Photo: The resin secreted by a damaged conifer to protect the site of the damage has its own characteristic chiral signature within that of the overall emissions of the plant. Credit: Lykourgos Bougas
Chiral compounds require sensitive techniques to differentiate between enantiomers, and complex mixtures such as consumer products and plant emissions will have distinct chiral signatures offering clues to their characteristics and identity. Current methods for differentiating enantiomers, such as optical polarimetry, mass spectrometry (MS) and nuclear magnetic resonance (NMR) require extensive calibration to achieve accurate and precise results, and may still face issues in reliably detecting trace levels of chiral compounds in complex mixtures. Researchers at Johannes Gutenberg University Mainz (JGU) and the Max Planck Institute for Chemistry (MPIC) have now developed a new approach for rapid chiral analysis that provides precise measurements of substances’ chiral signatures without the need for calibration before each run.
The new chiral analysis technique combines gas chromatography (GC) and cavity-enhanced polimetry (CCP) to separate and analyze gaseous mixtures. The detection system developed by the researchers is highly sensitive to the specific optical rotation of a chiral compound and also performs refractometric measurements to determine the relative concentrations of specific compounds. After separation, the compounds are transferred to a small cavity and exposed to polarized light, and the system precisely and accurately measures the induced rotation of the polarized light. The use of a four-mirror cavity enhances the chiroptical signals due to a large number of passess through the cavity, increasing the sensitivity of the technique. The method does not require calibration before the run, which speeds up the process, and can also be performed in ambient conditions.
The team demonstrated the potential applications of their technique in identifying counterfeit perfumes, as well as detecting plant damage in order to monitor plant health in agriculture and environmental studies. The researchers used their GC-CCP system to compare four authentic high-quality commercial performs with low-cost counterfeits, and found they could easily differentiate the real perfumes from the fakes based on their chiral signatures in a single rapid measurement for each product, without the need for sample preparation. They also used the system to measure the volatile organic compound (VOC) emissions of a young coniferous plant (Pinus heldreichii) before and after sustaining damage in the form of one of its branches being cut off. The team observed an immediate change in the chiral signature of the plant’s emissions after damage, which was consistent with previous research on how plants react to damage. This research was published in Science Advances.
“Our new chiral-analysis approach provides us with precise results, faster and at better sensitivities than traditional techniques, without the need for any calibration before each measurement run. Additionally, our technique has been combined with gas chromatography for the first time to separate the individual components in a complex mixture,” said JGU physicist Lykourgos Bougas, lead author of the paper. “As a result, the chiral form of each constituent present in a complex blend of gases can be directly and accurately identified.”
In addition to the identification of counterfeit substances, the technique could be used to continuously monitor the health of crops or forests, with changes in chiral signatures potentially alerting to sources of distress like pest infestations, water shortages or disease. The method could also help contribute to a better understanding of the role of chiral VOCs in climate change, the authors wrote. This work was completed as part of the EU-sponsored ULTRACHIRAL project, an effort to address the needs of the scientific community in regard to the sensitive measurement of chirality.