Prepare samples containing Host Cell Protein in a known concentration.
I wrote this, but then looked up Google Ngram:
"At a known concentration" is more prevalent.
In Google Books, I found this example:
Calibration Gas means a gas standard containing Hg0 or HgCl2 at a known concentration that is produced and certified in accordance with an EPA traceability protocol for certification of Hg calibration standards.
Would in a known concentration in my sentence be wrong? If not, what is the difference between the two expressions?