Deuterated formaldehyde

Formaldehyde (FH₂), deuterated formaldehyde (FD₂), and ACS-grade formic acid (FA) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Gelatin was purchased from Becton Dickinson (Franklin Lakes, NJ, USA). All other chemicals and solvents were purchased from Fisher Scientific (Pittsburgh, PA, USA). Amicon Ultra 10 kDa 0.5 mL molecular weight cutoff (MWCO) devices and 10 μL C18 Ziptips were purchased from Merck Millipore (Billerica, MA USA), and 100 μL C18 Ziptips were purchased from Agilent Technologies (Santa Clara, CA, USA). Acidified methanol was prepared using 90/9/1 water/methanol/acetic acid. ACS-grade solvents and Milli-Q water (Merck Millipore, Billerica, MA, USA) were used for sample preparation, and Optima-grade solvents were used for MS analysis. Peptide standards were custom synthesized by GenScript Corporation (Piscataway, NJ, USA).