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The development of a novel smart mid-infrared sensing methodology for residual solvents

The potential of a novel polymer modified mid-infrared technique as a ‘smart’ sensing methodology is demonstrated. Diffusion of a penetrant (analyte molecule) was monitored into a Teflon® AF2400 membrane through observation of one of its infrared absorption bands. During the diffusion of select analytes, mid-IR polymer bands were observed to experience a red shift (reduction in absorption frequency). The rate of appearance of these bands matched that of analyte diffusion. As these bands are specific to certain analytes, and their intensity is analyte-dependent, monitoring of these shifted bands forms the basis of a ‘smart’ sensing regime. The suitability of this smart sensing methodology for the enhanced detection of several residual solvents is presented. A fivefold increase in sensitivity through the monitoring of these bands was realized for the detection of ethylbenzene. One of the aims of this work was to determine whether the cause of the polymer band shifting is chemical or optical in nature. Result data presented support the hypothesis that polymer/diffusant interactions cause this band shifting. This is demonstrated by the fact that a penetrant (tetrahydrofuran), which affected a band shift in the polymer, displayed a blue shift (increase in absorption frequency) in its own spectrum. Ethanol did not cause a polymer band shift and displayed no band shifts in its absorbance spectrum. The relative absorbance of the shifted polymer bands is compared between analytes and does not demonstrate a correlation to analyte refractive indices supporting a polymer/diffusant interaction hypothesis.