Xylem 6-Series Multiparameter - Manual (Page 280)

Principles of Operation Section 5
YSI Incorporated Environmental Monitoring Systems Manual 5-42
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 just before each sample is taken during
long term unattended monitoring studies. The number of wiper movements and the frequency of the
cleaning cycle for the unattended mode can be set in the sonde firmware. Generally, one wiper movement
is sufficient for most environmental applications, but in media with particularly heavy fouling, additional
cleaning cycles may be necessary.
LIMITATIONS OF ACCURACY FOR THE BGA-PE SENSOR
As mentioned above, the measurement of PE-containing BGA from in vivo fluorescence measurements will
almost always be less accurate than determinations made using either cell counting or spectrofluorometric
quantitation of molecular PE after its extraction from cells. Some of the reasons for this accuracy
limitation with in vivo BGA-PE measurement include the following:
 Interferences from other microbiological species such as chlorophyll-containing phytoplankton
 Interference from sample turbidity.
 Differences in the general fluorescence intensity of different PE-containing BGA species
 Differences in the effect of temperature on the fluorescence intensity of different PE-containing
BGA species
 Effect of the variation in ambient light conditions on BGA fluorescence and differences in this
effect between different PE-containing BGA species.
In addition, when present in high concentrations, colonies of BGA can often be seen with the naked eye
and may resemble fine grass cutting or take the form of small irregular clumps or pinhead-sized spheres.
When BGA colonize into these forms, the sensitivity of the YSI sensor in terms of the fluorescence per cell
of BGA is reduced because it has been designed to detect microscopic, free-floating cells and not large,
macroscopic floating particles. Thus, the sensor is likely to underestimate the total amount of BGA
present in the water when clumps are present.
Users should take careful note that these limitations mean that any in vivo sensors such as BGA-PE and
chlorophyll will be significantly less quantitative than any of the other sensors offered for use with YSI
sondes and make it impossible for YSI to provide an actual accuracy specification in cells/mL for the 6132
PE Probe.
ESTIMATION OF THE BGA-PE RANGE IN CELLS/ML
As noted in the above section, the use of in vivo phycoerythrin fluorescence to estimate the cell content of
PE-containing algae has significant limitations. These limitations also make the designation of a range (or
full scale sensor reading) for any BGA-PE sensor less than quantitative. The range estimate for the YSI
6132 sensor is based on the fact that its reading in an empirical sample of PE-containing algae is very
similar to that of the industry standard fluorometer from Turner Designs which is configured for BGA-PE.
In the estimation experiment, the Turner Cyclops was fixed on its middle range and then its voltage reading
in a BGA-PE culture (a Synechococcus sp.) was determined. The YSI 6132 sensor was placed in the same
culture and its sensitivity adjusted to show the same percent of full scale deflection as the Turner sensor on
its middle range. Since Turner Designs has designated the middle range of its sensor as 0-200,000
cells/mL, the YSI sensor is estimated to have the same range. Naturally, this range is only an estimation
for both the YSI and Turner sensors because of the general limitations of in vivo fluorescence
measurements described above.