'asymmetric' peak responses
Posted: Thu Jul 09, 2015 11:22 am
Hi,
I am running VFAs on an old Varian 3400 instrument, which was brought out of retirement a couple of years ago.
It seemed to work fine at first, but I am increasingly seeing an asymmetric effect on peak response, i.e. when the response is greater (than normal) for early eluting peaks, later eluting peak response is depressed, and vice versa. My internal standard is the last peak out, so correcting for IS response works fine for the last few peaks, but is disastrous for early peaks. Injections are generally pretty reproducible, but every now and then this effect shows up.
Method details:
• VFAs in aqueous solution on Stabilwax-DA 30m/0.25mm/0.25um column + 10m guard column
• Hydrogen carrier at 16 psi, split ratio 1:40
• Injector 220C Detector(FID) 260C Oven 110 – 185C @ 15C/min
• 1 ul injection
I'm by no means a GC expert, and so far I have tried only things that are relatively easy to do:
• Replacing septum/liner
• +/- injector temp
• +/- column/split flow
• +/- initial column temp
• Trimming column / adjusting column insertion depth / replacing column
without any noticeable improvement (in fact my experimentation generally makes things worse!)
I ran this analysis about a year ago and the effect was present then but not bad enough to spend time trying to fix it.
Now the effect is more pronounced, and chromatography is not as good, with significantly broader peaks. Peaks are narrower and more well-formed with increasing retention time. First eluting peak (acetate) is broad and tails. Sensitivity also seems less for early peaks (areas less than expected).
In between times I had an amino acid column on there and the effect was similar although not as pronounced, but I ended up having to use 2 internal standards (early and late eluting) to cater for the asymmetric effect. Interestingly, at first I had problems seeing any response at all for late eluting AAs until I dropped the injector temp from 250 to 225, whereupon they popped up. Fiddling with injector temp doesn't seem to help my current problem though.
Any thoughts as to what might be going on gratefully accepted!
Rick
I am running VFAs on an old Varian 3400 instrument, which was brought out of retirement a couple of years ago.
It seemed to work fine at first, but I am increasingly seeing an asymmetric effect on peak response, i.e. when the response is greater (than normal) for early eluting peaks, later eluting peak response is depressed, and vice versa. My internal standard is the last peak out, so correcting for IS response works fine for the last few peaks, but is disastrous for early peaks. Injections are generally pretty reproducible, but every now and then this effect shows up.
Method details:
• VFAs in aqueous solution on Stabilwax-DA 30m/0.25mm/0.25um column + 10m guard column
• Hydrogen carrier at 16 psi, split ratio 1:40
• Injector 220C Detector(FID) 260C Oven 110 – 185C @ 15C/min
• 1 ul injection
I'm by no means a GC expert, and so far I have tried only things that are relatively easy to do:
• Replacing septum/liner
• +/- injector temp
• +/- column/split flow
• +/- initial column temp
• Trimming column / adjusting column insertion depth / replacing column
without any noticeable improvement (in fact my experimentation generally makes things worse!)
I ran this analysis about a year ago and the effect was present then but not bad enough to spend time trying to fix it.
Now the effect is more pronounced, and chromatography is not as good, with significantly broader peaks. Peaks are narrower and more well-formed with increasing retention time. First eluting peak (acetate) is broad and tails. Sensitivity also seems less for early peaks (areas less than expected).
In between times I had an amino acid column on there and the effect was similar although not as pronounced, but I ended up having to use 2 internal standards (early and late eluting) to cater for the asymmetric effect. Interestingly, at first I had problems seeing any response at all for late eluting AAs until I dropped the injector temp from 250 to 225, whereupon they popped up. Fiddling with injector temp doesn't seem to help my current problem though.
Any thoughts as to what might be going on gratefully accepted!
Rick