Dr. Fukuda et al. from Towa Pharmaceutical Co., Ltd published a literature focusing on the testing method for N-nitroso duloxetine. Using LC-MS/MS they analyzed the trace amounts of NDSRIs by methanol extraction.
https://pubs.acs.org/doi/10.1021/acsomega.4c00136
You can check the Swissmedic method for Duloxetine. It is similar to Fukuda’s.