I found an excellent article in Pharmatech. It is about the importance of API quality. And nitrosamine problem is focused on in the latter part. The author interviewed some leading experts in this field such as @ASrinivasan. She stresses the importance of risk assessment in API synthesis and GMP compliance.
What Goes Wrong When API Quality is Compromised? (pharmtech.com)
PDF is here.
I strongly recommend you to read the whole article. It’s fantastic!!