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Headspace FID negative peaks
Posted: Tue Aug 07, 2012 8:58 am
by klimbaka
Dears,
I have a problem with reproducible negative peaks during analysis of residua solvents.
The system consists of Turbomatrix HS40 and a Shimadzu GC2014(FID).
The peaks are not present when injection with gas-tight syringe is performed.
Please advise what the cause might be.
You can see a chromatogram of blank water here:
Re: Headspace FID negative peaks
Posted: Tue Aug 07, 2012 11:23 am
by Don_Hilton
If you could give a bit more detail on what you are doing, it might help.
The only way to generate negative peaks in FID is to have a background level of something in the carrier stream that generates signal. And that background being displaced by something that gives no signal (like CO2, N2, Ar...).
So if you could tell what you are using for carrier gas and gas associated with you head space sampler. And then instrumental conditions for the sampler and for the GC, including what column you are using would help as well. If you could tell us what kinds of samples you are analyzing, that may give a clue as well.
Re: Headspace FID negative peaks
Posted: Tue Aug 07, 2012 11:48 am
by klimbaka
I hope the details would help:
The system is used for various analyses of residual solvents in pharmaceuticals.
Solvent: Water
Carrier gas & Heads pace gas: Nitrogen
Column Elite-624 60m
HS...........
Thermostatting time: 20 min
pressurization time: 1 min
withdraw time 0.2 min
Inject volume 1 mL
Needle tepm. 85 degrees
Transfer line tepm. 115 degrees
GC
Injector temp 180 degrees
Detector temp 240 degrees
Flow rate ~ 3mL/min
Program: 40-200 degrees in 6 minutes
The whole system was conditioned for 8 hr (capped detector).
Thanx
Re: Headspace FID negative peaks
Posted: Tue Aug 07, 2012 1:37 pm
by Peter Apps
On a 60 m column with nitrogen as carrier at the optimum flow rate the dead time should be about 6.5 min, which would mean that the whole chromatogram shows the period before the first peak elutes. You have a flow rate of 3 ml/min, what is the internal diameter of the column, and what is the inlet pressure ?
Peter
Re: Headspace FID negative peaks
Posted: Tue Aug 07, 2012 1:51 pm
by klimbaka
I may be wrong with the flow rate.
I'll double check it
Re: Headspace FID negative peaks
Posted: Tue Aug 07, 2012 2:15 pm
by Peter Apps
I may be wrong with the flow rate.
I'll double check it
A very good idea
If you are wrong about the basic parameters of the analysis, how right is the advice you get going to be ?
Peter