JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The nose is incredibly sensitive to certain odours and for a satisfactory analysis it is necessary to identify and include in the formulation constitu- ents that may be present in quantities less than one part per million and there are claims that much lower concentrations, even parts per billion, are of vital importance. For certain compounds, the nose is more sensitive than the flame ionization detector and in such cases it may be necessary to use spectrometer techniques. Interest in the gas chromatography of yapours began with the introduc- tion of instruments having ionization type detectors, for example those using strontium 90, or tritium. These detectors were far more sensitive than the earlier catharometers and there must be many who repeated the - at the time - fascinating experiment in which the head space yapours from a freshly punctured tin of ground roasted coffee were chromatographed. The interpretation of the chromatogram so obtained is a much more difficult problem. Retention times are often employed, but must be used with care particularly with packed columns and low retention times, and in all cases they should be measured on at least two stationary phases of different polarity. An unambiguous identification of each peak usually involves preparative chromatography whereby the compound producing each peak is separated and identified by physical means, such as a com- bination of infra-red, U.V. and nuclear magnetic resonance spectroscopy. Chemical reactions on a micro-scale are also used to aid complete identifica- tion. An important step forward was made when gas-chromatography was coupled with high resolution mass spectroscopy. With this technique it is possible to determine the molecular weight and probable chemical formula for each peak in the chromatogram and in certain cases it is possible to deduce the chemical structure. Thus, high resolution capillary chromato- graphy columns have been coupled to a fast-scan mass spectrometer to give unambiguous identification of the components of fruit volatiles (1). The work is still tedious because of the number of peaks involved, but it has been made possible by the use of computer techniques which will carry out the necessary calculations and tabulate the molecular weights and chemical formulae. This techn. ique is expensive and probably beyond the resources of most perfumers, quite apart from the fact that it is not the final answer. Many perfumers use gas chromatography to assist in the identification of the components of an essential oil or perfume. The usual procedure is to put the sample to be examined on to a chromatographic column and to divide the separated components at the exit into two streams. One stream

THE ANALYSIS OF ODORIFEROUS VAPOURS passes to a detector and the peaks are recorded in the usual way. The rest of the stream is used for smelling. Others use a non-destructive detector, such as a catharometer, so that the whole of the exit gas can be used for smelling. Notes of the odours are usually dictated and either recorded on magnetic tape or written directly on to the chromatogram. The technique is not without difficulties of which perhaps the most important is that an organoleptically essential constituent may be present in such a small proportion that it is not recorded on the gas chromatograph. It is suspected too that some odoriferous compounds are so unstable that they do not survive the chromatographic separation. The simplest procedure for vapour analysis is to take a measured volume of air from the free space in a closed vessel containing the odorifer- ous material, such as food, fruit or perfume. The air is then injected directly on to the chromatographic column. The limitation of the method is that it is relatively insensitive to high boiling, but perhaps strongly odoriferous constituents of the vapour. Furthermore, there must be a loss of resolution that is related to the volume of the sample and the gas flow rate. With sensitive detectors and temperature programmed dual columns to com- pensate for bleeding of the stationary phase, it is possible to distinguish around 20 peaks in a 5 ml sample of the vapour above fresh pineapples {2). Greater sensitivity can be obtained by concentrating the vapour by freezing. Thus, in the analysis of tobacco smoke it has been found that the direct injection of 30 ml of smoke on to a column at 30øC gave a chromatogram with around 18 peaks. However, if the column was cooled to minus 65øC, then all the constituents collected on the inlet to the column without spread- ing and were not released until the column was warmed. In this way the same volume of tobacco smoke produced around 130 peaks (3). It was found when using 304 m of 0.76 mm I.D. open-tubular column coated with methyl silicone that unacceptable broadening of the peaks occurred when a vapour sample measuring more than 0.5 ml was injected, a volume which unfortunately contained insufficient material for a satisfactory analysis (4). The resolution was improved by trapping the components in a 20 ml sample of vapour by passing it through a short length {about 75 mm) of coated capillary tube cooled in solid carbon dioxide and acetone. By means of a switching valve, the capillary was connected in series with the inlet to the glc column and heated rapidly to 130-140øC in a current of hot air. Ex- cellent resolution was obtained and the procedure was used in a study of apple variety and the changes that take place during processing. Other workers obtained similar improved resolution by cooling the first inch or