ABSOLUTE OPTICAL CHIRAL ANALYSIS

• Detection of chiral compounds at trace levels in the gas phase in real time: for instance, a concentration of monoterpenes at the ~1 ppmv levels)
• Sample removal and calibration are not needed
• Enhanced signal-to-noise ratio and insensitive to any frequency shifts of the cavity modes

• Rapid quality control of mixtures containing chiral (volatile) compounds
• Counterfeit analysis of drugs and cosmetics, e.g. perfumes
• Identifying adulterated chemical mixtures
• In situ observation of chiral volatile emissions from biological organisms, e.g. plants, under abiotic stress

Techniques for analyzing the chirality of a material such as optical polarimetry, mass spectrometry, and nuclear magnetic resonance require extensive calibrations for accurate analysis and typically fail to reliably detect chiral compounds at trace levels within complex mixtures. Chirality-sensitive optical techniques such as photoionization, femtosecond, microwave, and superchiral light-based spectroscopy provide a higher sensitivity, but currently cannot be used for chiral sensing in ambient air. However, all the above-mentioned techniques are not suited for in situ real-time studies of important chemical, biological, and medical dynamical processes. Optical rotary dispersion and circular dichroism can address these purposes in high-concentration samples, particularly liquids; for lower-concentration samples cavity-enhanced chiral polarimetry (CCP) based on ring-down measurements was developed. Unfortunately, this approach has reached only poor chiral detection limits within relatively long integration times; which hinders the use of ring-down based CCP to real-time chiral analysis.

In order to make CCP suitable for real-time chiral analysis, the herewith presented device for an optical chiral analysis of a chiral material uses a continuous-wave laser source and an adequate optical frequency metrology. The device comprises: a light source configured to continuously provide a first and a second continuous wave laser beam (14,16); an optical resonator including a sample container (22) for housing the chiral material to be analyzed and a Faraday rotator (26); light-guiding means for guiding the first beam and second beam; and detector means for detecting resonance frequencies of the first beam and the second beam. The optical resonator can be set up in a bowtie configuration; and the light-guiding means is adapted to guide the first beam and the second beam through the Faraday rotator and the sample container in opposite directions; cf. the upper part of Fig. 2. Depending on the direction of the magnetic field applied to the Faraday resonator, the optical resonator has an R mode and L mode for the first beam and the second beam, respectively. When introducing a chiral material into the container (22), each of these modes splits into a pair of nodes (R1, R2) and (L1, L2), respectively. The nodes of each pair of nodes are separated by 2f_c, wherein the resonant frequency f_c is dependent on the circular birefringence introduced by the chiral material (φ_c); cf. the lower part of Fig. 2. The detector means comprises a control loop so as to automatically control the first and second beam into a resonance mode of the optical resonator; and after reaching the resonance mode it reads out the resonant frequency of this mode for the determination of the total refractive index and the chirality of the chiral material in the sample container.

The sensitivity of a device built in accordance with the above is ~3.4´10^-7 rad/√Hz (20 μdeg within 0.5 s of integration time). Considering the typical value for the specific optical rotation of monoterpenes ~100 deg (gr/ml)^-1 dm^-1, this limit corresponds to a gas sensitivity of ~35 ppmv/√Hz. A chiroptical sensitivity of ~10^-13 rad/√Hz, which corresponds to a gas concentration sensitivity of <10 pptv/√Hz, can be attained by enhancing the finesse of the optical cavity, using higher-quality intracavity optics, improving the detection schemes.

Lykourgos Bougas et al., Absolute optical chiral analysis using cavity-enhanced polarimetry, Sci. Adv. 8, eabm3749 (2022), 3 June 2022

• EP 4124848 A1
• PCT application filed