how can you tell by using this test that a substance contains protein
Biuret test In the BCA test, Cu+ forms a deep purple complex with bicinchoninic acid (BCA),[7] which absorbs around 562 nm, producing the signature violet color. The water-soluble BCA/copper complex absorbs much more strongly than the peptide/copper complex, increasing the sensitivity of the biuret test by a factor of around 100: the BCA assay allows to detect proteins in the range of 0.0005 to 2 mg/mL). Additionally, the BCA protein assay gives the important benefit of compatibility with substances such as up to 5% surfactants in protein samples.