What background does the backing foil give?
The mylar film has traces of calcium and phosphorus in a known ratio. If the protein has either calcium or phosphorus (e.g. from bound DNA) in it, the background can be deconvoluted. However, this increases the error, and if the accuracy is critical, another backing may be required. Polypropylene is clean of any trace elements, and it is the film we are currently using most of the time.
