Xylem 6-Series Multiparameter - Manual (Page 279)

Principles of Operation Section 5
YSI Incorporated Environmental Monitoring Systems Manual 5-41
EFFECT OF CHLOROPHYLL ON BGA-PE READINGS
While the orange LED used in the 6132 PE Probe is not ideal for excitation of the chlorophyll in non-BGA
phytoplankton, some fluorescence of environmental chlorophyll will always be induced by the `PE Probe.
Because the filter system for the 6132 photodiode is not perfect in excluding chlorophyll fluorescence, a
minor interference on PE-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 with a YSI 6025 sensor will have chlorophyll interference
characterized by a factor of about 20 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 600 cells/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 20 cells/mL per ug/L of chlorophyll is only a gross
approximation.
EFFECT OF TEMPERATURE ON BGA-PE 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 PE-containing BGA readings
if the site temperature were significantly different from the calibration temperature. This temperature error
can be reduced by employing a PE temperature compensation routine (“PE 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 PE-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%
PE 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 PE-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-PE 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 PE data obtained with a fluorescence sensor that has no
capability of mechanical cleaning is likely to be compromised. The YSI 6132 probe is equipped with a