GE Analytical Instruments ©2006 14-5 DLM 14291 Rev. A
measurement of nitrite or nitrate in serum, plasma and other fluids. Contact
Sievers for specific references for these applications.
Apparatus for Headspace Analysis
The purge vessel is used as a injection port for headspace measurements using
the same configuration as is used for nitrite analysis (page 12-1) except no
reducing agent is used. A few milliliters of water can be added to the purge
vessel to help visualize the gas flow. Adjust the flow rate of gas into the purge
vessel to produce a cell pressure of 4-7 torr.
Sample Collection
In headspace analysis, the sample is allowed to equilibrate in a sealed
container for a fixed period of time, typically 5-30 minutes. During the
equilibration, NO will partition into the gas above the liquid and reach an
equilibrium concentration. After the equilibration period, a gas-tight syringe is
used to withdraw the gas sample for injection into the purge vessel. The
sample sizes can be adjusted to achieve the desired sensitivity but typically 0.1
to 5 cc of gas can be removed from the sample container.
The best procedure is to first fill the gas-tight syringe with a volume of NO-free
gas. The syringe is then inserted into the sealed container and the plunger
depressed to inject the NO-free gas into the sealed container. The plunger is
then withdrawn to remove an equal volume of headspace gas. If the container
is not first pressurize, it will not be possible to withdraw gas from the sealed
container. Since removal of the gas will change the equilibrium, normally only
one sample of headspace can be collected from a sealed container.
The gas sample is then injected into the purge vessel and the Liquid program
used to collect the data and integrate the peaks. In order to obtain a sharp NO
peak, a sample should be injected as fast as possible.
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