DEVELOPMENT OF A FIBER OPTIC SENSOR FOR MEASUREMENT OF PCO2 IN SEA-WATER - DESIGN CRITERIA AND SEA TRIALS

Measurement of the partial pressure of CO2 gas in sea water (PCO2) is usually accomplished by gas chromatography or infrared spectrometry. Both techniques require large. complex and power-demanding apparatus. In this paper we explore the possibility of developing small, low-power sensors. We have developed and tested a prototype pCO2 sensor for seawater based upon the fluorescence of a combination of dyes encapsulated within a gas permeable silicone membrane at the tip of a single optical fiber. The optical module (Douglas Instruments) delivers 30 Hz chopped white light to a filter and is passed through a dichroic mirror. This light is then focused on to a 220-mu-m optical fiber. The fiber, approximately 2 m long, was terminated with a standard coupler equipped with a small silicone nipple. The internal volume of the sensor tip (about 10-mu-l) was filled with a combination of a fluorescent indicator and two absorbing dyes so as to achieve the required sensitivity. HPTS (hydroxypyrenetrisulfonic acid) was chosen as the fluorescent species; Neutral Red and DNPA (2-(2,4-dinitrophenylazo)-1-naphthol-3,6-disulfonic acid) were selected as absorbers. Illumination at lambda(ex) = 450 nm yielded fluorescence at lambda(ex) = 530 nm, and fluoresced light was returned through the same fiber, reflected at 90-degrees by the dichroic mirror, passed through an interference filter and focused on to a sensitive silicon photodiode. Experiments carried out both in the laboratory on standard solutions and at sea show a precision of 3% in the range 400-500 ppm pCO2. To our knowledge, this is the first demonstration of an optical pCO2 sensor for detecting oceanic signals. This technology is complementary to optical detection of pH and points the way towards full characterization of the CO2 system within this measurement framework.