Magazine article Oceanus

Chemical Sensors in Marine Science

Magazine article Oceanus

Chemical Sensors in Marine Science

Article excerpt

Chemists might debate the definition of a chemical sensor. but within the context of this article we consider a chemical sensor to be an instrument or probe that can be placed directly into the process under study to measure a bulk chemical property (such as salinity), specific chemical compound, or an element. Measurements made inside the process are often referred to as "in situ" measurements as opposed to the traditional approach of removing a sample for later laboratory analysis. In situ chemical measurements provide important advantages over sampling when studying almost any chemical process. The advantages, which will be described in more detail later, include fewer sampling artifacts, continuous and immediate (often called real-time) information, better spatial resolution, remote and unattended operation, and lower costs. These benefits have made in situ chemical sensors very popular in industrial chemical manufacturing, biomedical monitoring, and environmental sciences.

Many of the same chemical measurement challenges familiar to the chemical industry are encountered in the marine sciences. In fact, the ocean might be considered a huge reaction vessel, with seawater carrying nutrients, dissolved gases, organic compounds and trace elements as feedstock for biological and geochemical processes. An industrial approach would place instruments "on-line" in as many places as possible, and use the information obtained to generate predictive models that describe the process's behavior under varying operating conditions. But how can we study the expansive, highly dynamic, heterogeneous ocean in sufficient detail to understand biogeochemical processes? Oceanic biogeochemical pathways for many chemical species, particularly those considered important with regard to climate change, such as carbon and sulfur, have been subject to significant scrutiny through conventional ship-based sampling. Seawater sampling has evolved from bucket collection to the more sophisticated sampling-bottle rosette that accompanies a ship's CTD (Conductivity, Temperature, Depth) profiler. Sediment samples are obtained by a variety of mechanical devices such as grabs and box corers. Yet even with intensive sampling, chemical data remain sparse in space and time relative to the large variability of most biogeochemically important chemical species. Ship-based surveys are biased toward more accessible (mid-latitude) regions and fair weather, and, due to their sporadic nature, they often miss important events such as phytoplankton blooms. In addition, samples can change from their original state during the period between collection and analysis. To study the oceans more effectively, chemical species need to be measured in situ on towed platforms, remotely operated or autonomous vehicles, and moorings.

One of the first chemical sensors developed for oceanography, the conductivity cell (see Oceanus, Spring 1991), has had an enormous impact not in marine chemistry but in the field of physical oceanography. Conductivity instruments can now be moored and towed to measure seawater salinity, providing great insights into ocean circulation. What tools do marine chemists have that are capable of directly probing the ocean? Until recently, most chemical measurements were chained to the laboratory bench because adapting them as in situ sensors posed serious technical challenges. Typically, a sensor must accurately resolve very small changes at low concentrations, selectively detect chemical species in the complex ionic seawater matrix, operate at low power, and be reliable, small, and inexpensive. Integrating chemical protocols and oceanographic instrument design has been problematic. Additionally, the time frames of funding and that required to make an instrument really work are often incompatible. However, through a combination of new technologies and considerable determination on the part of a few researchers, there is now a suite of proven in situ techniques. …

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