318 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 2.--Gas chromatography of octvl (Cs) aidehyde. Figure &--Gas chromatography of 1:1:1 mix- ture of methyl, ethyl and isopropyl benzoates 190øC. ANALYTICAL USES or GAS CHROMATOGRAPHY One important use of gas chromatography is for the detection of impuri- ties. Its use in detecting 1 per cent of heptaldehyde in octyl aldehyde is shown in Fig. 7. The sensitivity of the instrument and the sample size have been increased to such an extent that the octyl aldehyde peak runs off the chart. Under these conditions, the heptaldehyde peak shows up very clearly between the peaks for the very small impurities that were in the octyl aldehyde (Fig. 2) and the major peak. It is obvious that the latter materials are present in less than 1 per cent concentration since their peaks are much smaller than that of the heptaldehyde. Figures 8-11 illustrate the use of gas chromatography for quantitative analysis of a mixture of supposedly equal amounts of C7, Cs, C9 and aldehydes. This analysis could not be done by any other analytical technique with the possible exception of mass spectroscopy. The quantity of each aldehyde present is, strictly speaking, represented by the area under the corresponding peak. However, estimating the area under a peak of this sort in a rigorous way is a rather tedious and time-consuming job. For most purposes, one can use the approximation that the area under the peak is proportional to the height of the peak times the width of the peak at Figure 4.--Gas chromatography of 1:1:1 mixture of methyl, ethyl and isopropyl benzoates 228øC. Figure 5.--Gas chromatography of 1:1:1 mix- ture of methyl, ethyl and isopropyl benzoates 167 ø C.

GAS-PARTITION CHROMATOGRAPHY 319 Figure 7.--Gas chromatography of octyl Figure &--Gas chromatography of methyl, (Cs) aldehyde plus 1% heptyl (C0 aide- ethyl and isopropyl benzoates 190øC. hyde. TABLE I--QuANTITATIVE ANALYSIS or MIXTURE or STRAIGHT-CHAIN ALDEHYDES, C7: , C8, C9 AND C10 X Found, % Calculated, % C7 23.3 4- 0.45 25 Cs 28.2 4- 0.4 25 Co 24.4 4- 0.25 25 C•0 24.1 4- 0.15 25 half its height. Using this method, results (Table I) accurate to within a few per cent for each aldehyde were obtained. However, the reproduci- bility of the method is better than this it should be possible to get at least 1 per cent accuracy on this kind of a mixture by use of calibration chroma- tograms. Figure 8.--Gas chromatography of 1:1:1 mixture Figure 9.--Gas chromatography of of C7, Cs, Co and C•0 aldehydes. heptyl (C7) aidehyde.