The role of free electrons in MALDI: electron capture by molecules of α-cyano-4-hydroxycinnamic acid

Eur. J. Mass Spectrom. 10, 477–486 (2004)
DOI: 10.1255/ejms.650

The role of free electrons in MALDI: electron capture by molecules of α-cyano-4-hydroxycinnamic acid

S.A. Pshenichnyuk^* and N.L. Asfandiarov
Institute of Physics of Molecules and Crystals, Ufa Research Center, Russian Academy of Sciences, October Avenue 151, 450075 Ufa, Russia. E-mail: sapsh@anrb.ru

ABSTRACT:

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 10 µs 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.

Keywords: MALDI, negative ions, free electrons, low energy electron attachment, temperature effect

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