Ammonium-adipic acid clusters were prepared using solutions of the following reagents: >99.0% adipic acid from Merck Millipore, HPLC grade water from Fisher Scientific, 99.9% deuterium oxide (D 2 O) from Sigma Aldrich, HPLC grade 99.8% acetonitrile, LC/MS formic acid in H 2 O from Fisher Scientific, and 95 wt. % formic acid from Sigma Aldrich.