Geoscience Reference
In-Depth Information
internal quartz tube and any tubing that connects the sensor to the pump that is required
with this design type. In contrast, the flat-faced sensor utilizes two intersecting excitation
and emission cones as its sample volume and is directly related to the optical aperture or
geometric size of the excitation source and photodiode detector. These sensors are ideal for
waters with high concentrations of DOM, where sensitivity is less of an issue and because
cleaning of the optics head is easier than with the flow-through design. Analysts should be
cautioned though that the presence of particles can affect the size of the viewable sample
volume cone, altering signal intensity.
There is a fourth design, based on laser-induced fluorescence, which utilizes laser light
sources, and fused silica fiber optics (
Figure 6.7d
). These systems exhibit high sensitivity
and low detection limits, which are advantageous in environments with dilute concentra-
tions. Internal quenching in highly fluorescent waters is virtually eliminated by normal-
izing to the Raman scattering signal of water (Rudnick and Chen,
1998
). In flow through
systems (e.g., Chen and Bada,
1990
), a reduced sample volume is ideal for measurements
that require small sample size (e.g., porewaters, rainwaters) or benefit from it (e.g., pumped
flow-through sampling). One drawback however is the inability to submerge the entire
sensor, therefore a fiber optic probe (up to 50 m in length) is deployed
in situ
or water is
pumped to the sensor placed in a flow through cell.
6.3.2 Light Sources and Detectors
Original
in situ
sensor designs utilized xenon flash (or pulsed) lamps due to their high-
energy output in the UV and visible regions and long lamp life. But these have been mostly
replaced by light-emitting diodes (LEDs) as light sources, resulting in significant reduc-
tion in cost and power consumption, warm-up time and size of the sensor package. This
transition was made possible due to advances in LED technology; specifically the ability
of LEDs to produce nearly monochromatic excitation light with high signal even at lower
UV wavelengths and nearly infinite lamp life. There are some applications that still require
xenon lamps (measurement of protein fluorescence below 275 nm, low UV polycyclic
aromatic hyrdocarbons and crude oil, and excitation over numerous wavelengths); how-
ever, LEDs can now be used to excite FOM from the low-300-nm up to the 700-nm region
with narrow bandwidths and high spectral resolution. Alternatively, lasers can be used as
light sources, as in LIF sensors and their corresponding applications, which can provide
increased sensitivity of the measurements. Nitrogen and HeCd gas lasers excite at 337 and
325 nm, respectively; and solid state lasers such as the Nd:YAG at 266 and 405 nm.
As with advances in light source technology, improvements in detectors have yielded
savings in instrument size and power consumption. PMTs were originally used in most
fluorometers where the resulting current is proportional to light intensity, and the counting
of individual photons is possible. Owing to the amplification of electrons per each photon,
low light levels can be measured yielding highly sensitive detection. At present most photo-
multiplier detectors have been replaced by photodiode detectors within field sensors. These
detectors are rugged and offer low cost, low power consumption, and small size. There
