Calibration-free optical chemical sensors

Calibrations are necessary for most chemical sensors because the response i s not consistent between sensors nor stable over time. If chemical sensors could be designed to have identical behavior from sensor to sensor and no d rift, the need for sensor calibrations would be reduced. In the present pap er, the feasibility of calibration;free optical chemical sensors is explore d, An indicator-based pCO(2) (partial pressure of CO2) sensor is designed t hat has excellent sensor-to-sensor reproducibility and measurement stabilit y, This superior level of performance is achieved by using the following st rategy: (1) renewing the sensing solution, (2) allowing the sensing solutio n to reach equilibrium with the analyte, (3) calculating the response from a ratio of the indicator solution absorbances, and (4) through careful solu tion preparation, wavelength calibration, and stray light rejection. Three pCO(2) sensors are calibrated, and the response curves are essentially iden tical within the uncertainty of the calibration. Long-term laboratory and f ield studies are presented that show the response has no drift over extende d periods (months), The theoretical response, determined from thermodynamic characterization of the indicator solution, also predicts the observed cal ibration-free performance. Other absorbance-based sensors, such as optrodes , can be designed and operated in a similar fashion, making calibration-fre e optical chemical sensors available for a wide range of biomedical, indust rial, and environmental applications.