The unusual fragmentation of long-chain feruloyl esters under negative ion electrospray conditions

Long-chain ferulic acid esters, such as eicosyl ferulate (1), show a complex and analytically valuable fragmentation behavior under negative ion electrospay collision-induced dissociation ((-)-ESI-CID) mass spectrometry, as studied by use of a high-resolution (Orbitrap) mass spectrometer. In a strong contrast to the very simple fragmentation of the [M + H]⁺ ion, which is discussed briefly, the deprotonated molecule, [M - H]^- , exhibits a rich secondary fragmentation chemistry. It first loses a methyl radical (MS² ) and the ortho-quinoid [M - H - Me]^-• radical anion thus formed then dissociates by loss of an extended series of neutral radicals, C_n H_2n + 1 ^• (n = 0-16) from the long alkyl chain, in competition with the expulsion of CO and CO₂ (MS³ ). The further fragmentation (MS⁴ ) of the [M - H - Me - C₃ H₇ ]^- ion, discussed as an example, and the highly specific losses of alkyl radicals from the [M - H - Me - CO]^-• and [M - H - Me - CO₂ ]^-• ions provide some mechanistic and structural insights.