HeadSpace Use in GC Analysis

[Carlos Teixeira]

Hello my Fellows,

What is the better difference in temperature between the headspace oven temperature (for the samples equilibrium) and the Boiling Point (BP)
of any analyte?

Suppose that the Analyte has BP of 170oC: what is better equilibrium temperature? 150oC or 170oC?

Tanks very much.

Have you a good good day!!


Carlos Teixeira

[chromatographer1]

Of course such a general question has to be answered generally.

In general, the higher the equilibration temperature the quicker the equilibration.

However, there are many factors which enter into a temperature choice:

Matrix, reactivity, components present, system materials, etc.

[GoldLeader]

To elaborate somewhat on chromatographer1's response (if I may):

- The specific temperature is not as important as the fact that standards and samples use the same temp/time/matrix.

- You must certainly consider your matrix here; if you're trying to determine DMSO (bp=189) in water (bp=100) I certainly wouldn't use a temp near 189 as that would produce tremendous pressure inside the HS vial from the boiling water, although lower temps may not be enough to volatilize the DMSO...

-Depending on the nature of your samples you may find that addition of some salt into your matrix (I sometimes use sodium sulfate) will let you equilibrate at a much lower temp than your bp and give you more reproducible results.

Hope this helps a bit.

-Jeff

[Carlos Teixeira]

My Dear,

Tank you very much for your precious help.

Have you a good and interesting day.

Carlos Teixeira

[mkigls]

I'll go a bit further. In general, you should use the lowest temperature possible to acheive the required sensitivity. This will minimize transfer of the bulk matrix to the chromatographic system. Remember that the partial pressure and the boiling point are only loosely related. As previously stated, salting out is an excellent way to acheive increased partial pressure of a non-polar analyte from a polar matrix. In general (there are exceptions) headspace works best when the bulk matrix is polar and the analytes non-polar or polar with high vapor pressures.