If you're looking for studies, there are quite a number of them written in various medical journals that you could look into. One that I remember and have seen quite often was the testing of over 15 different branded types of protein powder. All of which contain 4 different heavy metals. Some examples listed were:
Eas Myoplex (Arsenic, Cadmium)
Muscle Milk-Chocolate (Cadmium, Lead)
Muscle Milk-Vanilla (Lead)
The article then went on to show that drinking solely protein powders as a means of reaching your protein intake could in fact cause long term health conditions.