Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: Golden_4_Life on February 22, 2010, 12:01:25 PM
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In working with samples dissovled in methanol, and, operating the GC at 170 degrees Celsius for 10 minute run time, injector at 245 C, detector at 224C, He flow rate of 25 ml/min--I observed a huge-presumably--solvent peak at the 0.895 minute mark. It partially obscured an analyte peak that had Tr ~ 1.0 mins because the trailing edge of the sovlent peak was grading a long ways into the analyte. The appearance was such that the analyte had to be integrated on the trailing shoulder of the solvent peak--and by doing manual integration in this fashion significant error is incurred. How do I get rid of this huge solvent peak?
:delta:
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Can you dissolve your analyte in ethanol instead of methanol? Ethanol may have a different enough retention time from methanol that you can get a clean analyte peak.
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Doesn't Ethanol retain longer on-column than Methanol--according to their structures and vaporizations? I fear that using Eth will not solve the problem--alas.
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"Doesn't Ethanol retain longer on-column..."
Yes that's point.
Perhaps ethanol will have a retention time of 1.8 min (or whatever) with your GC parameters. That would be good since you analyte should still come out at about 1 min. So perhaps with ethanol, your analyte peak may come out first followed later by the ethanol peak. Since you can run a standard injection of ethanol and identify the peak as ethanol, who cares where it is as long as it's out of your way.
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I concede that you make a well-reasoned point Stewie; thanks for advice, I will attempt the suggested run this week. Although I will not be using the ethanol that is depicted in your avatar (as shown posted with your message board!).
:delta:
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Let me know how it goes..
It probably would make sense to just inject some ethanol first using your GC parameters to get an idea of what the retention time will be. If it's too close to 1 min then it's probably not worth your time to make up a solution of analyte in ethanol (and therefore end up wasting your analyte).
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On the present method, methanol eluted at 1.2 mins and ethanol eluted at 2.02 minutes. The query analyte eluted at 1.85 mins; it is now coming out before the Ethanol and appears as a peak that just borders the front side of the peak (instead of the tailing side as was before) although it is still somewhat occluded.
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Hmm...
Try propanol? I don't know what your analyte is or if the solvent peak really matters to you or not. But it would seem that if ethanol is almost out of your way then perhaps propanol will finally be completely out of the way.
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Just turn on and turn off integration function before and after solvent peak retention time. If you use GC software such as chemstation, do not report solvent peak. Good luck.
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Successfully used Acetone and the solvent peak appeared in like 0.25 mins which was noticeably ahead of the first analyte peak, which was previously obscured by the use of methanol.
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:)
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Just fed you a "mole snack" Stewie.
I cannot seem to wrap my head around this other issue: what are the factors that determine a peak's ''area" count when assayed by an FID specifically? Conventional wisdom says that it ought to be primarily concentration of the analyte and molecular weight (i.e., total Carbon count). However, when I inject a sample that is highly concentrated and consisting of only 6 Carbons (Q) vs. an analyte that is comprised of 12 Carbons but of only 1/2 the concentration (R), then would not you hypothesize that Q compound to give a much larger area count?
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I think what you said is a bit over my head ???
I'm a total synthesis organic guy, and I don't remember much of my analytical stuff. :-[
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Hi
What you write is right but only for the same homological seria. Presence of oxygen, OH, S, N, NH, etc, double bond, aromatic ring etc change the correlation. the amount of sample injected and the conditions in FID ( ratio H2 to Air and Makeup gas) and the kind of makeup gas.
For your analys you can try longer or different polarity column.