Dual-Kinetic Relaxation Spectroscopy for Discrimination of Volatile Organic Mixtures via Finite-Size Surface Gratings

Discrimination of chemically similar volatile organic compounds (VOCs) in gas mixtures remains a challenge for traditional optical refractive index sensors. In this work, we used a surface holographic grating relaxation technique to monitor the transport kinetics of VOCs. By analyzing the decay of the diffraction efficiency, we extract two independent relaxation time constants. These constants arise from the interplay between the local diffusion of the gas molecules through the material and the geometric aperture losses of the sensing surface (diffusion of molecules in air underneath the grating sensor). These parameters provide a unique kinetic fingerprint for individual species. Mapping these values in a 2D inverse-time state space reveals distinct, nonoverlapping clusters. Furthermore, in the presence of multicomponent gas molecules, by mapping the relaxation lifetimes of pure volatiles and their binary mixtures, we identify a predictable transition in the state space. This approach enables the discrimination between individual volatile compounds and mixtures (e.g., methanol-ethanol) and, in principle, allows their respective concentrations to be quantified and monitored over time with a high degree of sensitivity. This dual-parameter approach uses the finite-size effects of the probe beam, traditionally considered an experimental limitation, to enhance sensor selectivity, offering a robust framework for miniaturizing complex gas-phase analysis.