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- Posts: 37
- Joined: Fri Aug 28, 2015 8:05 pm
We have maybe a dozen GCs with molesieve columns doing permanent gas analysis. A few have PLOT molesieve columns, the rest are packed 5A. Carrier is argon, detection is TCD, injection is by gas sampling valve at atmospheric pressure.
On a few of the GCs (3 packed, 2 PLOT), the hydrocarbon gas (C1-15, CO2, N2) standard chromatogram exhibits an ugly tailing blobby peak eluting after hydrogen & just before the oxygen. On a couple of the GCs, the tail co-elutes with oxygen, causing a problem.
However, every other standard we run (non-hydrocarbon standards, e.g. He H2 O2 N2, medical air, pure CH4) never contains this interference. Only the hydrocarbon standard & occasionally samples, and only on select GCs.
It isn't a valve upset as there is no valve rotation other than at inject. And it's on random instruments, not all. All GCs have the same method, same instrument parameters.
Since it's elution time is repeatable, it is only in some of the GCs and not all, we can't figure out what it could be, or how to rid ourselves of it. How could it be a standard component if it doesn't appear on all GCs? Furthermore, can't think of something that will elute between H2 & O2 on a molesieve column.
We've been able to work around this problem for awhile because it eluted before oxygen. Now it's co-eluting with oxygen on a couple of GCs and we have to rid ourselves of it. Baking out the column doesn't affect the blob. Nor does a brand new column.
Anyone ever see this problem before? Any ideas what it might be or how to get rid of it?
On a few of the GCs (3 packed, 2 PLOT), the hydrocarbon gas (C1-15, CO2, N2) standard chromatogram exhibits an ugly tailing blobby peak eluting after hydrogen & just before the oxygen. On a couple of the GCs, the tail co-elutes with oxygen, causing a problem.
However, every other standard we run (non-hydrocarbon standards, e.g. He H2 O2 N2, medical air, pure CH4) never contains this interference. Only the hydrocarbon standard & occasionally samples, and only on select GCs.
It isn't a valve upset as there is no valve rotation other than at inject. And it's on random instruments, not all. All GCs have the same method, same instrument parameters.
Since it's elution time is repeatable, it is only in some of the GCs and not all, we can't figure out what it could be, or how to rid ourselves of it. How could it be a standard component if it doesn't appear on all GCs? Furthermore, can't think of something that will elute between H2 & O2 on a molesieve column.
We've been able to work around this problem for awhile because it eluted before oxygen. Now it's co-eluting with oxygen on a couple of GCs and we have to rid ourselves of it. Baking out the column doesn't affect the blob. Nor does a brand new column.
Anyone ever see this problem before? Any ideas what it might be or how to get rid of it?
