The Role of Free Electrons in Matrix-Assisted Laser Desorption/Ionization: Electron Capture by Molecules of α-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 α-cyano-4-hydroxicynnamic acid, has been investigated in the gas phase at different temperatures ranging from 140°C to 260°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 μs. It was found that at 140°C the main decay channel of the long-lived temporary molecular anion of α-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°C. It was shown that the escape of the CO₂ molecule from the parent molecular anion is a slow process. It takes [M]⁻ about 10μs 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 α-cyano-4-hydroxicynnamic acid significantly. A possible role for the findings in typical MALDI MS experiments is discussed.