A UPLC system equipped with a binary solvent delivery system was sourced from Waters Corp. (Milford, MA, USA), coupled with an auto-sampler and a UV detector as previously reported [22 (link)]. The quadrupole time-of-flight mass spectrometer (Q-Tof Premier™, Waters Corp., Milford, MA, USA) was used. The lock mass used was a reference solution of leucine-enkephalin ([M − H] − m/z 554.2615) in the form of a spray.
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