Geoscience Reference
In-Depth Information
data. Both the flow-through and the open-faced sensor designs can be used in most natural
waters. However, differences amongst sensors (peak wavelengths, bandpass of interfer-
ence filters, gain settings, flexibility of the sensor platform, and automated programma-
ble internal dataloggers) should be taken into consideration when selecting a sensor for
a specific application. For any deployment, it is recommended to perform manufacturer
recommended cleaning and maintenance procedures and frequent intercalibration checks
with a benchtop fluorometer or standard material. Fluorescence data should be verified
with ancillary field measurements to avoid discovering voltage or temperature dependency
on the data stream during final data analysis, and data should be recorded when possible to
avoid loss of measurements due to unforeseen events.
6.5.1 Factors of Concern
6.5.1.1 Particles
The presence of inorganic and organic particles in natural waters can be problematic for
any
in situ
optical measurement of dissolved species. Field fluorometers are subject to
output bias from particle interferences through increased scatter of light within the sam-
ple volume, particularly in moderate to highly turbid systems. The result can either be
inhibition or contribution to fluorescence signal. In a study by Saraceno et al. (
2009
), two
WET Labs flow-through fluorometers were deployed in filtered (in-line 10 µm and 0.2
µm) and unfiltered modes within an agriculturally impacted California watershed, Willow
Slough, demonstrating that the presence of particles in this system inhibited fluorescence
during peak discharge events (~50% underestimation of dissolved species), but not during
periods of little freshwater influx to the system (
Figure 6.12
). Conversely, the presence of
particles can contribute to the fluorescence signal, such as bloom events containing phyco-
bilipigments that fluoresce in the green portion of the spectrum, interfering with the humic
fluorescence signal. It is important to note that a simple field proxy (e.g., turbidity) is not
always adequate for particle prediction with respect to quenching of the sensor output.
Particles may absorb excitation or emission light or in fact, reduce the apparent sample
volume in front-faced instruments. FOM sensors in particular are sensitive to variations in
turbidity and can easily be affected by particle quantity or quality, yet particle quality is an
often overlooked parameter. Some manufacturers have bench-tested sensors to show tur-
bidity rejection to levels as high as 400 NTU (nephelometric turbidity unit). The problem
with this is that turbidity standards used (e.g., high-quality polystyrene beads) are typically
monochromatic and absorb well below the excitation or emission bandpass of the sensor.
Particles in natural systems, however, are not homogeneous and do not always behave in
a predictable manner. There is much work to be done to understand fully the limitations
and potential impacts of particle size and quality on
in situ
fluorescence measurements. To
evaluate the contribution of particles on fluorescence measurements, analysts can operate
two sensors simultaneously in filtered and unfiltered modes during field operations. If only
one sensor is available, fluorescence data can be collected within a laboratory whereby
