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 and capillaries coated with adsorbents (Ter Heide, 1985; Kaiser, 1991). However, the most common method for trapping flower volatiles is the use of adsorbent traps. The traps are small glass tubes containing activated charcoal, Tenax®, or a similar porous polymer through which the air is pumped. The volatiles from the air are adsorbed onto the trap while air and water pass through unretained, thus permitting the volatiles from many litres of air to be concentrated on the adsorbent. The flower is enclosed in a modified bell-jar or flask to prevent the volatiles being swept away by air currents and to isolate the flower from any contaminants that may otherwise drift into the sampling area. The traps are inserted into the enclosed space of the bell — jar and attached to a small, portable pump until sampling is complete.

The traps are then removed, sealed and returned to the laboratory for analysis. The duration and flow rate used for sampling are adjusted to match the expected concentration of volatiles in the air, the capacity of the traps and the intended method of desorption. Traps designed for thermal desorption contain only a few milligrams of adsorbent and sampling can be completed in a few minutes. In contrast, solvent desorption can be carried out on traps containing any amount of adsorbent from a few hundred milligrams to several grams. The traps have a finite capacity for individual compounds, dependent on the amount of absorbent and the dimensions of the trap. Different materials are retained to varying degrees according to their volatility and polarity, and it is therefore important to determine the capacity of the traps to ensure that none of the volatiles are lost during sampling.

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 the samples can be stored for long periods 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 technique has been successfully applied to a great range of plants and flowers for fragrance analysis, pollination studies and other botanical investigations (Kaiser, 1993; Knudsen et al., 1993; Bicchi and Joulain, 1990). The sample size can be limiting when it comes to identifying a new material. Although modern instruments are extremely sensitive and only a few nanograms are required for acquisition of a mass spectrum, several hundred micrograms are required for 13C NMR and isolating materials on this scale, even using GC-trapping, requires more sample than is usually available by headspace collection.

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.

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