Chromatography Forum

I have a question regarding the operation of Agilent Headspace Sampler 7697A.

Is the unit able to read the current vial pressure during the equilibration step, when the vial is in the oven? I thought at one time I saw the pressure increasing during vial equilibration, but now it is staying at a constant pressure < 1 psi for the same types of samples that I saw a pressure increasing over time. Or that may have been a dream.

I am currently equilibrating for 3 hours on a sample that has high volatiles, and the pressure has stayed at 0.2 psi the entire time.


When this step ends, and it pressurizes the vial, then I get a current pressure reading as the vial pressure increases to the pressure setpoint.

The 7697A can't read vial pressure during the vial equilibration process. The vial is a sealed system. The only time it is possible to get an idea of the vial pressure is at the end of the equilibration process when the vial is punctured by the sample probe. The "actual" vial pressure reading will give you an idea what the vial pressure was the moment the vial moved onto the probe, followed by an increase in the actual vial pressure as it brings the vial up to the vial pressure set point.

Thanks so much for your help. I was thinking that, and now I know that my memory was flawed.

Vendor support couldn't give me a direct answer.

3 hours is an awfully long time to equilibrate - what are you analysing ?

Peter

Yeah, 3 hours is a really long time. If the sample is highly volatile, should your oven-thermostating time be short?

If you are concerned about pressure in your vial, and you have a highly volatile sample, I would suggest shortening oven time or decreasing your oven temperature. If you are concerned about vial pressure being kept, too, there is a built-in leak test that you can do on that new Agilent Headspace model you have.

This is a new model, and phone tech support may or may not have been thoroughly trained yet. If you have questions specific to the 7697, I would ask very probing questions to the person on the phone and find out their level of training on the model! You could always ask to be transferred to a more senior support person. I worked for Agilent for a number of years. They have great support (and this is a great product!), but you sometimes have to make sure that you are talking to the 'right' person


Dave

Also...

What pressure reading are you looking at that is "<1psi" or 0.2psi during your oven-equilibration time? I can't think of any pressure settings that should be less than one during a run.

Although it has been a while since working with this model...

Dave

I have done a 5 hour equilibration on headspace before, so a 3 hour is not that unusual depending on the sample.

Hi Roger

I would agree that 3 hours might be appropriate for very viscous liquids or finely divided solids, but it is certainly a lot longer than the tens of minutes that are plenty for ordinary samples. With longer equilibrium times interactions among sample components, and absorption into the septum become more significant.

Peter

The "actual" vial pressure reading will give you an idea what the vial pressure was the moment the vial moved onto the probe....

Would it be correct to say the pressure reading at the moment of puncture is the Static Vial pressure? I use to the older Tekmar fixed loop systems.

Hi Charles

You are correct. You can also set the pressurization time to zero (or something, different headspacers have different ways of controlling things) so that it has a bit of time to settle.

Peter