The 1-fluoro-2,4-dinitrobenzene (FDNB) [3] was from Fluka, and the Oasis (30 mg) solid-phase extraction (SPE) cartridges were from Waters.
[3] Nonstandard abbreviations: FDNB, 1-fluoro-2,4-dinitrobenzene; SPE, solid-phase extraction; APCI, atmospheric pressure chemical ionization; NMR, nuclear magnetic resonance spectroscopy; and OPA, o-phthaldialdehyde.