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www.sea-technology.com October 2017 / st 13 sis method (NEMI Method, SM4500- NO3-F). The SUNA V2 nitrate sensor was calibrated with nitrate-spiked seawater at Sea-Bird Scientific's state- of-the-art sensor development labs. Absorbance spectra were corrected for seawater bromide using in-situ temper- ature and salinity data. At the Jefferson Head Station, in the upper water column (less than 30 m), the nitrate concentration is very low because it is rapidly consumed by phy- toplankton, which are fueled by the penetration of the sun's rays into the seawater. Below this, the nitrate con- centration is larger because upwelling replenishes nutrient concentrations from deeper, nitrate-rich water. Dif- ferences between the upcast and the downcast can be attributed to verti- cal movement of water. The reproduc- ibility of the sensor is demonstrated by the negligible hysteresis between the upcast and downcast. The SUNA V2 nitrate sensor accuracy was also excellent, to within ±0.18 µM of ni- trate when compared to discrete bot- tle samples taken from 20- to 300-m depth (the accuracy specification of a 10-mm cell SUNA V2 with T and S cor- rection is ±2 µM). Nitrite is included in the bottle samples and can contrib- ute from 0 to 2.2 µM of the total ni- trate and nitrite. The 1-m bottle sample had poor agreement with the reported value from the SUNA V2 nitrate sen- sor (Δ3.5 µM from bottle sample). This could either be due to high variability in the surface layer, as the SUNA and the bottle samples are not collocated. Conclusion With the appropriate calibrations and corrections, the SUNA nitrate sen- sor is a robust tool to measure nitrate concentrations in oceanic and fresh- water environments. In this case study, SUNA measure- ments from Puget Sound corrected for bromide match the accuracy of mea- surements made with traditional wet chemistry techniques to within ±0.18 µM. While care must be taken when processing SUNA data, updates to the reference spectrum and seawater cor- rections can be applied by the user with Sea-Bird Scientific's UCI graphi- cal data processing software if concur- rent temperature and salinity measure- ments are available. These corrections help SUNA measurements made in seawater approach those made with discrete measurements, while retain- ing the high vertical resolution of opti- cal-based chemical sensors. Scientists and engineers at Sea-Bird Scientific continue to refine SUNA technology, calibrations and processing to mini- mize sensor drift and interference from other chemical species for greater ac- curacy and precision in a wider range of environments. Acknowledgments The authors would like to acknowl- edge Stephanie A. Jaeger (King County Department of Natural Resources and Parks), Laura Collins and Dave J. Mur- phy for their help in writing this article. References For a list of references, contact kmartini@seabird.com. ST Kim I. Martini is a senior oceanographer at Sea-Bird Scientific whose work includes the characterization and optimization of sensors, Navis customer sup- port and customer training. Martini received her Ph.D. in oceanography at the University of Wash- ington and went on to do post-doctoral work at the University of Alaska. She came to Sea-Bird from NOAA PMEL, where she led the development of the Ecosystems & Fisheries-Oceanography Coordi- nated Investigations (EcoFOCI) group towed instru- ment platform effort. Ian D. Walsh is the science director and senior oceanographer at Sea-Bird Scientific, with respon- sibilities over the biogeochemical and optical in- strument lines. Walsh oversees Sea-Bird Scientific's training classes and online training. He has a Ph.D. from Texas A&M; University and an extensive back- ground as a seagoing oceanographer. Charles W. Branham is the senior chemist at Sea- Bird Scientific, with responsibilities including oceanographic sensor design with a focus on the characterization, optimization, calibration and validation of the pH sensor line. Branham received his Ph.D. in process analytical chemistry from the University of Washington. His research interests are focused on developing stable, robust, precise and fast-responding sensors for the analysis of complex systems. w w w. n ke- in s t r ume nt at io n .c om A PPLI CA T I ON CA SE Monitoring station Water quality control Environmental monitoring Bathing water monitoring Temperature, ConducƟvity, Depth, Turbidity, pH, Redox, CDOM, Dissolved oxygen, Chlorophyll a, Blue Green Algae, Hydrocarbon. SA MBAT S EA MulƟparameter probe ENVIRONMENTA L MONITOR ING WATER QUALITY CONTROL "The advantage of measuring a dissolved species using absorption over wet chemistry is manifest in two major ways: There is no need to handle a sample and no need to prepare, store and deliver reagents."