Development of an in-situ long pathlength spectrophotometric sensor for oceanic chemical analysis

An oceanographic in-situ sensor has been developed for chemical analysis via conventional absorbance spectroscopy. Laboratory analyses with the Spectrophotometric Elemental Analysis Sensor (SEAS-I) have demonstrated detection limits of 0.2 nmol/dm/sup 3/, 0.5 nmol/dm/sup 3/ and 1.5 nmol/dm/sup 3/ for Fe(II), nitrite and nitrate respectively. The SEAS-I has a 5 meter optical pathlength and an internal cell volume of approximately 1 cm/sup 3/. The optical cell in SEAS-I is a flexible liquid core waveguide which is coiled within a volume of 80 cm/sup 3/. The absorbance response of this long pathlength cell conforms to Beer's law. The molar absorbance characteristics of measured species are identical to those obtained using conventional shout pathlength spectroscopy. SEAS-I is a self-calibrating self-contained system which pumps seawater, meters in reagents, spectrophotometrically analyzes resulting solutions and stoves data. The sensor acquires 2048 absorbance channels between 200 and 850 nm. Complete spectra, temperature and depth information is obtained once per second. The system can be operated via computer or in a standalone mode with internal data storage, and consumes less than 7 watts of power. This sensor is packaged in a 16" long/spl times/4.5" diameter aluminum housing capable of withstanding depths to 500 m. By changing reagents and software, SEAS-I is capable of determining a large number of seawater constituents at nanomolar and sub-nanomolar levels. Deployment of multiple units on underwater vehicles can provide real-time data for oceanographic monitoring. Software modifications make SEAS-I amenable to long-term moored operation.