Xylem 6-Series Multiparameter - Manual (Page 330)

Chlorophyll Measurements Appendix I
YSI Incorporated Environmental Monitoring Systems Operations Manual I-3
Note: Be sure to carry out1-point calibration of the sensor before the study, using deionized water to make
certain that the sensor reads zero in chlorophyll-free water.
If the sensor is post-calibrated using the values from laboratory analysis of grab samples, will my
field data be totally accurate?
No. They will only be as accurate as possible within the limitations of the fluorometry method.
The
limitations for all in vivo chlorophyll determinations described in Section 5, Principles of Operation will
almost certainly compromise the accuracy of your data even if you employ the best calibration method
possible. For example, different phytoplankton species are likely to fluoresce differently in vivo even if the
actual chlorophyll content is the same. Thus, unless the biological species are perfectly homogeneous with
regard to site in a sampling study and with regard to time in a monitoring study, inaccuracy will occur if
only a single phytoplankton calibration is performed. In addition, the fluorescence intensity of many
phytoplankton types shows a diurnal cycle even though the same amount of material is present. Under this
limitation, the time of day at which calibration samples are taken would have to be identical to the time of
field measurement for the chlorophyll values determined by the two methods to agree consistently. This
level of synchronization is usually not practical.
The key point to remember is that in vivo chlorophyll determinations made with any fluorometer are likely
to be less accurate than those measured by laboratory analysis of individual samples. The advantages of
the in vivo method are its convenience and ability to continuously track changes in relative phytoplankton
values via indirect fluorescence readings
Can I use a dye standard to calibrate my 6025 chlorophyll sensor to increase the accuracy?
The use of the dye solutions described in the Principles of Operation section above will usually not
significantly enhance the accuracy of your field readings relative to extractive analysis of grab samples.
The primary utility of the dye is to check for sensor drift during deployment by reanalyzing the dye solution
after sonde recovery. The use of the 2-point dye calibration procedure may make it easier to quantify the
predeployment dye value and, if so, may be preferable. Remember, however, that no real enhancement of
sensor accuracy is attained by using the dye as a calibrant.