Chromatography Forum

I've noticed that when I fill a headspace vial with a standard gas mixture, the area of the peaks are dependent on the pressure in the headspace vial before sampling.

I thought the sampler I'm using (Agilent 7697A) vents to atmospheric pressure in the loop loading/pressurization step, so why does the small amount of overpressure in the standard vial make any difference?

Should I purge the vial with the calibration gas and then vent it to atmospheric pressure, or should I leave a small amount of overpressure in the vial?

What's the proper way to prepare a headspace vial standard from a calibration gas?

thx

From your first line it sounds like you are pumping gas into a headspace vial for sampling? Below I'm going to list out what I know of the 7697A sample loop loading. Hopefully some of it will be useful for you.

It is important to have an idea of the "natural" vial pressure in the vial after the vial equilibration is finished. The headspace oven temperature, length of time, sample properties, amount of sample, and how much shaking is used will influence what the "natural" vial pressure is before the vial is punctured by the sample probe. The nice thing about the 7697A is when the vial is punctured by the sample probe for a split second you can see the actual pressure of the vial before the vial is filled with extra gas.

The Agilent 7697A headspace uses the pressure difference between the vial pressure and atmospheric pressure to create the flow rate that fills the loop.

The key parameters for loading the sample loop are:
- Vial Pressure (should be higher than the "natural" pressure of the vial)
- Loop Fill Ramp Rate
- Final Loop Pressure (often 5 to 10 psi lower than the Vial Pressure setting)

During the loop loading proccess the vial is vented to atmosphere at a controlled flow rate (Loop fill Ramp Rate) until the pressure in the vial and sample loop are at the Final Loop Pressure. During the loop loading process, the only time the vial will reach atmospheric pressure is if the Final Loop Pressure was set to 0 (most of the sample is vented).

A couple of mistakes I have seen done that should be avoided. Setting a Vial Pressure that is less than the "natural" vial pressure after headspace vial equilibration. This will let sample out of the vial before the sample loop loading process has began and reduce the amount of sample that could have gone to the GC (plus this will likely contaminate the system). The second is setting the Vial Pressure and Final Loop Pressure to the same pressure setting. In that case there is no actual flow rate through the loop and the loop was loaded only by gas diffusion.

Getting the maximum results from the sample loop loading process involves making sure the Vial Pressure is above the "natural" pressure of the vial, and having enough pressure difference between the Vial Pressure and Final Loop Pressure to produce the flow rate that sweeps the volume of the sample loop two or three times.

I cap a headspace vial and fill it with a calibration gas by inserting two needles into the septum. One needle is long and reaches to the bottom of the vial and through this needle the gas is flowing. A second short needle is inserted in the septum to allow the gas to escape and flow to occur.

After purging for 30 seconds, I withdraw the short needle causing flow to stop, and the vial pressurizes to the setting on the supply gas regulator...about 20 psi.

Should I then run that vial with the 20 psi pressure in it, vent the vial to a some intermediate pressure, or vent it to atmospheric pressure before running it?

I presume that you are using the headspacer to add extra gas to the vial before sampling ? (this being the default for the hardware). The headspacer adds gas until the vial pressure reaches a preset value, if you have more pressure already in the vial then less gas is added. The added gas dilutes the sample in the vial, so with more initial pressure you have less dilution and therefore bigger peaks.

Having two needle holes through the septum raises the possibility of leaks, so your safest procedure will be to vent the vial to atmosphere before it goes into the headspacer. This will stop leaks (but not diffusion) and you will get a consistent dilution of the sample because it will need a consistent volume of pressurization gas to bring it up to the set pressure.

Are you heating the vials ?, how long do they wait before being sampled ?

Peter