Chromatography Forum

Hello all,

If this is in the wrong place, please feel free to move it mods.

Due to the baseline issues with GC-MS system over the past month (there is a separate thread regarding this), I have had to send some samples to a colleague at another university for GC-MS analysis. I extracted the cuticle surface chemicals from 30 mg of mites using 1 ml of hexane, which I then blew down to 200 ul under a stream of nitrogen. My colleague insisted that I run my extractions on a GC-FID prior to sending them to to him for GC-MS analysis to confirm that I had peaks (mite cuticle extractions are extremely tricky due their size and he didn't want to waste our time). Perhaps rather foolishly I decided to run my samples overnight using an auto sampler and then package the samples up to post the following morning without changing the vial caps after the septa have been pierced. Fast forward to today and he has emailed me to say that my samples are useless as the pierced PTFE/silicone septa have allowed some solvent to evaporate and also contaminated the sample. I had a quick google around and only found one source regarding this ([url]http://www.americanlaboratory.com/914-Application-Notes/35726-How-Pierced-PTFE-Silicone-Septa-Affect-GC-MS-Experiments/[/url. Basically, I want to know how common this is and whether any of you have experienced it before?

Thanks,

Joe

"Experience is simply the name we give our mistakes." Oscar Wilde

This is a common problem, and a hardy perennial on the forum. There was a thread on it within the past couple of weeks.

Peter

You would most likely have contamination of the sample with siloxanes from the silicone septa. The teflon is used as a liner on the inside to keep the solvent from extracting the siloxanes from the rubber, once it is pierced you lose that barrier. Hexane and Methylene Chloride also evaporate through the puncture easily, water, methanol and acetonitrile would evaporate at a much lower rate and could probably be near the same volume after a day or so at room temp.

If you know specifically what compound you are expecting to see then the mass spec analysis could give you a positive or negative result for that, even if you have to add back a little hexane to redissolve any target analytes, but if you are looking for all unknown peaks for identification it would be much more complicated.