Xylem 6-Series Multiparameter - Manual (Page 273)

Principles of Operation Section 5
YSI Incorporated Environmental Monitoring Systems Manual 5-35
EFFECT OF CHLOROPHYLL ON BGA-PC READINGS
While the orange LED used in the 6131 Phycocyanin Probe is not ideal for excitation of the chlorophyll in
non-BGA phytoplankton, some fluorescence of environmental chlorophyll will always be induced by the
Phycocyanin Probe. Because the filter system for the 6131photodiode is not perfect in excluding
chlorophyll fluorescence, a minor interference on PC-containing BGA readings from chlorophyll-
containing phytoplankton will result. Laboratory experiments indicate that a suspension of phytoplankton
from Scenedesmus quadricauda which had its chlorophyll content measured using a YSI 6025 sensor will
have chlorophyll interference characterized by a factor of about 77 cells/mL of PC-containing BGA per
ug/L of chlorophyll. For example, at a chlorophyll value of 30 ug/L from Scenedesmus quadricauda, a PC-
containing BGA reading of 2310 cell/mL will be observed over and above the reading actually due to the
presence of BGA. Note, however, that the chlorophyll interferences from other algae species are likely to
be significantly different from that used in the test, and so the quoted value of 77 cells/mL per ug/L of
chlorophyll is only a gross approximation.
EFFECT OF TEMPERATURE ON BGA-PC READINGS
YSI experiments have indicated that the fluorescence of phytoplankton suspensions can show significant
temperature dependence, both due to a change in BGA fluorescence and to a change in probe output. In the
absence of compensation, this effect would obviously result in errors in field PC-containing BGA readings
if the site temperature were significantly different from the calibration temperature. This temperature error
can be reduced by employing a phycocyanin temperature compensation routine (“PC tempco”) resident in
the sonde firmware under the Advanced|Sensor menu where the factor in “% per degree C” can be input
by the user.
The value of this factor can be estimated as follows using a single suspension of PC-containing BGA under
laboratory conditions. In the experiment, the cells/mL value of the suspension is measured at both ambient
temperature and then at a much colder temperature by cooling the suspension in a refrigerator.
Change in Temperature = 21 C at ambient temperature – 2 C in refrigerator = 19 C temperature change
Change in Fluorescence = 100,000 cells/mL at 21 C – 120,000 cells/ml at 2 C = 20,000 cells/mL change
% Change in Fluorescence = (20,000/100,000) x 100 = 20%
PC Tempco Factor = 20%/19 C = 1.05 % per degree °C
CAUTION: This example is hypothetical only. Actual tempco factor values must be determined by
the user.
Note that the use of this empirically derived compensation does not guarantee accurate field readings since
each species of PC-containing BGA is likely to be unique with regard to the temperature dependence of its
fluorescence. Changes in fluorescence with temperature are a key limitation of the in vivo fluorometric
method (see below) which can only be reduced, not eliminated, by this compensation. In general, the best
way to minimize errors is to calibrate with standards of known BGA composition that are as close as
possible in temperature to that of the environmental water under investigation.
EFFECT OF FOULING ON BGA-PC READINGS
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 sometimes be removed in short term sampling applications by
simply agitating the sonde or by manually activating the wiper. For studies longer than a few hours where
the user is not present at the site, the quality of the PC data obtained with a fluorescence sensor that has no