YSI ADV6600 - Manual (Page 133)

Section 9. Principles of Operation
ADV6600
Y S I
Environmental
Page 123
As for chlorophyll,
rhodamine WT fluoresces,
that is, when irradiated
with light of a particular
wavelength, it emits light
of a higher wavelength (or
lower energy). The
ability of rhodamine WT
to fluoresce is the basis
for all commercial
fluorometers capable of
measuring the analyte in
situ. Fluorometers of this
type have been in use for some time. These instruments induce rhodamine WT to fluoresce by
shining a beam of light of the proper wavelength into the sample, and then measuring the higher
wavelength light which is emitted as a result of the fluorescence process. Most rhodamine systems
use a light emitting diode (LED) as the source of the irradiating light that has a peak wavelength of
approximately 540 nm. LEDs with this specification produce radiation in the visible region of the
spectrum with the light appearing green to the eye. On irradiation with this green light, rhodamine
WT in the water emits visible light with a higher wavelength than that of the exciting beam, i.e. the
rhodamine fluoresces. To quantify this fluorescence, the system detector is usually a photodiode of
high sensitivity that is screened by an optical filter that restricts the detected light. The filter
minimizes (a) the exciting light being detected when it is backscattered off of particles in the water
and (b) the interference from other fluorescent species such the chlorophyll in phytoplankton.
Without the filter, turbid (cloudy) water or water with high levels of phytoplankton would appear to
contain rhodamine WT, even though none were present. The following diagram can be used to
better understand the principles of the YSI rhodamine WT system.
Field optical measurements are particularly susceptible to fouling, not only from long term build up
of biological and chemical debris, but also to shorter term formation of bubbles from outgassing of
the environmental water. These bubbles can generally be removed in short term sampling
application by simply agitating the sonde manually. However, for studies longer than a few hours
where the user is not present at the site, the quality of the data obtained with a rhodamine WT sensor
that has no capability of mechanical cleaning may be compromised. Like the 6026 /6136 turbidity
and 6025 chlorophyll probes described above, the 6130 rhodamine WT probe is equipped with a
mechanical wiper that makes it ideal for unattended applications. The wiper can be activated in
real-time during discrete sampling operations or will function automatically during long term
unattended sampling studies. The number of wiper movements and the frequency of the cleaning
cycle for the unattended mode can be set in the sonde software. Generally one movement is
sufficient for most environmental application, but in media with particularly heavy fouling,
additional cleaning cycles may be necessary.
Optical Fiber
Photodetector
Light Source
540 nm
Optical Filter