Every now and then you get lucky trying to id these contaminants. But generally, be prepared
for a lot of work.
First, find out how the customer identified the contaminant. Then repeat the process they used
to identify it. Repeat it multiple times to make sure you know how to do it.
Then go back and start testing individual ingredients, packaging, etc. one by one. You can test
them individually. But you may have to test them by leaving one at a time out of your formulation.
Once you have identified the source, try to identify the individual component. For example, it may
be a by product of a vitamin. And be prepared. Your suppliers have many "proprietary" parts of the
product they supply. Be prepared to get all the people in your company to put pressure on them.
Other wise, they wont tell you what the contaminant is. Without that information, identifying the
risk the contaminant poses is almost impossible.
Every now and then, you get an interference that is a reaction product of two or more ingredients. This
makes life very difficult. Don't pass over that bridge until you have to.
Good luck.