Pet Forum / Mammals / Rats / December 2007

Teklad 2018 had no detectable amounts of mercury while certain shipments of
Purina 5001 had enough that it was of "potential biological significance."

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1280388

"[...] The Purina 5001 diet is an open diet; that is, its ingredients are
subject to change, depending on the source of the raw materials. Fish meal
is one of the ingredients, and it is possible that methylmercury present in
tuna scraps, for example, may have been the source of the fish meal used in
the batch provided by the vivarium. The supplier gives the limit of
detection as 0.02 ppm (20 ng/g), so the problem apparently escaped
detection. Even so, such levels are excessively high for experiments on
mercury, especially those focusing on low-level dose-response outcomes. We
were unprepared for the results in the present study because, in an earlier
methylmercury study with mice (Stern et al. 2001) also fed the Purina 5001
diet, we detected no mercury in control dams or pups.

"[...] More recently, in our ongoing attempts to find a suitable,
mercury-free diet, we analyzed samples of the Teklad 2018 diet (Harlan
Teklad, Madison, WI), which does not contain fish meal. We ground four
pellets in a mortar to obtain a fine powder, which was then digested with
sulfuric acid. No mercury was detected."