We are given, a certain food has a gluten ratio of
.
We need to convert milligrams per liter into micro-grams per milliliter.
1 mg = 1000 micro-grams.
And 1 L = 1000 milliliter.
Substituting 1 mg = 1000 micro-grams and 1 L = 1000 milliliter, we get

= 13 µg/mL