There are no guidelines set down by the FDA for trace-element contaminants in pet food. A $40 billion dollar a year industry in the United States, yet the FDA has not established toxic levels of heavy metals in pet food.
There is a newly published paper in Spectroscopy Magazine titled "Analysis of Toxic Trace Metals in Pet Foods" It is a startling reality of the true conditions of commercial dog and cat foods. As evidenced in this paper, commercial pet foods were found to contain toxic levels of lead, mercury, arsenic, and even nuclear waste (uranium, beryllium, and thorium).
Click here for a link to the first article, written by Susan Thixton.
Click here for part two.
And here is a video produced by Spex CertiPrep, the company that did the analysis: