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Reproducibility Issues after system maintenance

BChristle

I am using a splitless method to analyze a solution of a material that contains non-volatile polymer and volatile monomer I am running a sequence in which use one method to  volatalize and analyze the monomers and a few  high boiling solvents followed by a second method that degrades the polymer and removes it. I can see that method 2 is pretty effective in cleaning out the liner from the polymer and resulting contamination. We tuned the method untile we had very nice overlaying chromatograms and a very good calibration result.

Then I replaced the filament, the guard column and the MS transferline (both fused silica, deactivated) and all the the connections and ferrules from these to the MS, the PUU (which is used to connect to the analytical column on both sides) and the inlet. After that the calibrations still look fine (area ratios of standard to volatile components), but the overlays don't look good anymore. I even observe shifts in retention time.

When I use split methods for other materials, the overlays look much better.

There is no leak based on the tune and the tune looks very good in general. 

I replaced the liner+O-ring, septum and syringe step by step. Still the same issue.

I also just recently replaced the vent line filters.

A consultant of ours suspects the vent valve to maybe not seal properly. Also, I was wondering if it could be the gold seal that needs to be replaced. 

Also it would be great if someone could explain to me a bit more how the sample is injected and volatalized and then released from the inlet in a split/splitless mode. How long is the material heated and not released in the inlet, How long does the vent valve open before it closes again? How far does the syringe go into the liner? Is there a part of the liner that is not heated as much or is the whole liner heated homogenously in the inlet?

My system is an Agilent GC 6890N - MS 5975C. 

I used both of these liners: https://www.agilent.com/store/productDetail.jsp?catalogId=210-4004-5, https://www.agilent.com/store/productDetail.jsp?catalogId=210-4022-5 I know they say split, but I was told they both work for splitless as well and since I have some split and some splitless methods that worked pretty well for us.

Thanks. Bettina

  • 0 in reply to BChristle

    It's a dance with many partners!  Would you please share the latest tune report?  You said "pumping for 2 days" below. What is the vacuum gauge reading?

    The N2:O2 ratio could be from some combination coming from your gas, leak or leaks, screwed up tune parameters due to too high flow, too low flow, poor vacuum, and on and on.  

    After a GCMS is tuned, the simplest way to test it is to have a known peak of interest that you inject. Then you evaluate the chromatogram for retention time, chromatographic peak shape, and height/area - which checks mostly the sample, syringe, inlet, flow rates, column, heaters, etc, but also the MS sensitivity.  You check the spectrum of the peak for relative abundances of ions across the range.  If you set the range to start around 15amu, you'll also see water, nitrogen, oxygen, etc.   So one injection takes a good picture of the system right then.  If that peak of interest is in the right place, at the right time, is in the typical range of height/area plus or minus whatever you deem acceptable, and the spectrum is correct plus or minus whatever you deem acceptable, the system is ready to run.   This is how some EPA methods work. You inject DFTPP, evaluate the peak and if it meets criteria the system is ready to go.  That tests more than just the ion source.

    In that test if the spectrum is starting to tilt, losing higher masses relative to lower ones, or the total sensitivity is lower than your limit, then run tune.   Some operators running clean drinking water applications only tune once every six months or more!  

    I doubt you need to do anything with the MS right now, but when you're ready to clean the source you can see videos on how to do it here:

    Eliminate the Fear of Mass Spec

  • 0 in reply to paulsalverda

  • 0 in reply to BChristle

    Thank you Paul. I like the title of the document you shared. I am definitely on the way of loosing my fear, but at the same time it makes me very upset at times. I'll keep dancing I guess.

  • 0 in reply to BChristle

    I've been working on these a very long time. You'll get it!  Let me know what you think about the source cleaning videos.

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