There are several ways to investigate differences between perfumes: people can smell and compare them, or researchers can separate and analyse their ingredients. In a study published in 2022, Lykourgos Bougas and colleagues passed light through perfume vapour and used changes in the light to capture characteristics of individual products.
The study was not primarily about ranking perfumes. Its focus was a technique for rapidly measuring molecular chirality in gases. Perfume was chosen as an application to test whether the technique also worked with complex mixtures.
Probing mirror-image molecules with light
Some molecules, like our right and left hands, cannot be superimposed on their mirror images. This property is called chirality, and the two forms are called enantiomers. They contain the same atoms but differ in their three-dimensional arrangement.
These molecules can rotate the plane of linearly polarized light. The two enantiomers rotate it in opposite directions, providing a clue to which form predominates in a mixture and by how much. In dilute gas samples, however, the signal is extremely small.
The team used an apparatus with four mirrors that circulated light repeatedly through the sample. Continuous laser light and a method for precisely tracking optical frequencies allowed them to read small optical-rotation signals. They also measured changes in refractive index simultaneously, separating information about concentration from information about the mixture’s chiral imbalance.
A central aim was to obtain optical-rotation measurements without having to calibrate the instrument against a reference sample each time. The researchers first measured mixtures of known composition in combination with gas chromatography, which separates components. In a sample containing both limonene enantiomers, optical rotation distinguished their opposite contributions even where the refractive-index signal appeared as a single feature.
Comparing four perfumes with inexpensive smell-alikes
The perfume experiment used the eaux de parfum Chanel Coco Mademoiselle, Dior Miss Dior, Thierry Mugler Angel and Lancôme La Vie Est Belle. These were compared with inexpensive smell-alike products bought from retailers. The authors used them as models for testing methods of detecting imitation or adulteration; the experiment did not establish that they were illegal counterfeits.
Air containing perfume vapour was drawn into the instrument, which measured optical rotation and refractive index for the mixture as a whole. In a comparison between Coco Mademoiselle and one smell-alike, the former produced a positive optical-rotation signal, while the latter produced no clear signal. Their refractive-index signals also differed.
An absent signal does not necessarily mean that no chiral molecules are present. Separate compositional analysis identified relevant components in that smell-alike, and the authors considered their concentrations to be below the instrument’s detection limit. More generally, opposite optical rotations can also cancel in mixtures.
Comparisons involving the other perfumes likewise revealed characteristic signal differences among the products tested. Different bottles of the same perfume produced similar signals. Rather than identifying every individual ingredient, the approach uses the overall characteristics of a mixture for discrimination.
In the continuous perfume measurements, each data point took 0.5 seconds. However, the product-comparison figure combines at least seven measurements per sample obtained while drawing air for approximately 20 seconds. This does not mean that a device had been developed to guarantee the authenticity of an unknown perfume in 0.5 seconds. Nor were these comparisons of seven independent manufacturing batches.
The amount can recover without the composition recovering
The technique’s significance extends beyond product identification. The researchers also examined volatile emissions from one individual of the pine species Pinus heldreichii before and after cutting a branch. These dilute emissions had to be collected before analysis, so even the online measurements required 15 minutes of sampling.
Immediately after cutting, concentrations rose sharply and then approached their earlier levels. The concentration-adjusted optical-rotation signal followed a different time course, however, becoming more negative than before. Changes in enantiomeric composition remained that would not have been apparent from the amount alone. Comparing the signal with separately collected resin, the authors interpreted the later pattern as an increasing contribution from the resin covering the wound.
Investigating plants’ responses to injury and the origins of atmospheric emissions requires tracking not only total amounts but also their stereochemical composition. This experiment was a step toward that broader observational goal.
Summary
The study demonstrated a way to measure small optical-rotation signals in gases alongside information about concentration. It offers possibilities both for analysing enantiomers after separating components and for comparing intact mixtures such as perfumes. An important contribution is the connection it makes between product quality control and changes over time in plant emissions using the same measurement principle.
However, the results do not show that a perfume with a larger signal is better, or that more natural ingredients make a product superior. Because synthesis can also selectively produce one enantiomer, the method cannot provide a universal natural-versus-synthetic classification. Fragrance preferences, changes on skin and safety were not studied, and discrimination performance needs testing across more products and conditions. Further sensitivity improvements and wider industrial uses discussed by the authors remain future possibilities.
The research was supported by the European Horizon 2020 ULTRACHIRAL project. Bougas and Williams disclosed that they were inventors on a patent application related to this work.
References
Bougas, L., Byron, J., Budker, D., & Williams, J. (2022). Absolute optical chiral analysis using cavity-enhanced polarimetry. Science Advances, 8(22), eabm3749. DOI: 10.1126/sciadv.abm3749 / Full text. © 2022 The Authors; exclusive licensee AAAS. Original article: CC BY-NC 4.0.
