YSI 6000UPG - Manual (Page 110)

6-10
The slope, offset, and isopotential point drift slowly, and the probe should be recalibrated
periodically.
All ion selective electrodes are subject to the interaction of species with the sensor membrane
which are similar in nature to the analyte. For example, chloride ion binds in this way to the nitrate
membrane and produces positive nitrate readings even when no nitrate is present in the medium.
Fortunately, most fresh water does not usually contain significant quantities of ions which produce
a large interference on the nitrate reading, such as azide, perchlorate, and nitrite. It usually does
contain some chloride and carbonate ions, but the interference from these ions is relatively small.
For example, if the all of the ionic content of water with a conductivity of 1.2 mS/cm (Sal = 0.6)
were due to the presence of sodium chloride, the nitrate reading would be erroneously high by about
1.6 mg/L. If the conductivity in this sample were all due to sodium bicarbonate, the sensor output
would indicate the presence of only 0.2 mg/L of non-existent nitrate from the interference.
Even though the interference from chloride is relatively small and thus tolerable at low salinity, the
large quantity of this species in salt or brackish water creates an interference so great as to make
the sensor unsuitable for these media.
Despite the potential problems with interferences when using ISEs, it is important to remember
that almost all interfering species produce an artificially high nitrate reading. Thus, if the
6000
UPG indicates the presence of only small quantities of nitrate, it is unlikely that the reading is
erroneously low because of interference. Unusually high nitrate readings (which could be due to
interfering ions) should be confirmed by laboratory analysis after collection of water samples.
Ion selective electrodes have the greatest tendency to exhibit calibration drift over time of all the
sensors available on the 6000
UPG . This drift should not be a major problem for sampling studies
where the instrument can be frequently calibrated. However, if a nitrate sensor is used in a
longer term deployment study with the 6000
UPG , the user should be aware that drift is almost
certain to occur. The extent of the drift will vary depending on the age of the probe, the flow rate
at the site, and the quality of the water. For all monitoring studies using ion selective
electrodes, the user should acquire a few “grab samples” during the course of the deployment for
analysis in the laboratory by chemical means or with another nitrate sensor which has been
recently calibrated. Remember that the typical accuracy specification for the sensor (+/- 10 % of
the reading or 2 mg/L, whichever is larger) refers to sampling applications where only minimal
time has elapsed between calibration and field use.
CALIBRATION AND EFFECT OF TEMPERATURE
The nitrate sensor should be calibrated using solutions of known nitrate-nitrogen content
according to the procedures detailed in Sections 3.2 and 4.2. If a two point calibration protocol
is used, the temperature of the standards should be as close as possible to that of the
environmental medium to be monitored. The recommended calibration procedure is one
involving three solutions. Two of the solutions should be at ambient temperature while the third
should be at least 10 degrees Celsius different from ambient temperature. This protocol
minimizes the effects of taking readings at temperatures which are significantly different ambient
laboratory temperatures.