YSI 600DW-B Sonde - Manual (Page 238)

Principles of Operation Section 5
4. Store the concentrated standard solution in a darkened glass bottle in a refrigerator to retard
decomposition. The dilute standard prepared in the previous step should be used within 5 days of its
preparation.
When rhodamine WT standards are required in the future, perform another dilution of the concentrated dye
solution after warming it to ambient temperature. Our experience has indicated that the concentrated solution
that has been kept at cold temperatures is much more stable than the dilute solution stored at room
temperature.
EFFECT OF TURBIDITY ON RHODAMINE WT READINGS
As described above, the filters in front of the photodiode in the YSI 6130 rhodamine probe prevent most of
the green light which is used to excite the rhodamine molecules from reaching the detector after being
backscattered off of non-fluorescent particles (turbidity) in environmental water. However, the filter system
is not perfect and a minor interference on rhodamine WT readings from suspended solids may result.
Laboratory experiments indicate that a suspension of typical soil measured with a YSI 6026 sensor will
have turbidity interference characterized by a factor of about 0.03 µg/L per NTU. For example, the
turbidity of the water must be above 100 NTU to produce an apparent rhodamine WT reading equal to 3
µg/L. In very cloudy water, the user may wish to use the independently-determined turbidity value and the
above compensation factor to correct measured chlorophyll values using, for example, a spreadsheet.
EFFECT OF CHLOROPHYLL ON RHODAMINE WT READINGS
While the green LED used in the rhodamine WT sensor is not ideal for excitation of the chlorophyll in
phytoplankton, some fluorescence of environmental chlorophyll will always be induced by the rhodamine
sensor. Because the filter system for the rhodamine photodiode is not perfect in excluding chlorophyll
fluorescence, a minor interference on rhodamine WT readings from phytoplankton may result. Laboratory
experiments indicate that a suspension of phytoplankton measured with a YSI 6025 sensor will have
chlorophyll interference characterized by a factor of about 0.10 µg/L Rhodamine WT per ug/L of
chlorophyll. For example, the chlorophyll content of the water must be above 30 ug/L chlorophyll to
produce an apparent rhodamine WT reading equal to 3 µg/L. In water with a high algal content, the user
may wish to use the independently-determined chlorophyll value and the above compensation factor to
correct measured rhodamine values using, for example, a spreadsheet.
YSI Incorporated Drinking Water Monitoring Systems Operation Manual 5-25