Going back to your original question, you say you're making the isocyanate of an amino acid.
I saw a procedure in orgsyn, but they actually start from an amino ester.
So is it the amino acid or ester you're using?
The difference might be important. Have a look at http://www.sigmaaldrich.com/aldrich/bulletin/al_techbull_al176.pdf
, they say you don't get the isocyanate, but a cyclic 'anhydride'.
On the other hand, if you're using an amino ester, you could try two different solutions:
1 - changing the alcoholic moiety of the ester to a heavier of lighter one, so that you can have a sufficiently different bp wrt TP.
2 - starting from the amino acid and see if the cyclic anhydride is easier to separate from TP, while still reacting as you want afterwards.
By all means, it's a tricky problem, and even commercial websites selling TP admit that its separation is sometimes awkward.