Geoscience Reference
In-Depth Information
6.5.2 Sensor Choices for Specific Environments
6.5.2.1 Optically Dilute Systems
Environments that are weakly absorbing with little scattering properties are considered
optically dilute. Examples of these systems include open-ocean, oligotrophic lake, and
groundwater environments. Biofouling and particle interference in these systems would
be minimal, so there would be little complication with deploying either a flow-through or
flat-faced sensor. The main advantage to the former is a higher signal-to-noise ratio, which
becomes increasingly important in low-DOM waters.
6.5.2.2 Optically Thick Systems
Environments that are strongly absorbing due to either dissolved or particulate matter are
considered optically thick. Examples of these systems include coastal ocean, estuary, river,
eutrophic lake, and wastewater environments. The occurrence of self-shading can be an
issue in these waters, where at high concentrations incident light is prevented from reach-
ing organic material within the optical sampling volume, thereby decreasing the apparent
fluorescence response. Signal-to-noise ratio is not an issue in these environments, but fil-
tration and biofouling are. Applications where filtration is necessary may require a flow-
through sensor, but note that some flat-faced sensors are also available with flow-through
chambers. Analysts should make note that the sensitivity and sample control volume of
the flow-through design is unequalled. A major consideration when using flow-through
sensors is maintenance of the internal cuvette, which is prone to biofouling. This can be
problematic in optically-thick systems, thereby making flat-faced sensors equipped with a
bio-wiper to reduce fouling a better choice in systems with fewer particles.
6.5.2.3 Turbid Systems
Strongly scattering environments are a special case because all turbid waters are considered
optically thick, but not all optically thick waters are turbid. In highly turbid environments
(e.g., greater than 100 NTU), a flow-through sensor deployed with a microporous mem-
brane particle filter may be the best choice to avoid interferences from particles (Belzile
et al.,
2006
; Downing et al.,
2009
; Saraceno et al.,
2009
). Although flat-faced sensors are
very attractive as they do not require the added power requirements of a pump and filter
assembly, the effects of both quantity and quality of particle interferences on these sensors
is not fully understood.
6.5.2.4 Energetically Flashy Environments
Some systems undergo extreme fluctuations in water flow, wave energy, tides, suspended
particles, temperature, and anthropogenic discharge and can be termed “flashy” environ-
ments. These systems can also exhibit large DOM concentration gradients, and the presence
of other fluorescing organic species (natural or anthropogenic) which can lead to interfer-
ing fluorescence signals. Flashy environments may also necessitate a sensor with a rapid
response time, where the sensor optics and associated electronics are designed to react
