Anal. Chem. 1994,66, 3352-3361
Submersible, Osmotically Pumped Analyzers for Continuous Determination of Nitrate in Situ Hans W. Jannasch,' Kenneth S. Johnson, and Carole M. Sakamoto Monterey Bay Aquarium Research Institute, 160 Central A venue, Pacific Grove, California 93950, and Moss Landing Marine Laboratories, P.0. Box 450, Moss Landing, California 95039
A continuous flow analyzer powered by osmotic pumps has been developed to monitor nitrate concentrations while submerged in freshwater or seawater. Osmotic pumps driven by a NaCl gradient propel both sample and reagents through a miniature continuous flow manifold. The analyzer operates with sample and reagent flow rates of about 12 and 1 pL h-l, respectively (43 and 0.7 mL month-'), shows a linear response from 0 to 20 pM nitrate with a detection limit of 0.1 pM, and has a 90%responsetime of approximately 30 min. The analyzer can automaticallystandardize itself by correcting for baseline and sensitivity drift by periodically injecting and analyzing known standard solutions. The system has operated continuously for month-long periods in freshwater and seawater and can be easily adapted to a variety of colorimetric analyses. Potential applicationsfor this analyzer include environmental and oceanographic monitoring and process control. Electrochemical sensors such as pH and oxygen electrodes' as well as newer technologies including solid-state ChemFET's,* fiber optical p r ~ b e s , ~and . ~ biosensors5 have been developed for monitoring dissolved compounds in aquatic environments. With few exceptions,6 however, these sensors are still not stable nor sensitive enough for most long-term environmental monitoring requirements. Common difficulties associated with sensors are detector drift, the inability to selfcalibrate, a lack of sensitivity and selectivity (especially in a seawater matrix), and the formation of organic films and biofouling on the sensor surface. Environmental chemists are, therefore, still limited to the collection of samples followed by transport to the laboratory for analysis. This lack of chemical detectors for long-term environmental monitoring of dissolved chemicals poses a series of problems, including (a) the limited number of data points that can be obtained by conventional methods due to the effort, time, and expense associated with collecting and transporting samples in an appropriate manner; (b) the potential for sample contamination and alteration during sample transfer, transport, or storage; and (c) the lag time in obtaining the desired data.',* An approach which circumvents these issues is to adapt continuous flow analyzers to operate autonomously for (1) Langdon, C. Deep-sea Res. 1984, 31, 1357-1367. (2) Wohltjen, H. Anal. Chem. 1984, 56, 87A-103A. (3) Saari, L. A. Trends Anal. Chem. 1987, 6, 85-90. (4) Barnard, S.M.; Walt, D. R. Nature 1991, 353, 338-340. ( 5 ) Schultz, J. S.Sci. Am. 1991, 64-69. (6) Wallace, D. W. R.; Wirick, C. D. Nature 1992, 356,694-696. (7) Johnson, K. S.;Coale, K. H.; Jannasch, H. W. Anal. Chem. l992,22,1065A1075A. (8)D,ickey, T.; Marra, J.; Granata, T.; Langdon, C.; Hamilton, M.; Wiggert, J.; Siegel, D.; Bratkovich, A. J. Geophys. Res. 1991, 96, 8643-8663.
3352
Analytical Chemistry, Vol. 66, No. 20, October 15, 1994
extended periods. Chemical analyzers, where sample and reagents are pumped through a reaction manifold and detector, can determine analyte concentrations with well-proven irreversible chemistries. They differ from sensors, which are dependent on diffusion to transport the analyte onto or through an easily fouled surface and which generally require reversible chemistries. Continuous flow analyzers that can operate underwater have been used to measure concentrations of nutrient compounds, including nitrate and silicic acids%l3and trace elementsI4J5to depths of several thousand meters in the ocean. These submersible analyzers, however, use either peristaltic or piston pumps to propel both sample and reagents, which require significant quantities of power ( N 10 W) and reagents (> 150 mL d-l) when run in a continuous mode. The tubing in peristaltic pumps also requires periodic replacement. Such analyzers are, therefore, best suited for short-term, highresolution spatial mapping of dissolved chemicals. This lackof chemical sensors or analyzers that are capable of unattended long-term deployments has led us to begin developing an unsegmented continuous flow analyzer specifically for monitoring dissolved species in aquatic environments for longer times. Our initial results, shown in this paper, describe a chemical analyzer designed to operate while completely submerged for periods of 1 month or longer for the determination of dissolved nitrate. This system uses osmotic pumps16 to propel both the sample and reagents through a microconduit, flow-injection-style manif01d.l~ Nitrate is determined by reduction to NO2 in the presence of a Cd surface, followed by reaction of the NO2 to form an azo dye that is detected colorimetrically.'* The system has a number of advantages, including (a) a system for automatic in situ calibrations using standards with known nitrate concentrations; (b) mechanical simplicity and ruggedness to ensure reliability; (c) minimal use of electrical power (
