YSI ADV6600 - Manual (Page 128)

Section 9. Principles of Operation
ADV6600
Y S I
Environmental
Page 118
The limitations of the in vivo method are outlined below and should be carefully considered before
making chlorophyll determinations with your ADV6600 chlorophyll sensor. Some of the sources of
inaccuracy can be minimized by combining the data from the YSI 6025 with data from extractive
analysis of a few samples acquired during a sampling or monitoring study. However, the in vivo
studies will never replace the standard procedure. Rather, the estimates of chlorophyll concentration
from the easy-to-use YSI chlorophyll system are designed to complement the more accurate (but
more difficult to obtain) results from more traditional methods of chlorophyll determination.
9-16.1. In Vivo Measurement
One key characteristic of chlorophyll is that it fluoresces, that is, when irradiated with light of a
particular wavelength, it emits light of a higher wavelength (or lower energy). The ability of
chlorophyll to fluoresce is the basis for all commercial fluorometers capable of measuring the
analyte in vivo. Fluorometers of this type have been in use for some time. These instruments
induce chlorophyll 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 chlorophyll systems use a light emitting diode (LED) as the source of the irradiating
light that has a peak wavelength of approximately 470 nm. LEDs with this specification produce
radiation in the visible region of the spectrum with the light appearing blue to the eye. On
irradiation with this blue light, chlorophyll resident in whole cells emits light in the 650-700 nm
region of the spectrum. To quantify the 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
prevents the 470 nm exciting light from being detected when it is backscattered off of particles in
the water. Without the filter, turbid (cloudy) water would appear to contain fluorescent
phytoplankton, even though none were present. The following diagram can be used to better
understand the principles of the YSI system.
Most commercial
fluorometers fit into two
categories. In the first
category are benchtop
instruments that generally
have superior optical
flexibility and capability
but are relatively
expensive and are often
difficult to use in the field.
In the second category are
sonde-type fluorometers
that have a fixed optical
configuration, but are less expensive, can be more easily used in the field, and are usually
compatible with data collection platforms. The use of a pump is recommended for some sonde
fluorometers and this can result in the need for large capacity batteries for field use.
Optical Fiber
Photodetector
Light Source
470 nm
Optical Filter