I dont really know what can be an impurity in the process of tryptamine production but it will be most likely one of the "common" impurities.
Its impossible to say what those 2% are without testing it.
In most organic chemistry the impurities are always some side products that form when the main product breaks down due to reasons like UV radiation or heat, etc..
100% purity is impossible. Just leaving any compound in the air will reduce the purity by a lot.
Even when the compound does not react with air moisture can be attracted and reduce the purity.
It is probably either water, a solvent, or a precursor chemical that is very similar. It might be tryptamine or something that was used a starting chemical.
If the process isnt 100% efficent you end up with a rest that is difficult to seperate due to the fact that the compound is so similar in size and struture.