Feature ArticlesA Compact, Low-Power In-Situ Flow Analyzer for Marine Applications
By Renaud Vuillemin
Head of Observation Service
de Banyuls, Laboratoire Arago
Universit' Pierre et Marie Curie
Banyuls sur Mer, France
Dr. Luca Sanfilippo
Continuous monitoring of highly dynamic systems like coastal waters requires frequent sampling in a precise place in order to catch short-term events that might have a strong impact on the ecosystem, such as exceptional phytoplankton blooms or changes caused by storms. The lack of monitoring systems that provide continuous observations of the marine environment, especially concerning chemical species like nutrients in the coastal areas and shelf seas of Europe, is a serious hindrance.
Oceanographers require instruments for in-situ dissolved nutrient analyses. In-situ measurement techniques have many advantages, including avoiding the need to ship storage bottles to land-based laboratories, reducing the risk of sample contamination and allowing for high-frequency measurements. Further-more, additional and collateral sampling can be performed when special phenomena are identified.
Need for Automated Observatories
Autonomous observatory station implementations should rely on technologies that are multipurpose, multiscale, intelligent, adaptive, automated, network-enabled, interoperable, low cost and low maintenance. Most criteria are mandatory in an operational oceanography framework, but also for climate research and water-quality surveys over the long term. Automatic sensor calibration, data-quality-check procedures and techniques with limited human intervention are preferable.
Operational oceanography networks are routinely collecting in-situ and remotely sensed hydrodynamic data, including some multidisciplinary (i.e., biological, optical) marine environment parameters. Remote sensing data offer regular depictions of the sea surface, while in-situ data provide full-water-column spatial information.
The ocean environment is by definition hostile to sensors, and there is room for improvement in many areas. Biofouling limits the period an instrument can be deployed, but anti-fouling paints are usually not environmentally friendly, and except for cabled systems, platforms usually have a limited lifetime because they run out of power.
However, new models of batteries (e.g., capacitors, fuel cells) could extend this lifetime significantly. Nanotechnologies could also decrease the energy demand of miniaturized optical and geochemical sensors (i.e., microarrays, cameras, etc.). Addition-ally, improved storage capacities and communications have also contributed to the decrease in the recovery/redeployment rate of moorings.
New developments in the fields of underwater positioning systems; acoustic, optical and magnetic communications; and inductive battery recharging are expected in the near future.
More sensors are expected to be developed for chemical-parameter measurement, and technologies should be flexible enough to accommodate future innovations.
Due to these considerations, the Observatoire Oc'anologique de Banyuls (OOB) established a technical and scientific collaboration with SYSTEA regarding in-situ analyzers. Testing of new ultraviolet (UV) sensors (like Halifax, Canada-based Satlantic's ISUS V3 nitrate sensor or Oldenburg, Germany-based TriOS's ProPS UV photometer) for nitrate measurement showed that the instruments did not adapt well to the oligotrophic conditions in the Mediterranean Sea.
In-flow analysis remains the best way to detect low concentrations of nitrates and other dissolved chemical species, so OOB asked SYSTEA to deploy its water in-situ analyzer (WIZ) probe in the area to allow it to perform in-situ analysis.
Instrumented platforms are required to monitor environmental changes in real time and to better assess biological processes. Incorporation of meteorological, hydrodynamic and biogeochemical measurements at time scales relevant for the processes related to the microbial loop is a necessary and fundamental advance for the improvement and validation of coupled hydrodynamic-biogeochemical models.
In order to perform this analysis, the OOB designed and manufactured the Microbial Observatory from Labora-toire Arago (MOLA) buoys and installed them at two observation stations in Banyuls sur Mer to become autonomous observatories for long-term deployment and high-frequency sampling with real-time access to data.
The platform is a MOBILIS (Aix-en-Provence, France) buoy (three meters in diameter with a height of 5.5 meters). A pyramidal structure has been developed and integrated to support the external sensors (Milford, New Hampshire-based AIRMAR's meteorological station and global positioning system), position light, power supply (solar panels and batteries), data logger unit and the Wi-Fi remote communication device. A Sea-Bird Electronics (Bellevue, Washington) SBE16 conductivity, temperature and depth probe is placed below the flotation elements on the central mast to measure surface salinity, temperature, fluorescence and turbidity data.
The SYSTEA WIZ is an advanced analytical probe for in-situ applications. It includes a 1.5-milliliter microloop flow reactor—a multibeam-optical-fibers-based colorimetric detector coupled with a microfluorimeter—which enables an extremely low consumption of reagents and standard solution. A compact, plug-in reagent container allows an immediate reagent and calibration solution changeover in the field.
The polyvinyl chloride body is 140 millimeters in diameter and 720 millimeters in height, including the reagent container. This compact cylindrical design allows for it to be deployed like conventional in-situ water-quality-monitoring probes. Power consumption has also been dramatically decreased compared with the previous SYSTEA Nutrients Probe Analyzer Plus/Pro models (three watts in standby mode and six watts during analysis, with a maximum consumption of one ampere, 12 volts direct current).
The two main analytical innovations are the use of a fluorimetric method for ammonia detection, ensuring high selectivity and sensitivity-minimizing matrix effects, and the use of UV photo reduction and subsequent determination of reaction products as nitrites are applied for nitrate detection.
Results are directly provided in concentration units: all measured values are stored with date, time and sample optical density, and the same data are remotely available through an RS-232 serial communication port.
Reagent consumption is limited to about 50 to 60 microliters of each reagent per single analysis for orthophosphate and nitrite and 225 microliters for ammonia and nitrate. The hand-held reagent container is designed to contain up to 500 milliliters of reagent and standard solution to ensure the instrument can perform about 500 complete field analyses. Autocalibration is conducted using concentrated standard solutions contained in the same reagent container.
In August, the WIZ probe was integrated onto the MOLA buoy in Banyuls harbor using its own power supply.
Data transmission was performed using a Global System for Mobile Communications programmable modem to manage the probe and to send data via short message service every four hours to a second phone, automatically forwarding the data to an Oracle Corp. MySQL database, which is Internet-accessible through a browser-based application. The probe will soon be plugged through an RS-232 serial port into the buoy's data logger in order to recover data using the Wi-Fi connection like the buoy's other sensors, automatically storing data in the same laboratory database.
To avoid biofouling in the sampling line, an online filter was added with a copper wire around it.
The WIZ was fixed on the buoy at two meters' depth at two points to avoid any rotation of the probe.
The WIZ nutrients in-situ probe was configured to automatically measure four nutrient parameters (ammonia, orthophosphate, nitrate, nitrite) in seawater using an analytical sequence and standard wet chemical methods. The sampling frequency was adapted to the amount of reagent and standard that filled the flexible bags.
A two-week field test was planned, and a four-hour sampling frequency was chosen to automatically measure the four nutrients. Online working standards were 100 micrograms per liter for ammonia, 100 micrograms per liter for orthophosphate, 20 micrograms per liter for nitrites and 150 micrograms per liter for nitrates.
Very low values were obtained, typical for this time of the season and of the oligotrophic water in the C'te Vermeille, France, area. The values showed that orthophosphate (ranging from eight to 20 micrograms per liter with a median of 13.5 micrograms per liter) and nitrite (ranging from zero to 3.8 micrograms per liter with a median of 2.32 micrograms per liter) concentrations in the harbor were relatively stable. Due to a strong storm during the night of August 27, an increase in the ammonia and nitrate average concentration was observed (up to 22.5 micrograms per liter for ammonia, with a median of 6.4 micrograms per liter, and up to seven micrograms per liter for nitrate, with a median of 0.5 micrograms per liter).
The data show the WIZ probe's capability to work in very low concentrations, very close to the limit of detection with good accuracy.
Regular in-situ calibration checks were made using onboard concentrated standard automatically diluted by the probe before the measurement. Even though no chloroform was placed in the internal standard to prevent calibrant degradation, good stability was observed for orthophosphate and nitrate, with a small decrease in the nitrite standard's accuracy during the first three days. It then showed substantial stability until the end of the test, however.
Ammonia was quite stable for the first six days, but then its accuracy started to deteriorate. This could explain the intercomparison results obtained on two different dates with a laboratory autoanalyzer method on a sample taken in the same place at the same moment.
These results are quite encouraging, since concentrations were in a very low range and no particular prevention measures were taken for the standards. In future deployments, the internal concentrated standard solution will be prepared with a higher ion concentration, and chloroform will be added to evaluate stability and determine how long the same standard can be used in situ, especially for ammonia.
After the two-week deployment, a visual check of the condition of the filter revealed that the protective copper wire placed around the filter prevented the sample line from being clogged by biofouling.
The in-situ deployment of the WIZ probe in the Banyuls sur Mer harbor successfully demonstrated the capability of such a system to work in an automated mode with high accuracy. The probe's two-week autonomous deployment, analyzing four nutrient parameters continuously at four-hour intervals, also showed that the probe could be deployed even longer in a highly oligotrophic environment with minimal maintenance, as there was found to be minimal biofouling in the sample line.
The probe's low power consumption permits it to be adapted to the MOLA buoy without perturbing the autonomy of the system. The compact design and the reagent container provide a low weight and small size and allow a quick, simple and easy reagent changeover, minimizing maintenance service on the buoy, which can be performed by a single person.
Longer tests are planned in order to better evaluate the system, with a focus on the accuracy and repeatability of its measurements.
The authors extend a very special thanks to the OOB Observation Service development team for their involvement in this experiment (Laurent Zudaire, Arnaud Catania, Christophe Mariotti, Cyrielle Tricoire, Eric Carbon's and Louise Oriol) and to Pompeo Moscetta, managing director of SYSTEA and designer of the WIZ.
For a complete list of references please contact Renaud Vuillemin at firstname.lastname@example.org.
Renaud Vuillemin joined Laboratoire Arago in 2007 and is responsible for the Observation Service and the Banyuls Observatory float. He has worked in the field of in-situ chemical analyzers for a large number of various marine applications since 1997.
Luca Sanfilippo is a senior environmental consultant who has been working with SYSTEA since 1996 as a project manager and marketing manager. He has significant experience in the development and management of integrated water-quality-monitoring systems for different field applications, including seawater monitoring.