Real-time optical sensing of exhaled acetone concentration utilizing non-Fickian Nafion diffusion inside a flow-through sample chamber

Abstract Rapid in-situ chemical analysis of flowing gas streams is of interest in a wide range of applications but requires deconvolution of the time-scales associated with the analyte source concentration, its accumulation within a sampling chamber, and its detection by a sensor. A mathematical analysis is presented on the use of a flow-through sample chamber for rapid, in-situ breath analysis utilizing analyte diffusion through a Nafion membrane optode. We show that this approach yields apparently non-Fickian (anomalous or Case II) transport that varies from t1/2 to t as t → 0 with constant inlet concentration. Such behavior arises due to the transition from membrane-limited to sample chamber-limited transport dynamics depending on test conditions. The model is validated utilizing experimental data obtained from the color response associated with the Friedel-Craft acylation of acetone vapor with resorcinol reagent immobilized in Nafion membrane solid-state catalyst. Reduction of optode membrane thickness and increase in membrane humidification yield an optical response limited only by sample chamber material accumulation. At this limit, the exhaled breath signal for acetone obtained from a healthy individual is found to vary as t2 (apparently Super Case II transport). Utilizing a simplified material balance on the human lung, this observation is ascribed to a constant acetone exhalation rate as opposed to a constant exhaled acetone concentration. This conclusion is shown to have broad implications on the use of exhaled breath biomarkers for medical diagnosis, in particular, lung physiology and permeability.