alpha-Cyclodextrin-modified infrared sensing system for rapidly determining the enantiomeric composition of chiral compounds.

This paper describes a new infrared (IR) sensing method for rapidly determining the enantiomeric compositions of chiral compounds through the use of a chirality-selective compound immobilized on the surface of the evanescent-wave sensing elements. alpha-Cyclodextrin (alpha-CD) was selected as this agent and it was immobilized on a zinc selenide sensing element to allow different analytical signals to be generated for each compound of a pair of enantiomers. Theoretical working equations were developed to monitor the response of the ATR-IR spectroscopic sensor during this process. The difference between the formation constants of enantiomers was crucial to the sensitivity when determining the enantiomeric compositions. Control of the pH of the solution could be used to effectively increase the difference between these formation constants. Based on the static sensing system with optimal conditions, the ratio of the values of the signals of the two enantiomers was <0.4. A linear relationship between the analytical signals and the mole fraction of the enantiomers was obtained under conditions in which the concentration of the chiral compound was sufficiently low. Based on the detected time profiles, this sensing method had a fast response: the detection time could be <10min. With a flow-cell configuration, the time to finish one determination can be shorter than 10min with similar sensitivity and accuracy as in static sensing system.