Xylem 6-Series Multiparameter - Manual (Page 270)

Principles of Operation Section 5
YSI Incorporated Environmental Monitoring Systems Manual 5-32
described above. The YSI 6131 sensor is designed for these in vivo applications and its use allows the
facile collection of large quantities of data in either spot sampling or continuous monitoring applications. It
is important to remember, however, that the results of in vivo analysis will almost certainly not be as
accurate as those from the certified extractive analytical or cell counting procedures.
The limitations of the in vivo method are outlined below and should be carefully considered before making
BGA determinations with your YSI sonde and sensor. Some of the sources of inaccuracy can be
minimized by combining the data from the YSI 6131 with data from standard laboratory 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 BGA concentration from the easy-to-use YSI Phycocyanin
Probe are designed to complement the more accurate (but more difficult to obtain) results from more
traditional methods of BGA determination. The YSI 6131 sensor is ideally suited for the monitoring the
relative changes (temporally or spatially) in the PC-containing BGA population.
Measurement of PC-containing BGA In Vivo
One key characteristic of phycocyanin-containing BGA is that the cells fluoresce, that is, when irradiated
with light of a particular wavelength, they emit light of a higher wavelength (or lower energy). The ability
of phycocyanin to fluoresce while present in whole BGA cells is the basis for all commercial fluorometers
capable of measuring the analyte in vivo. These instruments induce phycocyanin to fluoresce by shining a
beam of light of the proper wavelength into the sample, and then measuring the longer wavelength light
which is emitted as a result of the fluorescence process. The YSI 6131 Phycocyanin Probe uses an orange
light emitting diode (LED) for the excitation process. On irradiation with this orange light, phycocyanin
molecules resident in whole cells emit light in the 600-700 nm region of the spectrum. To quantify the
fluorescence, the system detector is a photodiode of high sensitivity that is screened by an optical filter that
restricts the detected light. The filter prevents the orange 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
significant amounts of PC-containing BGA, even though none were present. The following diagram can be
used to better understand the principles of the YSI system.
The unique YSI 6131 BGA system consists of a probe which is compatible with the optical probe ports of
all YSI 6-series sondes. The output of the sensor is automatically processed via the sonde firmware to
provide readings in either generic fluorescence units (RFU) or cells/mL of PC-containing BGA. Like all
YSI optical probes, the 6131is equipped with a mechanical wiper to periodically clean the optical face
either by manual or automatic activation. With this feature, the YSI 6131 sensor can be deployed in
environmental water for several weeks without the need for service and is ideal for providing continuous
monitoring of potentially-hazardous PC-containing BGA.
Optical Fiber
Photodetector
Light Source
(Orange LED)
Optical Filter