The role of free electrons in MALDI: electron capture by molecules of alpha-cyano-4-hydroxycinnamic acid.
Low-energy (0-12 eV) electron attachment to molecules of a typical matrix substance used for matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS), namely alpha-cyano-4-hydroxicynnamic acid, has been investigated in the gas phase at different temperatures ranging from 140 degrees C to 260 degrees C by means of electron capture negative-ion mass spectrometry (ECNI MS). The yield of negative ions, formed by electron capture, was measured as a function of incident electron energy for four different temperatures. The long-lived parent molecular anion, [M]- (m/z 189), was observed in the negative-ion mass spectra of the substance under investigation. Its autodetachment lifetime was estimated to be approximately 600 micros. It was found that at 140 degrees C the main decay channel of the long-lived temporary molecular anion of alpha-cyano-4-hydroxicynnamic acid is a formation of the [M-COOH]-; fragment negative ion (m/z 144) with an intensity of 37.2% in percentage terms in respect of the total anion current. There are also [M-H]-, [M-CO2]- and [CN]- fragments in the spectra with intensities of about 7.7%, 21.6% and 3.1% at 140 degrees C. It was shown that the escape of the CO2 molecule from the parent molecular anion is a slow process. It takes [M]- about 10 micros to decay on carbon dioxide molecules and [M-CO2]- fragment anions. Increasing the temperature of the target molecule alters the negative-ion mass spectra of alpha-cyano-4-hydroxicynnamic acid significantly. A possible role for the findings in typical MALDI MS experiments is discussed.
Institute of Physics of Molecules and Crystals, Ufa Research Center, Russian Academy of Sciences, October Avenue 151, 450075 Ufa, Russia. sapsh@anrb.ru
S A
SA
Pshenichnyuk
N L
NL
Asfandiarov