Chemical microsensors
It is a common phrase, 'the salty sea'. Common salt, sodium chloride, makes up the bulk of the dissolved substances in seawater, but many more chemical substances are present, right down to trace quantities of gold and rare earth metals. A number of substances are essential to life in the oceans. For example, the main nutrients of nitrate, phosphate and silicate. Others are essential trace elements for life, including iron.
Chemicals also act as tracers of certain activities in the ocean. For example, high levels of dissolved manganese can indicate proximity to hydrothermal vent sites. Dissolved gasses such as carbon dioxide are intimately linked with the ocean’s role in climate.
With so many aspects of oceanography requiring information on the chemistry of the oceans, it is perhaps surprising that the standard approach to obtaining measurements of the concentrations of substances is to capture a sample of water and perform an analysis on a ship or back in a laboratory. We have very few instruments that can give accurate and reliable measurements of dissolved chemicals in situ within the oceans. There are many reasons why this is so, including:
- Many substances are present in low concentrations.
- Other substances present may interfere with the measurements, some techniques lack the specificity required.
- Instruments may find it difficult to cope with biofouling, with the high hydrostatic pressure, or the chemical reagents used may be unstable for use in the long term.
- For some substances, knowing the oxidation state is also important, and it may be important to know what fraction on a substance may actually be available to marine plants (whether the substances are bioavailable).
Many groups are now developing in situ chemical analysers calling upon the latest developments in microfluidic and optical technologies, for example at Monterey, California and St Petersburg, Florida.
The Sensors Group [GG1] at NOC specialises in chemical sensors for:
- Several of the major nutrients important for life in the oceans. These are: nitrate, nitrite, ammonia, and phosphate. We are not yet tackling silicate.
- The trace nutrients, present at low concentrations yet essential for life, such as dissolved iron.
- Chemical tracers for phenomena in the oceans, for example, dissolved manganese, which is a tracer of hydrothermal vent emissions.
Wet-chemical analyzers
Many of these sensors are based on the principle of Flow Injection Analysis (FIA), using techniques originally invented in the late 1950s, and implemented in large laboratory instruments.
The challenge at NOC and other laboratories making these devices for use in the ocean is to condense all of the parts into a small hand-held device. The collage below shows the present state of our art, from the micro-machining of fluid flow channels to a finished instrument.
Purely optical chemical sensors
Charles Clayson described a version adapted for use in seawater of the well-known technique of measuring dissolved nitrate concentration through its absorbance of UV light. Unfortunately, many constituents of seawater other than nitrate also absorb in the UV, as shown by the curves in the absorbance against frequency graph opposite. Here ALNS is the absorption in low nutrient seawater and ANO3 is absorbance due to nitrate. The challenge was to implement an instrument that looked at the area of the spectrum where the ratio of nitrate absorbance to that due to other substances was greatest, shown by the peak in the graph around a wavelength of 225 nanometres (nm).
The current version of the instrument is shown below, with the optical path superimposed.
Data from this instrument when mounted in a SeaSoar [GG1] towed, undulating vehicle is shown here, with low nitrate (blue) in the upper ocean, in summer, having been consumed by plant growth. These results were published in 2010 by Pidock et al.
External links
Review articles at
http://www.annualreviews.org/doi/abs/10.1146/annurev.marine.010908.163817
http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=4302222
