Potential over-fragmentation?

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

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So I have a problem I've been trying to fix for a while now with a 7890/5977 running HS-SPME. I work mostly with wine samples and am having an issue where every peak in the chromatogram has a spectrum that looks like typical hydrocarbon fragments (regular loss of 14m/z), the biggest peak keeps showing up as 1-tetradecene which I know is not naturally found in wine. Had an Agilent tech come out and he said it was the SPME fiber so I changed that but still have the same problem. At this point, I've changed the column, replaced the filaments, cleaned the source, did a full clean of the inlet, new gold seal, new liner, new septum, etc etc but am still seeing almost nothing but hydrocarbon peaks in my chromatograms. The only other thing I can think of is that maybe the compounds are getting over-fragmented in the mass spec somehow and just look like hydrocarbons even though they're not? (wines tend to be full of esters). Air blanks have no peaks, no injection blank looks like a no injection blank.

Edit: Mass spec tunes fine though on the low end of spec, air/water check is fine

Edit2: I've re-run samples that had been run previously that didn't show a bunch of hydrocarbons and now they do
What is the filament current set to?

The default is normally 35uA, but you can usually get away with lower currents.

If you do a repeller ramp the 69, 219 and sometimes the 502 should give gaussian profiles that come close to aligning at maximum, the 502 sometimes though will still be increasing out to max repeller voltage. As you lower the emission current the maximum response for each mass on the repeller will occur with less repeller voltage. You may loose just a little sensitivity at low mass as you lower repeller voltage but you will fragment less and have more abundance at the higher masses.
The past is there to guide us into the future, not to dwell in.
I once had some samples from solvent contaminated water that had been used to strip paint from aircraft and then evaporated. I was running P&T for 8260 VOC's; purguing at 70C to improve oxygenate recovery. The tetradecane ruined my trap and it took a complete cleaning of P&T and autosampler lines as well as the GC inlet to get rid of the signal.

Perhaps your wine has been in contact with painted surfaces and has solvated or picked up the tetradecane from the paint?
That's a good point, I wish we had another GC handy to run it on and see if it looks the same or not... might just have to send it out to a commercial lab and see if they can confirm
Another thought, try injecting pure ethanol and see if the peaks appear.

Since they didn't appear from an air blank or just blank run, maybe there is something stuck in the system that the ethanol from the wine is knocking loose. Inject it from a needle, just 1ul, not from the SPME fiber.
The past is there to guide us into the future, not to dwell in.
James_Ball wrote:
Another thought, try injecting pure ethanol and see if the peaks appear.

Since they didn't appear from an air blank or just blank run, maybe there is something stuck in the system that the ethanol from the wine is knocking loose. Inject it from a needle, just 1ul, not from the SPME fiber.


Not that it would necessarily make any difference to this good suggestion, but as a variant on this blank one could try making up a 10-15% solution of EtOH in water and then injecting that via the SPME fiber.

I only suggest that because it should in theory better aproximate the sample matrix for the wine. Depending on the exact SPME fiber used, you might or might not get a lot of water adsorbing on to it(I honestly don't know-I've only ever done SPME with GC-MS and don't generally scan low enough to see water even though I've done it with aqueous samples), but that could also play a factor.
So just a follow up in case anyone else encounters this problem - it seems as though it is indeed contamination (which I suppose makes sense since everything tunes fine and there's not much noise in the tuning). Ran a blank with 15% EtOH and it did indeed look like the same contamination I was seeing in the samples, so it appears it's something that requires solvent to leech in to the system. We have an unused GC so we're going to completely switch out the inlet and see if that fixes things. Ran the samples on another GC and no sign of diesel-looking peaks (though the other GC is running a polar column, so there's still the chance that there's something weird about the sample itself).
laytox wrote:
So just a follow up in case anyone else encounters this problem - it seems as though it is indeed contamination (which I suppose makes sense since everything tunes fine and there's not much noise in the tuning). Ran a blank with 15% EtOH and it did indeed look like the same contamination I was seeing in the samples, so it appears it's something that requires solvent to leech in to the system. We have an unused GC so we're going to completely switch out the inlet and see if that fixes things. Ran the samples on another GC and no sign of diesel-looking peaks (though the other GC is running a polar column, so there's still the chance that there's something weird about the sample itself).


Could there be septa fragments in the port causing the problem? If so you may want to try a duckbill septa like the Merlin Microseal to prevent the SPME needle from coring the septa.
The past is there to guide us into the future, not to dwell in.
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