Identification of Iris Scent Volatiles Using Dynamic Headspace with PDMS Foam Trapping and GC-TOFMS

The alluring fragrances of flowers are the primary inspiration for new perfumes. In the quest to develop novel synthetic aroma chemicals, perfumers have increasingly relied upon the assistance of analytical chemists to help them identify major chemicals responsible for floral fragrances. Gas chromatography with mass spectrometry detection (GC/MS) has been the method of choice for identifying fragrance chemicals in natural products. Gas chromatography is the tool for separating the numerous fragrance chemicals once they are extracted from floral scents, while mass spectrometry is used to identify the structures of the chemicals, usually by matching the mass spectrum of an eluting chromatographic peak with the mass spectra in an electronic database library. The challenge facing the analytical fragrance chemist is how to extract a representative profile of fragrance chemical that mimics the optimum aroma of the flower being studied. How and when the fragrance chemicals are extracted profoundly affects the chemical profile obtained. Chemists have learned that picked flowers can have an entirely different fragrance profile than growing flowers.1 Additional factors that determine the fragrance chemicals present include temperature, moisture and soil conditions, as well as the flower’s stage of life (i.e., its maturity). The time of day in which sampling is performed is also critical. Optimum sampling time normally occurs when the plant’s primary pollinator is most active. Floral scent analysis normally involves sampling at specific time intervals over a 24-hr period to determine maximum levels of key fragrance chemicals. The peak olfactive moment is defined as the maximum scent emission. The composition of aroma chemicals present at the peak olfactive moment is determined and is then the basis for formulating the reconstituted fragrance. Understanding the biorhythm of the particular flower being studied is fundamental to determining the peak olfactive moment and is critical to the successful artificial synthesis of the desired floral scent. To accurately reconstitute the scent of a flower, it is necessary to capture, analyze and identify the most significant aroma-contributing chemicals that are present at the peak olfactive moment. Methods Used to Isolate Floral Scent Chemicals The analytical techniques used to extract/isolate fragrance chemicals can influence the types and quantities of chemicals collected for GC/MS analysis. Extracting picked flowers with solvents and/or distillation techniques is not advisable. Solvent extracts and floral distillates often lack the delicate aroma of the flower at the peak of its life cycle. Extraction techniques that require heat can generate artifacts by hydrolysis, oxidation or thermal breakdown of plant metabolites. Headspace extraction techniques on living flowers are preferred. They can be classified as static headspace and dynamic headspace. The first headspace technique applied to collection of floral scents was static headspace and was initially applied by Dodson and Hill in 1966.2 They were attempting to identify the aroma chemicals that attract Euglossine bees to several species of orchids. To extract the fragrance chemicals, the researchers placed orchids in a sealed jar for 30 min, then used a gas-tight syringe to withdraw a sample of the air and injected it into a GC. Static headspace methods suffer from poor sensitivity for less volatile floral scent chemicals. While crude compared to today’s sophisticated sampling techniques, Dodson and Hill’s approach helped direct fragrance scientists to the development of modern sensitive and accurate sampling methods. Examples of the most common techniques currently used to extract fragrance chemicals from flowers include: Dynamic headspace extraction: This technique usually involves enclosing the scent emitter in a suitably shaped glass vessel. With a vacuum pump, the scented air is then drawn through a Tenax trap (or a trap filled with some other adsorbing or absorbing material). The chemicals responsible for the scent are preconcentrated on the sorbent trap, eluted off the sorbent with heat (or sometimes with a small amount of organic solvent) and analyzed by GC/MS. Solid-phase microextraction (SPME): SPME was developed in 1988 by Janusz Pawliszyn at the University of Waterloo in Ontario, Canada. The technique was commercialized in 1993 by Supelco. With SPME—a solvent-free sample preparation method—a fused silica fiber coated with a polymer film is exposed to the air E xt ra ct io n te ch ni qu es