A new look at the mechanism of chemical ionization fragmentation of demethylated choline esters

The mechanism of fragmentation in the chemical ionization mass spectra of thermally demethylated choline esters has been investigated using various deuterated analogs of acetylcholine as models. The reagent gases employed included CH₄, C²H₄, NH₃ and N²H₃. Prominent [MH]⁺ or [M²H]⁺ ions were observed respectively in all cases. A 1,3‐hydrogen rearrangement in the acetyl portion followed by loss of ketene was seen in CH₄ chemical ionization spectra, as well as cleavage of the CH₂N bond after protonation on the nitrogen. Alpha cleavage produced ions analogous to the m / z 58 ion which is found as the base peak in the electron impact spectra of demethylated choline esters. A major difference between the fragmentation after electron impact and chemical ionization was seen in the m/z 72 region. The McLafferty rearrangement observed after electron impact was not found after chemical ionization. Rather, a mechanism involving protonation of the carboxyl group followed by simple cleavage of the CH₂O bond was consistent with the present results for the variously labelled analogs and reagent gases. Finally, in the chemical ionization mass spectra of demethylated acetylcholine little retention of the H⁺ or ²H⁺ was found in the observed fragment ions.