Sea Technology

OCT 2017

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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10 st / October 2017 www.sea-technology.com turns more rapid data delivery over longer time periods than wet chem- istry, the user needs to understand the framework of the measurement to understand how to interpret the data and, most importantly, the limi- tations of the technique. In this article, we describe SUNA technology, optical nitrate theory, SUNA calibration by Sea-Bird Sci- entific, and examples of data collec- tion and validation techniques. Our intent is to highlight that while the measurement is robust, the optical technique requires a dutiful effort by the user to eliminate interference for delivering the best data. SUNA Technology The SUNA uses the optical ab- sorbance characteristics of nitrate to calculate concentration by measur- ing the response of nitrate to excita- tion by UV light across a range of wavelengths. A deuterium lamp il- luminates the sample volume with UV light. The water sample in the illuminated volume absorbs a vari- able fraction of the photons entering the volume, and the intensity of the light passing through the volume is measured with a spectrometer. The ratio of the light entering the volume and detected by the spectropho- tometer yields the wavelength-specific absorbance. These absorbance spectra across a range of wavelength channels are used to calculate the dissolved nitrate concentration, which is reported in micromoles per liter (µM) or milligrams of nitrogen per liter. Calculating Nitrate Concentration The former explanation vastly simplifies the conversion from optical absorbance to nitrate concentration. Optical T he promise of "chemical-free" chemistry is the oxymoron that you think it is. But the de- velopment of a "wet chemistry- free" sensor for determining ni- trate concentrations in seawater by Ken Johnson and Luke Coletti gave the oceanographic commu- nity a tool that has turned into a robust and cost-effective way to measure one of the primary nutri- ents. Now licensed and marketed by Sea-Bird Scientific, the UV absorption method is the tech- nology behind the SUNA optical nitrate sensor, allowing research- ers and managers to get reliable nitrate concentration measure- ments without the limitations of traditional wet chemistry. The advantage of measuring a dissolved species using absorp- tion over wet chemistry is mani- fest in two major ways: There is no need to handle a sample and no need to prepare, store and deliver reagents. Just as photo- graphic film has given way to digital imaging, ease of use tends to overcome differences in fidelity or accuracy between older and newer technology. In the case of digital photogra- phy, the explosion of the "selfie" was enabled by the ease of taking, transmitting and disseminating the image. With the SUNA, the effort required to learn and assemble the needed sample processing has crumbled away with the ability to measure nitrate simultaneously with other parameters. However, while the SUNA instrument itself is a simple and elegant tool, there is a host of factors that must be con- sidered to measure nitrate concentrations with the accuracy and precision approaching that of the wet chemistry tech- nique. There are always trade-offs, and though the SUNA re- Measuring Nitrate in Puget Sound Using Optical Sensors Calibration, Correction Strategies for Obtaining Accurate Data By Kim I. Martini • Ian D. Walsh • Charles W. Branham The yellow circle shows the location of Jefferson Head Station and the purple circles show the loca- tion of the other offshore stations (total of 18 sites) monitored by King County Department of Natural Resources and Parks.

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