Calibration-Free Optical Chemical Sensors

Abstract

Calibrations are necessary for most chemical sensors because the response is not consistent between sensors nor stable over time. If chemical sensors could be designed to have identical behavior from sensor to sensor and no drift, the need for sensor calibrations would be reduced. In the present paper, the feasibility of calibration-free optical chemical sensors is explored. An indicator-based pCO 2 (partial pressure of CO 2 ) sensor is designed that has excellent sensor-to-sensor reproducibility and measurement stability. This superior level of performance is achieved by using the following strategy:(1) renewing the sensing solution, (2) allowing the sensing solution to reach equilibrium with the analyte, (3) calculating the response from a ratio of the indicator solution absorbances, and (4) through careful solution preparation, wavelength calibration, and stray light rejection. Three pCO 2 sensors are calibrated, and the response curves are essentially identical within the uncertainty of the calibration. Long-term laboratory and field studies are presented that show the response has no drift over extended periods (months). The theoretical response, determined from thermodynamic characterization of the indicator solution, also predicts the observed calibrationfree performance. Other absorbance-based sensors, such as optrodes, can be designed and operated in a similar fashion, making calibration-free optical chemical sensors available for a wide range of biomedical, industrial, and environmental applications.

Observed variability of Lake Superior pCO2

“…The mooring was equipped with a Submersible Autonomous Moored Instrument for CO 2 (SAMI-CO 2 ) (DeGrandpre et al 1995;DeGrandpre 2002, 2004) and a temperature sensor. The SAMI was calibrated for pCO 2 measurements over the range of 20.3-30.4 Pa by dilution of a 304-Pa CO 2 standard with CO 2 -free air (DeGrandpre et al 1995(DeGrandpre et al , 1999). An infrared analyzer (LI-COR Environmental Company), calibrated with National Institute of Standards and Technology-traceable CO 2 gas standards, was used to verify the SAMI calibration.…”

Natural ocean acidification at Papagayo upwelling system (North Pacific Costa Rica): implications for reef development

“…The water-depth varied approximately between 5-8 m depending on the tide, but sensors, hooked to the pier, moved up and down with the tide and were always at the same depth, 1.5 m below the surface. SAMI instruments measured pH and pCO2 spectrophotometrically by using a colorimetry reagent method (DeGrandpre et al, 1995(DeGrandpre et al, , 1999Seidel et al, 2008). Salinity from discrete samples was measured with a WTW probe (Cond3310) and used for correction of pH values.…”

Development of Calibration-Free Autonomous Chemical Sensors for Ocean Biogeochemical Measurements

“…The primary objective is to determine if other chemical sensors can be developed based on the calibration-free design discovered by the PI during the development of a pCO 2 sensor (DeGrandpre et al 1995(DeGrandpre et al , 1999. If the design proves applicable to other analytes, it will be possible to develop autonomous sensors for these analytes that have an identical sensor-to-sensor response with excellent long-term stability, potentially eliminating the need for periodic calibrations.…”

“…The method requires that an indicator chemistry be used that has two different colored forms in the presence of the analyte as described for the pCO 2 sensor in DeGrandpre et al (1999). The absorbances of the complexed and uncomplexed forms of the colored reagent (indicator) are combined to obtain a response that is only dependent upon the indicator equilibrium composition.…”