MS-grade H2O, organic solvents, and formic acid (FA) were purchased from Biosolve Chimie SARL (Dieuze, France). Solutions of ULN and its deuterated IS, ULN-d6, were provided by BioMed Valley Discoveries (Kansas City, MO, USA) with purities of 99% and 98.26%, respectively [16 (link)].
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