Can you share more on the specific API related nitrosamine you are interested in? Feel free to also share what you need with USP’s new pharmaceutical analytical impurities program which was established recently to create quality reference materials for pharmaceutical impurities. Your inputs can help shape the pipeline of new impurities materials being developed. You can email PAI@usp.org.
If the impurity is a NDSRI and UV active, you may check the response factor against the respective API. Use impurity and API in equal concentration (say, 0.1 mg/mL) in suitable diluent and check the peaks response with LC-UV. If the impurity is pure, you would get the response factor close to ‘1’. Make sure to do the salt correction wherever applicable.