Purina, along with the other pet food you get at the grocery store, is very poor quality. It is made with the waste from the human food industry, the not-fit-for-human-consumption by products.
Every pet owner should read this article from the Animal Protection Institute:
What’s Really in Pet Food
This is an excellent thread to help you find a better quality food:
http://www.yorkietalk.com/forums/yor...e-samples.html