HS-GC RSD problem with MeOH & IPA

[Peter Apps]

The headspace is connected via a transfer line to the inlet.

OK, let me be more explicit; how is the transfer line from the headspacer to the GC connected to the inlet of the GC ?. Is it plumbed into the carrier gas supply line, or connected to a needle that goes through the septum, or via a "volatiles interface", or some other way. If it is some other way it would be helpful if you could describe it.

Peter

[krickos]

Hi

In addition to Rod and Peters comments, you might have some contributing issues.

Methanol: As you use a HS oven temp above the bp of methanol (63°C?) and water as diluent you may quite likely get an unstable equllibrum just because of that. So lowering HS temp to 55 or so might be intresting as there is a very strong dependance between HS temp and response for polar solvents, particulary in water.

MIBK: High boiler (118°C?!) with polar function, in loop based HS it is strongly adviseble to use temps in loops and transferlines above the bp of solvents of intrest including diluent. Preferbly also inert material, not Ni, today agilent uses silcosteel as standard but if instrument is older it may be Ni based.

[GC_Man83]

It is plumbed into the carrier gas supply line. I don't think that the connection is the source of the poor RSD, as I have ran many other OVI methods on this particular GC, all with RSD's under 2% for a whole range of OVI's. These other methods howere don't use water as diluent.

[chromatographer1]

Hardware is a very important factor in HS analysis.

I would increase the transfer line temperature and the injection port temperature. I would insulate the line and all connections to avoid any 'cool spots' in the path of the sample.

I recommend (as always) that any bare metal surfaces be modified to have a fused silica coating.

best wishes,

Rod

[Peter Apps]

It is plumbed into the carrier gas supply line. I don't think that the connection is the source of the poor RSD, as I have ran many other OVI methods on this particular GC, all with RSD's under 2% for a whole range of OVI's. These other methods howere don't use water as diluent.

Between the end of the transfer line and the inlet proper you have a piece of unheated, undeactivated metal tubing. This is not good. Unfortunately it is not easy to fix either, so try the instrument settings and sample treatments that have already been suggested and see if they do the trick.

Peter

[GC_Man83]

Well folks,

I have taken the advice that you have given me and have been able to reduce my RSD from a nasty 15% to much more managable 1.1%. Thanks you very much for your informative and prompt responses It is very much appreciated.

[Peter Apps]

Excellent, thanks for the feedback.

Peter