ON MOLECULAR ION FORMATION BY HYDRIDE ABSTRACTION IN PLASMA DESORPTION MASS-SPECTROMETRY

Positive molecular ion formation in plasma desorption (PD) by hydride abstraction leading to [M - H]+ molecular ions is not a frequently observed phenomenon. An example of a compound that exhibits an [M - H]+ molecular ion in its PD spectrum is the neutral lipid cholesterol. Data on the initial radial velocity distribution of the [M - H]+ ion of cholesterol are presented and compared to data on the radial velocity distribution of the M.+ molecular ion of a structurally similar compound (ergosterol). The distribution for [M - H]+ cholesterol ions is broader and its centroid is closer to zero radial velocity than the corresponding distribution for ergosterol M.+ ions. These data support a model suggesting formation of [M - H]+ by a ''self'' chemical ionization type reaction.