Introduction

In perfumery, mass spectrometry is mainly used to identify molecules. This technique makes possible the study of compound traces. However, mass spectrometry is based on the fragmentation of the molecules but several molecules can have similar mass spectrum. This is particularly present in organic structures like the cis-trans diastereoisomers or isomers. To identify these compounds has a paramount importance in quality control as for safety because a cis and trans form can have completely different chemical properties. These differences allow the Kovats index to assist us in the discrimination of compounds having an identical mass spectrum.

Experimental conditions

A representative mixture of a perfuming composition containing the cis-trans forms of the Damascone alpha was introduced into a 20m capillary column of 0.180mm diameter and a film thickness of 0.18μm. The eluted compounds were penetrated into the Agilent MSD 5975B mass spectrometer, mounted in tandem with an Agilent 6890N chromatograph.

Results

The mass spectra show below that the cis and trans forms can not be distinguish by spectrum analysis. Indeed the matching between the two forms is between 95 and 99% which makes discrimination impossible with this technique or very hazardous.
However, the Kovats index of each compounds are different. Indeed, the cis form has a retention index of 1359 while the trans form has a retention index of 1412. Thus it is now possible to know precisely the structure of the molecule studied.

Conclusion

The combination of the Kovats index with mass spectrometry allows the precise identification unknown compounds. Indeed, this method is based on the affinity that a compound can possesses with the chromatographic column and whatever analysis conditions. Thus cis and trans forms having different affinities with the stationary phase will be able to be identified.
This technique also helps to identify unknown compounds by discriminating as much as possible same compounds possibilities.

Figure 1: Mass spectra of alpha Damascone, trans and cis form.

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