Hi,

I'm carrying out a BCA protein assay on samples (pieces of fabric impregnated with protein). The samples are prepared by soaking them in PBS solution and incubation at 50 degrees. 

On day 0, 3 and 7, a small volume of each samples' PBS solution is analysed with BCA on a spectrophotometer at 560nm.

On day 0, the absorbance is low, which is to be expected, as the protein hasn't worked its way out of the fabric. On day 3, all of a sudden the readings are quite high. On day 7, the result is lower than day 3.

Can anybody suggest why the reading on day 7 is lower than day 3, when the samples are taken from the same bottle - so should increase.

Thanks for any help.