Different baseline scales and the extra peak

[Cest]

Hi! I'm working on a GC 5890 and GC Chemstation Rev. A 10. 01 [1635] with a capillary column and I have some problems with the result.
All I have done: Replaced a new septum, cleaned the liner, cleaned the needle on the autosampler, trimmed and conditioned the column, cleaned the detector (including the collector), use fresh hexane.
I'm now just running hexane to check the peaks are good or not.

My questions:
1. The hexane peak comes out at 7.7-8 min with a nice sharp peak, but with a smaller peak right after.
I have no idea where the small peak comes from. I have done several hexane injections and the small peak always comes out after the hexane peak.

2. The scale of the baseline coming out at the real time "online plot" window when the sample is running is always different from the final result I pull out under the "Data Analysis" mode. For example, the baseline was as low as 5 when I saw it from the "online plot" window. But when the run was done and I switched back to the "Data analysis" mode and opened the data file from the subdirectory I saved, the baseline was about 500 or even higher. I don't know if it is because of the setting somewhere in the software.

Thank you very much if anyone could help!

[chromatographer1]

The scales of the chromatograms of the two formats are not the same. Look in your manual for scaling or formating the output on screen. It is a function set by the software. You must look it up to see how to adjust it to your liking. Self education is very rewarding, especially in this case.

The peak from your hexane is an impurity in the hexane, probably cyclohexane or heptane, possibly trimethylpentane.

Hexane is usually sold as 95% pure. If you still believe it is an artifact then inject another solvent, pentane, or acetonitrile for example and see if the artifact still appears. I don't think it will.

best wishes,

Rod

[PeterB]

A "no injection" run under the same conditions could tell you whether the extra peak is coming from your GC or not, but it's probably a hexane impurity.

[clwaldru]

Hi Cest,

How did you end up fixing the problem with the different scales between the online plot and the Data Analysis mode? Thanks.

Chris

Hi! I'm working on a GC 5890 and GC Chemstation Rev. A 10. 01 [1635] with a capillary column and I have some problems with the result.
All I have done: Replaced a new septum, cleaned the liner, cleaned the needle on the autosampler, trimmed and conditioned the column, cleaned the detector (including the collector), use fresh hexane.
I'm now just running hexane to check the peaks are good or not.

My questions:
1. The hexane peak comes out at 7.7-8 min with a nice sharp peak, but with a smaller peak right after.
I have no idea where the small peak comes from. I have done several hexane injections and the small peak always comes out after the hexane peak.

2. The scale of the baseline coming out at the real time "online plot" window when the sample is running is always different from the final result I pull out under the "Data Analysis" mode. For example, the baseline was as low as 5 when I saw it from the "online plot" window. But when the run was done and I switched back to the "Data analysis" mode and opened the data file from the subdirectory I saved, the baseline was about 500 or even higher. I don't know if it is because of the setting somewhere in the software.

Thank you very much if anyone could help!