I am working on bio-hydrogen production and during the GC analysis I have observed certain peaks of my interest, and one unknown peak. Hydrogen peak is subsequently followed by unknown peak. Out of interest I have injected the normal free atmospheric sample. I have observed the RTs of my unknown peak and air sample peak are matching. Can I draw a conclusion that my sample is contaminated with air since the reactant contains 60% DO?
Retention times are a good indication but not conclusive. As a next step, try spiking your sample with the "free air" and see if the area of the unknown peak increases.
I agree with Mr Henrik Van de Ven. Spiking either with free air or known concentration of other gas such as N2 would (provided that this is not the carrier car for your GC) separate the peaks at different detention time. Just my 2 cents :)
In support to above, for conformation of air component, alternately try your sample on hyphenated technique such as GC-MS and GC-HS-MS which can give you mass of component for which you can predict the component of air.
It may also be something coming off the column since it is being seen in the air sample and unknown sample. You may want to change out your inlet liner and gold seal to see if the contaminant is coming from there also. First, if you haven't already, try setting the to the high temp of the column for an hour or so to bake out the column, then inject with your original method, might be a contaminant on the original column.
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