Dear Sir. Regarding your question about the the measurement of molecular weight of polylactic acid with GC - MS. The molecular weight  of polylactic acid can be determined by GC or GC - MS after thermal degradation or pyrolysis. In pyrolysis, the polymer samples (introduced in the form of a solution or as a solid) break down into small fragments (e.g. monomers or oligomers) by supplying thermal energy in an inert atmosphere or vacuum . The small fragments can be separated and analyzed by chromatographic techniques such as GC or GC-MS. For more details the following below link may help you in your analysis:

http://dare.uva.nl/document/2/97798

Thanks

Thank for your answer sir. I was reading about pyrolysis GC-MS in this book. But in the pyrogram, didn't give an information about molecular wright of polymer. This is the book that i read. Thank sir. 

"My" GC-MS has limit for the detector of 1800 Da, which means that no more than  25 LA units (25-mer) can be measured, even if they pass the column.  The practical limit may be even much lower.  Usually for polymers, the people use MALDI. Less precise method and it measures indirectly the MW is the GPC/SEC. An advantage of the GPC is that you can go easily to MW of millions, which I think is unthinkable with any MS methods.

Is very hard works CG/MS with polymers. I think, the bes  .alternative technic is MALDI.

Quick answer: NO

Use GPC/SEC with ELSD (Evaporative Light Scattering Detector)

No. GC/ GC-MS helps to identify a copolymer composition, but not the molecular weight. As others have stated the best technique to analyze polymer molecular weight is SEC.

Thank All for your answer