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  • A Better Way to Take the Measure of Seawater

Photonics Spectra
Apr 2008
Hank Hogan

Time and tide wait for no man, the old saying goes. That presents a problem for those investigating the changing makeup of the sea. Renaud Vuillemin was a research engineer with the French marine research and oceanic development organization Ifremer, which has facilities at Brest and along the nation’s coast. He noted that researchers traditionally have been limited in time and space when it comes to determining the changing composition of seawater.


Fluorescent cells such as these help determine the chemical composition of seawater. Ultraviolet light enters the cell, causing various chemicals to fluoresce. From this emission, researchers determine the chemical species present in the water. Courtesy of Micro Module.

However, advances in photonic technologies have enabled such limitations to be overcome in the form of a new in situ chemical analyzer developed by Ifremer called Chemini, an acronym derived from “chemical miniaturized analyzer.”

“These analyzers are real in situ miniaturized laboratories using the same kind of analytical process as in a lab, such as flow analysis with optical detection via colorimetry or fluorometry,” said Vuillemin, who now is working at Observatoire Océanologique de Banyuls-sur-Mer.

The new tool is being put to use in both coastal and deep sea versions. Performance is similar to that of standard laboratory systems.

Sampling constraints

The problem with the traditional method for measuring the composition of ocean water is that it involves sporadic sampling followed by laboratory testing. Determining the chemical composition of seawater could be done only at certain spots and times, with results not available until later. Consequently, real-time readings are almost impossible to achieve.

Moreover, the sampling is done only every so often, with only weekly or monthly updates. As a result, rapidly changing situations can come and go without being traced in any detail. Likewise, the sampling technique may misread conditions if the composition changes over a short distance.

These constraints hinder the understanding of the ocean’s natural ebb-and-flow mechanisms. They also make it difficult to evaluate the effect of human activities on the marine ecosystem both in coastal areas and in the open ocean. Research organizations for years have been seeking a solution to the problems inherent in the traditional approach.

Field-deployable analyzers have been developed, with Ifremer coming out with an optics-based one a decade ago. This analyzer used colorimetry to determine spectral absorbance, and it utilized fluorometry to measure the fluorescence of chemical species, thereby pinpointing the chemical composition of the water.

Unfortunately, the colorimeter limited the sensitivity of the system. In addition, the illumination technology was based on a xenon lamp for fluorometry and on an incandescent lamp for colorimetry. Both consumed too much power and were too fragile. “These systems need to display low energy consumption, and the ability to sustain high pressure and long-term deployment,” Vuillemin said.

Vuillemin and his colleagues redesigned the instrument to take advantage of advances in photonics and other technologies. They used a multisensor component from Micro Module, a company based in Brest that develops optical measurement equipment. This module formed the basis for the Chemini.

A key improvement in the new analyzer was the switch to LEDs for illumination, with those emitting in the visible wavelength range for colorimetry and others in the ultraviolet range for fluorometry. The LEDs are inexpensive compared with the previous light sources, and they have the advantage of very long lifetimes and great robustness — characteristics that are required given typical oceanic conditions.

To enhance the ability to detect a small change in a noisy signal, the team exploited the ability of the LEDs to be pulsed. They modulated the LEDs and synchronized the signal measured from a photodiode to that modulation, amplifying and filtering the output before digitizing it to 20 bits. All of these features, Vuillemin noted, were implemented to minimize the noise and to maximize the measurement.

Frederic Berier, a manager at Micro Module, noted that this approach required exacting electronic controls to drive the LED and to acquire the low-level, high-dynamic-range optical signal. It also was necessary to pay attention to optical coupling and filtering. As for the illumination/sensor technology itself, he noted its use in other areas.

“Applications cover wide fields like environment, water quality, air quality, counterfeiting, physical properties of material and biomedical analysis.”

The development team has performed repeatability, calibration and comparison tests of the coastal and deep sea versions of Chemini for various chemical species. In every case, the miniaturized in situ analyzer performed similarly to laboratory systems. The new analyzers are now being deployed and are being commercialized by System S.p.a. of Anagni, Italy.

Contact: Renaud Vuillemin, Observatoire Océanologique de Banyuls-sur-Mer; e-mail:

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