Wax Column Peak Doubling

Hello, 

I am having a very strange problem with some analytes I am dealing with. The system I am using is an Agilent 8890 GC with a 7000D mass spectrometer. The injector is from Gerstel and consists of a thermal desorption unit (TDU2) and a cooled injection system (CIS) which is basically a PTV inlet. The column is a StabilWax-MS from Restek. The sample types are foods and we are analyzing flavor and fragrance compounds. 

While running a variety of samples I noticed peaks doubling. Some analytes were showing up twice at about ~0.5min-1min apart. Here are some images that illustrate the problem in a standard mixture. 

Here is the standard mix. 

Peaks look normal except if you look closely two of the peaks are 'doubling'. Specifically 2-methylpyrazine and 2-heptanol. 

These compounds didn't do this before and this problem only started happening after I had evidence of a contaminated inlet/column. I changed liner and problem went away temporarily but came back after running some samples and then checking standards again. Clipping column similar issue. So I replaced the whole column and I am in process of testing if that was the problem. Has anyone ever seen anything like this before? I don't have a lot of experience with Wax type columns but this behavior is something I've never seen before. It could be an issue with TDU/CIS and I'll know more after testing new column. Its just so strange to only be occurring with certain compounds. I was seeing a similar phenomenon with real samples and it was making data processing extremely confusing. 

  • I was able to solve this issue. Here is what I tried as far as troubleshooting maintenance. 

    1- Change liner (partially helped)

    2- Change column and clean inside of TDU and CIS (partially helped)

    3- Change GraphPack Adapter (no difference this time)

    4- Replace Dynamic Headspace needles and some o-rings. Completely dismantle TDU clean as much as I could and replace o-rings. (solved)

    Not exactly sure if it was dynamic headspace or TDU but most likely TDU because dynamic headspace sampler isn't really connected to injection flow path.

    Steps 1-3 were a waste of time / consumables so next time this occurs I'll start with TDU to truly isolate the issue. 

  • Delayed solution here. But I was able to solve this problem by switching from glass wool liners to glass bead liners. The glass bead liners have more surface area. I assume this issue was exacerbated by contamination but it seems it was related to the liner not having enough surface area to sufficiently trap and release certain analytes. Still weird and comes back sometimes but generally doesn't occur with glass bead liners. Thanks for advice. 

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