Start with what is specified in the contract documents.  Barring that, the material specification & grade, actual chemistry, form of material, traceability to the MTR and any unique identification you may put on the material.  You should also record the the date it was performed and indicate who did the sample.  If there are more than one pieces of material being tested, serialize your tests. 

We use an "alpha code" system to trace all material; so we identify the test by alpha code, followed by the number corresponding to the number of pieces sampled with the same alpha code.

This isn't really complicated.  Ask yourself what the reason for the PMI is in the first case, and answer all the questions that come to your mind. 

Charles.