GE MOA 280i, Operation And Maintenance Manual

Page: 153 / 211

GE Analytical Instruments ©2006
13-11
DLM 14291 Rev. A
the top of the purge vessel after purging for a minute. If it does not, add more
VCl
3
/HCl to bring the level of the liquid near the top of the purge vessel. A
tuberculin syringe with a small needle can be used to inject the reagent
through the septum without having to open the purge vessel. Before analyzing
samples, allow the reagent to purge for a few minutes while the NOA signal
drops.
Nitrate Contamination
The most common problem, particularly for low level (<1 µM) nitrate
measurement is contamination. If the glassware, pipettes, and other
equipment are contaminated with nitrate, then the actual concentration of
NO
3
-
in the standards will be higher than expected. Rinse all equipment with
nitrite-free deionized water before use to minimize nitrite contamination.
Another common problem is nitrate contamination in the water used to prepare
the standards. Check for contaminated water simply by injecting a sample of
the water. Most laboratory water systems can produce water with <1 µM nitrate
if properly maintained. Ion exchange resins must be replaced at regular
intervals, and the manufacturer’s recommendations should be followed to
ensure high purity water is produced. It is best to use water fresh from the
purification system and minimize its exposure to air to avoid contamination.
Replacing the Reducing Agent and Opening the Purge Vessel
In order to open the purge vessel or drain the reducing agent from the vessel,
it is necessary to bring the pressure in the purge vessel back to near
atmospheric pressure using the procedure below. To open the purge vessel:
•
Close the outlet stopcock on the purge vessel.
•
Continue purging the solution with the inert gas until the flow of gas, as
indicated by the gas bubbles, has stopped or slowed considerably.
•
Close the gas inlet stopcock to stop the flow of gas into the purge vessel.