Compact optical instrument for simultaneous determination of oxygen and carbon dioxide

A multi-analyte platform based on a portable instrument is presented that enables oxygen and carbon dioxide to be determined in sample gases. The use of four sensing channels (two channels for each analyte to provide redundancy) warrants high system reliability. The sensing scheme in case of oxygen is based on the quenching of the phosphorescence of the platinum octaethylporphyrin complex. In case of carbon dioxide, a secondary inner-filter effect is exploited that is caused by a pH indicator whose color is reversibly changed. The sensing membranes were placed directly on the detectors and on the light sources so to make additional optical element dispensable, reduce system costs, avoid problems related to optical alignment, optimize the efficiency of data acquisition, and enable facile replacement of sensors. The resulting microcontroller-based system is immune against optical and electrical interferences, contains simple digital signal processing circuitry, and has low power consumption. The response of the system to the two gases was modeled, and calibration curves are corrected for effects of temperature. The instrument was characterized in terms of cross-sensitivity and dynamic response. It can determine oxygen and carbon dioxide in terms of volume percentage.