorption (B) 5 min. adsorption (C) 10 min. adsorption.

Figure 12 Schematic drawing of an automated headspace collection apparatus used to...

The concentration of the volatiles emitted from a plastic bag was also found to increase as the ambient temperature increased. Figure 14 shows the total concentration of volatiles emitted from different plastic bags at 25°C and 45°C, respectively. From this smdy, roaster bags are recommended for use as headspace collection vessels as they emit relatively few volatiles even at the higher temperature. A more important criteria for selection of the roaster bag is that the major... 

Figure 16 Headspace collection using a Zenith trap and support apparatus.

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