I have a 1 mm thick plastic sheet, the plastic is not opaque.

Some gas pockets, I would say some bubbles, are inside the sheet; the size of the bubbles is about 1 mm.

What methods are available to know the composition of the gas that is inside the pockets?

Methods can destroy the sample.

  • 5
    $\begingroup$ Gas chromatography, possibly even GC/MS. Sample a bubble using a small syringe and put the sample through a GC/MS set up to identify common gases. $\endgroup$
    – matt_black
    Jul 14 '19 at 11:56