You weigh it before and after oxidation

For high enough specific surface (e.g. powder), either thermogravimetric analysis (TGA), or simply weighting the sample before and after annealed for surface oxidation, may be found accurate enough.

For rather accurate assessment with samples of quite low specific surface (e.g. plate-shaped), you may alternatively consider:

I ― In case you are able to highly selectively dissolve the oxide layer, but not the metal; it may be possible to determine its content in the resulting etchant solution, by means of quantitative chemical analysis.

II ― It could be possible to thermally reduce the metallic oxide to the metallic state by using (dry) N2/H2 or Ar/H2 mixture (e.g. 5% H2), if not pure H2 ─ if the risk can be accepted ─ at some suitable pressure; to analyse the exhaust gas for the mass of evolved H2O (e.g. weighting the water/ice collected at a suitable condensation/freezing trap).

III ― In some cases, the metallic oxide can be selectively and extensively hydroxilated (e.g. alkali treatment) to form M─OH surface groups, which upon thermal dehydroxylation, regenerate the oxide while evolving a stoichiometric amount of H2O, that could be accessed as above.

IV ― In some unusual cases, the oxide can be thermally volatilized/sublimated, or can be somehow selectively chemically converted to such a volatile substance; so that hereafter it can be deposited at a cold trap and weighted.

V ― Some solid state analytical methods could be also considered.

Answer suggested by Thomas Breuer is agreeable and will give accuracy in your data.

Hi Dr. Braun,

Thanks. But, I am talking about bulk samples. Complete copper sheet/coupon wont oxidize, only the surface would oxidize. I can probably do a cross sectional imaging and find the thickness of oxide and then use the density and make a rough approximation. But this works only if the bulk is smooth.

regards

You can determine the number of oxygen atoms at the surfcae using ion scattering techniques (RBS, ERD). Then calculate this number into a oxide mass assuming a composition ( e.g. CuO) or you determine additionally the chemical form, e.g. by XRD and use this chemical formula(e) (e.g. Fe2O3 / FeO) to calculate the mass.