Dimethylacetamide carryover?

Discussions about GC and other "gas phase" separation techniques.

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I am trying to validate a new residual solvent method using the FET method. We used to dilute out samples in DMSO, however, with all the different analytes, recovery wasn't going well, so now we are trying the full evaporation technique because we are testing flowers & concentrates (cannabis) and the FET method is best because we don't have matrix match.

I have been working off application notes from Restek & Emerald Scientific on how to approach this method. The emerald scientific methods states to use DMA as a diluent because it does not freeze like DMSO does, and the re-freezing/thawing of DMSO can cause the light gases to become less potent. So i got all of my standards in DMA. I got a brand new bottle of headspace grade DMA. Everything was going great until I started running blanks of DMA. The first air blank is completely clean, barely any peaks, great. The first DMA blank looks great, very minimal peaks. However, the 2nd DMA solvent blank has this very large, broad hump-like peak that elutes just before the actual DMA solvent peak. It gets progressively worse with every DMA blank I run. It's a problem because it interferes with my xylene peaks at the end of the run. I thought maybe it was the jar I put the DMA in, nope, the stock has the hump too. Once I run an air blank, it goes away and the next DMA blank is great and then the whole process starts again. The standard mixes in DMA I have are also showing the same pattern of the hump showing up right before the solvent peak. When I ran some blank DMSO runs, this did not happen at all.

How do i get the DMA from having this large interfering peak? Has anyone else had this happen? I am using an HP 5890 GC-FID attached to an HP7694 Headspace autosampler, ~20:1 split. I am using a capillary column 30m x 0.25mm x 1.4um. I've attempted many different temperature programs and it always shows up, no matter the difference. We have recently replaced the inlet liner, cut the inlet portion of the column, and have conditioned the column. I am stuck at this point! Help!

Kind Regards,
Make your GC runs longer to see if all impurities from previous injection leave the column. One of the possibilities is that they accumulate and leave in the 2nd run forming humps.
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