head pressure in static headspace

[emorgan]

hi,

i am running a Perkin Elmer balanced pressure static headspace sampler, going to an Agilent GC split/splitless inlet. The split ratio is set to 1:1 and the inlet set to constant pressure at 4.8 psi.

Usually, we have the head pressure set to give a 30mL/min flow out of the split vent. however, we recently got a new HS-40 and an engineer told us that if we reduced the head pressure it should improve the sensitivity of the instrument because we would be losing less out of the split vent. had the pressure set at 18.6 psi originally giving 30mL/min out of the split vent. we reduced this (over a few stages) down to 10psi, which gave 11mL/min out of the split vent.

this made no difference to the sensitivity of the instrument, which has, to be honest, baffled my head a bit. can anyone suggest a reason why changing the head pressure has no effect on the chromatography?

not particularly vital to my work, but just something that has been bugging me!

many thanks,

Beth

[chromatographer1]

I suspect your analyte is so volatile that the difference in pressure did not make much difference is the amount getting on the column.

That or the split ratio changed with the change in pressure.

Best wishes,

Rod

[emorgan]

yeh, the analyte is propionitrile in dimethylacetamide, so pretty volatile, might try with a less volatile analyte and see if it makes any difference!

[Peter Apps]

All alse being equal the size of the peaks (= sensitivity ?) depends on the split ratio. The split ratio is simply the ratio of gas going into the inlet to gas going into the column. Depending on how you have the inlet pressure controlled it is possible that reducing the head pressure at the headspacer reduces both the flow to the inlet from the headspacer and the flow out of the inlet to the split, leaving the split ratio unchanged.

Of course, if you are running standards and samples under the same set of conditions and basing your "sensitivity" on a peak area ratio, or a calibration you will not see any difference no matter what you do to the gas flows.

Peter

[Rouan]

Hi Emorgan,

If you look at the HS40 sampling you will see that the amount of sample being injected is determined by time (sample loop size in other headspaces) and vial pressure (dilution) By decreasing your headpressure on the vial the flow of your sample onto the transferline has been decreased and thus the amount of sample that you can inject in your specified time. So anything gained by less dilution in the vial is lost.

[Locutus_nor]

The HS40 has an option of using "High pressure" (P2) during injection (atleast our has). This high pressure is applied during the injection (injection time) to force the sample out of the vial, trough the transferline and into the inlet. It will also help you avoid double injections if you have high pressure in your vials.

If you have your GC inlet pressure at 4,8 psi I guess that any higher pressure at injection will go out the split vent (easily checked by measuring split vent flow during injection).

Lowering the carrier pressure (P1) will not have any effect on peak size, because the split is determined by the P2 pressure.

We use a direct connection between the transferline and column, avoiding this problem. But we get a pressure pulse at the start of every chromatogram.