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m/z=32 but no m/z=28

chjin9

Hello, I've been troubleshooting GCMS for a series of peaks that come out before general solvent peaks - at very early retention time.
Please see the image1 below. MS spectra of those peaks show m/z=14, 28, 32 for 1st,2nd peaks, then m/z=18 for third and then m/z = 18, 31, 44 for the 4th peak, which indicates, N2/O2, H2O and then MeOH, CO2. The largest peak is acetone.
I have opened inlet assembly, cleaned it, replaced liner septum and trimmed the column.
Air water check passed. But I still see those peaks after the cleaning though the intensity reduced. But what's even more strange is on the next day, when I ran a blank acetone, I see the peaks again and now MS spectra show only m/z=32 with no m/z=28 for the first two peaks (see image 2). The third peak shows m/z=31 and 44 which didn't change from before (see image 3). I'm curious how the ms detection has changed although the GC chromatogram looks comparable to each other. Air-water check has passed. If this is a common issue and there is any information available, please let me know. thank you


image 1



image 2



image 3

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  • 0

    The filament lifetime is compromised if it is on when the solvent peak is eluting.  The solvent delay is set so that the filament comes on after it, if acetone is your solvent then the solvent delay should be set to, oh, 2.5 minutes. This avoids any potential problems caused by the higher pressure in the ion source caused by the solvent.

    Are you injecting with a syringe? If so, there will always be a bit of air, maybe a tiny bit of water depending on how the syringe is handled and the relative humidity, and possibly whatever else is in the air at your facility.  

    Is this run using split mode or splitless mode?  What are the rest of your instrument parameters? Please upload the acqmeth.txt file that is underneath your xxxxx.M method subdirectory.

    Do you need to see these peaks at all? If so, why?

  • 0 in reply to paulsalverda

    thanks for the prompt reply. I agree with the solvent delay which we normally set and implement. Since we injected some type of sample last week, we noticed a baseline rise in the early retention time, and then we noticed this cluster of peaks under extended scan..As we haven't seen these peaks in the past, I'm trying to resolve the problem. Fyi, by the percent report, the "gas" peaks are 2.2% vs acetone 97.8%. To me, it looks higher than the air incursion during normal syringe injection. 
    Here are the acquisition parameters. sorry can't find a way to upload a file.

  • +1 in reply to chjin9

    Set the inlet mode to SPLIT, 50:1, save the method with a new/different name, and try again - maybe with a new syringe.

    The Front Inlet Splitless parameters are invalid.

    Purge flow 50ml/min @ 0.00 minutes.... means that at 0.00 minutes as the sample is being injected, the inlet reverts to split mode and 50 ml/min, so a 50:1 split.  But the very action of the flow/pressure changes in the inlet at 0.00 minutes could be causing these tiny peaks to show up.

    Valid splitless parameters would be 50 ml/min @ 0.75 minutes - so that after the sample has vaporized and been transferred onto the column with liner flow essentially equal to column flow that any leftovers are purged out at that time.

  • 0 in reply to paulsalverda

    thank you very much! Let me try it and update here.

  • 0 in reply to chjin9

    I see the clear difference between the split mode vs the splitless that now has the valid parameter. and the initial nitrogen elution seems a lot less problematic. thank you for your instructions!

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