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Gas-tight syringes equipped with shut-off valves are ideal for headspace

measurements. With these syringes, the shut-off valve can be closed and the

plunger depressed to pressurize the gas in the syringe before injection. After

inserting the needle through the septum of the purge vessel, the shut-off valve

is then opened and the plunger fully depressed to inject the sample. Gas-tight

syringes usually have large needles that can core the septum in the purge

vessel, resulting in clogging of the needle or frequent replacement of the

septum. Side-port needles are available for most gas-tight syringes that will

reducing septum coring and clogging of the needle.

Preparation of Standards for Headspace

There are several different techniques that can be used to prepare calibrations

curves for quantitating headspace samples. The simplest approach is to use NO

calibration gas. By injecting different volumes of the calibration gas, a

calibration of peak area versus moles NO injected can be prepared. The

amount of NO injected is calculated from the volume injected, the

concentration of the standard and the ideal gas law. This technique allows

quantitation of the amount of NO in the headspace above the samples, but

cannot be used to determine the concentration of NO in the liquid phase.

Another approach for standards is to prepare sealed containers with the volume

of water or media as is present in the samples. NO from a calibration gas is

then injected into the liquid phase and the container equilibrated. A sample of

the gas headspace is then withdrawn and analyzed in the same manner as the

samples. From the volume of the liquid in the container and the amount of NO

added, the concentration of NO in the liquid phase can be calculated. This

same approach can be used with NO-donor compounds rather than NO gas. A

known concentration of an NO donor (e.g., SNAP, SNP, nonoates) is added to

the sealed container, the sample is equilibrated and a headspace sample

collected as described above.

GE MOA 280i User Manual

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