gc does not quantify properly gas concentrations from vials

[marcelladionisio]

I have a major problem. By using Standard Gases direct to the GC I obtained a concentration that are near to the theoretical concentrations. But when I use the Headspace sampler with the same Gases the concentrations that I obtain is the 50% of each Gas. Could someone give me a possible explanation?
Thanks

[GasMan]

If you are using a Head Space sampler which uses a sample loop to inject the sample, you have to pressurize the vial by adding carrier gas. This will dilute the sample.

Gasman

[marcelladionisio]

Well, First I put the vials under vacuum and after that I inject the sample with a syringe. Finally, I place all the samples in the Headspace.

[CE Instruments]

Sorry but how are you determining that you have the right concentration in your standard gases ? As standards you are using them to calibrate ? The sample introduction by headspace will only be valid if you are ensuring that the same quantity is injected. This is not easy to do for gases and there are plenty of possibilities for losses and differences so volume injected cannot be valid. You should re-run your calibration for the headspace method.

[GasMan]

Please give us which model of Head Space Samper you are using.

Gasman

[marcelladionisio]

We are using Turbo Matrix 40 Headspace sampler from Perkin Elmer

[GasMan]

Sorry, I cannot help you further. I have only used the Head Space samplers from Agilent, and Perkin Elmer use a different technique of introducing the sample.

Gasman

[dlbenach]

Gasman is correct, the headspace dilutes your sample vial with carrier gas. The Turbo matrix uses a balanced pressure injection. The vial is pressurized with carrier gas to the same pressure as the inlet. This allows the vial contents to briefly replace the carrier gas during sample injection.

[marcelladionisio]

Thank you very much to all of you for your comments