Double peaks in GCHS

[ganna]

Hi there ppl,

I was performing analysis using GC-HS and always obtaining double the peaks expected in my chromatograms. To correct this I changed the injection septum, replaced the glass wool in the liner but still double the peaks were obtained.

To try and isolate the source I injected manually one of the solvents under analysis manually and obtained the 1 peak I was expecting. I then assumed the problem was coming from the HS so the needle was cleaned but this did not improve anything.

I then increased the flow rate of carrier gas (Nitrogen) from 12.5 cm/s to 33.5 cm/s and this seemed to have solved the problem as the expected number of peaks are being eluted.

Any suggestions as to why this happened?

[BG]

Hi ganna

I understood you have an HS system with syringe...........what is the injection speed of your autosampler, the injector temperature and eventually the depth of injection ??

[GC_Man]

Hi !

Probably, I can tell you the way your double peaks happened: I had the same problem some years ago !

You had double peaks because the injection took place twice: The first time an injection took place when the needle (after thermostatting) went into the vial ! The reason therfore is, that the pressure in the vial was probably too high (the pressure made by the solvent in the vial !!), so that this pressure was much higher than the pressure coming from the Nitrogen into the vial (in order to pressurize the vial !) !

Look at your chromatograms: Is the difference in time of two corresponding peaks always the pressurization time of your HS ?

When you increased the flow rate of Nitrogen, you also increased the head pressure, so that it was now higher than the pressure in the vial !

[ganna]

Thanks GC_Man that does seem to be the reason as I had checked the pressure in 1 of the vials & it was about 5psi which is about the same as the carrier gas pressure.

And you were also right about the difference between the 2 peaks being the same as the pressurisation time!!!

Thanks again my curiousity is happy...for now!!

[Mark]

The other thought on the double peaks is that initailly with such a low flow rate that when the syringe was injecting the sample the injection temporarily caused a high flow rate because of the injection speed. Raisisng the flow rate to the right flow now more closely matched the flow rate caused by the injection and no more (or much reduced) peak splitting.

Regards,
Mark