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. Author manuscript; available in PMC: 2018 Jun 16.
Published in final edited form as: Anal Methods. 2017 Jun 16;9:3876–3883. doi: 10.1039/C7AY00532F

Figure 1.

Figure 1

Structures of fentanyl analogs with * indicating isotopically labeled internal standard. Analogs with out matched internal standard used carfentanil as a suragate internal standard.