The role of free electrons in matrix-assisted laser desorption/ionization: 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 140degreesC to 260degreesC 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 mus. It was found that at 140degreesC 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 140degreesC. It was shown that the escape of the CO2 molecule from the parent molecular anion is a slow process. It takes [M](-) about 10 mus 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.