We are using HS-SPME-GC/C-IRMS. I have three images attached to this question. The first one is of an instrument blank. There are no peaks present, which is what we want. When we do a fiber blank (just the SPME fiber before it was introduced to any sample or standard) we get this big peak at 300 sec every time. This occurs even if it is a brand new fiber that is newly conditioned. The third attachment shows a typical run with the peak that we are interested in showing up at 334 sec. It seems very likely that the first peak is messing with out isotopic data for the peak of interest.
We use the exact same method on a GC-FID to find the concentrations of the compound of interest before running it on the GC/C-IRMS and we never get this peak. Has anyone had this happen to them before?
Thank you for the reply!
Yes, the instrument blank and the fiber blank is the exact same procedure. The instrument blank is without injecting a SPME fiber.
The peak at about 500 sec on the fiber blank is a backflush peak. We have tried moving the backflush to different times and that peak always is when the helium backflush kicks back on.
The peak shapes do look a lot better on the GC-FID. As for the width, I would say that they are about twice as wide on the GC/C-IRMS.
We use a constant flow of 2mL/min.
We have tried taking out this column and putting it in the GC-FID. Nothing weird happened, which leads us to believe that the column is fine. Both columns are exactly the same.
Based on the information provided I would guess the peak at 300s is real. Compound amount might however be too small to show up in your FID chromatogram (depending on your attentuation setting) or may just be another blip in the baseline. Your Delta V however is designed for optimum ion yield from the amount of CO2 entering its source which may explain why you can see a peak here.
Have you got access to a GC/MS to check if you see a peak there when running a blank with SPME fibre? If so, this may also give you an idea as to what this peak is.
By the way, what kind of GC column do you use. Something like a DB-5 or DB-1701 or is it a PLOT column?
I'm incredibly sorry for the huge delay in this reply.
We do not have access to a GC/MS and we don't even have consistent access to the IRMS. Right now it looks like we have been using a 23 ga microseal, but a 24 ga SPME. That first peak is probably atmospheric CO2 entering the system. We are waiting on an order to come in and I can let you both know if it fixes the issue.
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