YSI 600DW-B Sonde - Manual (Page 236)

Principles of Operation Section 5
5.14 RHODAMINE
WT
INTRODUCTION AND PRINCIPLE OF OPERATION
Rhodamine WT is a red dye that is commonly used in stream flow studies. The amount of the species at
various points (horizontal and vertical) in the water under examination is determined by utilizing the fact
that rhodamine WT fluoresces when irradiated with the proper wavelength of light and thus the YSI 6130
rhodamine WT sensor works on the same principles as described above for the 6025 chlorophyll sensor.
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.
Optical Fiber
Photodetector
Light Source
540 nm
Optical Filter
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
YSI Incorporated Drinking Water Monitoring Systems Operation Manual 5-23