What is a "pure odorant"?
I could not find a concrete definition after searching several sources, but after reading a few papers I think the term Pure Odorant refers to its Olfactory Bulb profile. Peanut Butter will stimulate specific Olfactory nerves, and only that set of nerves, whereas impure Odorants may stimulate part of the profile and also other parts of the Bulb depending upon the impurity.

Basically, Peanut Butter has a unique smell that you can't replicate, and thus cannot mistake in either flavor or scent when compared to similar compounds. You may  not even be able to tell it was Peanut Butter at all if not for the scent.

A Lemon would not work the same way, as the major Odorant in a Lemon is Limonene - which is present in all citrus fruits. You would activate the same neuronal pathways for Lemons that you would with Oranges, Limes, and Grapefruits. 

That's my best assessment.