Geoscience Reference
In-Depth Information
concentrations found in the study area. Therefore a major consideration should be instru-
ment gain. Many sensors are available with fixed gains and these settings may not be suit-
able for some environments. Undoubtedly a sensor optimized for linear, full-scale response
in optically dilute waters is most likely not suitable for use in certain wetlands or vice
versa. Some instruments use a logarithmic voltage response (e.g., Chelsea UV Aquatracka)
to eliminate the need for multiple fixed gain settings. In environments with high DOM
concentrations, internal quenching or absorption of light within the sample volume may
prevent fluorescence signals from reaching detectors that results in nonlinear fluorescence
intensities (
Figure 6.11
). A priori knowledge of measured optical properties in most study
environments is critical in the application of modern NOM sensors. At present, there is
no sensor that includes algorithms to optimize excitation or emission peaks, bandwidth or
electronic gain setting. There are, however, some sensors that include automatic gain set-
tings, albeit in fixed ranges.
6.5.1.4 Temperature Effects
All fluorescence measurements are subject to influences from temperature (Chen and Bada,
1992
). In addition to the issue of self-heating discussed in
Section 6.4
, variations in the
ambient water temperatures of the sampling environment can also impact fluorescence
values. This can be monitored with some instruments with internal and external thermis-
tors and some manufacturer's software offers temperature correction algorithms to apply
to data streams. As discussed in
Section 6.4
, correction procedures for temperature effects
can be conducted in the laboratory, and analysts should be aware that various deployment
styles and environments can influence temperature effects differently. Consider the case
where a vertically profiling sensor is deployed through a steep thermocline where rap-
idly changing external temperatures causes the internal temperature of the sensor to vary
quickly. Depending on the difference between the external and internal temperatures, the
effects on the sensor can be instantaneous or lag behind fluorescence measurements mak-
ing corrections for temperature challenging. Conversely, sensors deployed on a mooring
may undergo large fluctuations in environmental temperature, but over longer time scales
(i.e., weekly, monthly, seasonally) where the sensor has time to reach thermal equilibrium.
This can make for easier corrections to the data stream. Similarly, if operating a sensor
where water is pumped to the meter while submerged in a water bath or a holding tank, the
water bath can essentially serve to regulate the internal temperature of the sensor.
6.5.1.5 Biofouling
All sensors will biofoul if given enough time deployed in natural water systems. The extent
of biofouling varies as a function of environment, so analysts are encouraged to determine
how problematic this will be in their system, as fouling can severely interfere with meas-
urements (
Figure 6.13a
) (Davis et al.,
2000
). Historically, substances containing tributyl tin
(TBT) were used for long-term aquatic anti-fouling methods, but TBT has a negative envir-
onmental impact and is no longer in use today. Laboratory and field testing of TBT-based
products, antifungal agents, and silicon-based compounds on glass and acrylic surfaces
