Geoscience Reference
In-Depth Information
a concern. One example is the portable SMF2 spectrophotometer (Safe Training Systems
Ltd., Wokingham, UK), which utilizes a xenon flash lamp, bandpass and interference filters
for peak excitation wavelength of 280 nm. The fluorescence signal of tryptophan is mea-
sured between 350 and 360 nm. Measurements from this sensor can be calibrated using
diluted river or waste water samples to detect effluent in natural systems (Baker et al.,
2004
).
A great deal of effort has gone into transitioning fluorescence spectroscopy from the
laboratory benchtop to
in situ
field measurements. In
Section 6.3
, we will focus on the
most common submersible design types that are commercially available at the time of this
topic's publication that can offer analysts an appreciation of the options for their particular
applications.
6.3 Instrument Design Types
6.3.1 Sensor Configurations
There exists a wide range of sensor designs used by researchers to collect
in situ
OM fluo-
rescence measurements. The challenges of
in situ
measurements are the same - how to
collect high-quality fluorescence data that is inexpensive and limited by low-power UV
capabilities? Now we will focus on the most commonly designed, commercially avail-
able field sensor designs. Quite simply, an
in situ
fluorometer consists of five main optical
components: a UV light source, optical hardware to bring excitation light to the sample
volume, optical hardware to collect the emitted fluorescence from the sample, optical fil-
ters to separate the excitation and emission wavelengths of interest, and a photodetector
(
Figure 6.7
). These components are housed within pressure housings of two types of sen-
sor geometries: (1) the open-faced design, which can either be right-angle (
Figure 6.7a
)
or flat-faced (optical backscatter type;
Figure 6.7b
) with a fixed cell geometry and volume
and (2) the flow-through design, which may require a pump to pass the sample through a
quartz cuvette flow tube (
Figure 6.7c
). With either geometry, sensors can be configured to
collect single or multispectral measurements. Collection over multiple wavelengths greatly
improves the ability to characterize FOM as well as provide a measurement of the bulk
intensity or amount of FOM.
The flow-through geometry type tends to have greater efficiency in signal output com-
pared to the open-faced design, in part due to the absorption of light by the plastic (or epoxy
resin) facing of open-faced meters. Flow-through designs are an ideal choice for waters
with low OM concentration and few particles as the sensor geometry (sample volume) is
well defined as the width and length of the detection area is based primarily on the cuvette
and the geometric area of the excitation and photo diode detector assembly. This geometry
is also well suited for waters with high OM concentration as they have small pathlengths
and are therefore less susceptible to alterations in the sample volume cone that can erro-
neously reduce fluorescence signals. Limitations of flow-through designs exist for deploy-
ment in waters that are not optically clear due to difficulty in maintaining cleanliness of the
