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Prepare samples containing Host Cell Protein in a known concentration.

I wrote this, but then looked up Google Ngram:

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"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?

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  • at is not wrong. It just has a different meaning. When the samples have been prepared so that the protein is in a known concentration, we can state that the concentration level of the protein is at a known concentration. Dec 12 '16 at 11:54
  • Remember from the "in a circular motion" vs "with a circular motion" question that "in" refers to state, so "in" is fine here. The protein sample is "in" a concentration. "At" would refer to the identification of the concentration level, not to the concentration of the protein sample per se. Dec 12 '16 at 12:00
  • In is existential. At is epistemological. Dec 12 '16 at 12:01
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Depending on what you want to say, your situation can be described

Prepare samples containing Host Cell Protein to a known concentration.
you need to make samples to have a desired concentration of Host Cell Protein

Prepare samples containing Host Cell Protein at a known concentration.
the original concentration of Host Cell Protein used to make the samples is known (before dilution).

Prepare samples containing Host Cell Protein in a known (reagent) concentration.
cell protein samples were made using a prespecified reagent concentration

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