International Journal of Mass Spectrometry 220 (2002) 11–19 The origin of electrons in MALDI and their use for sympathetic cooling of negative ions in FTICR Vladimir Frankevich, Richard Knochenmuss, Renato Zenobi ∗ Department of Chemistry, Swiss Federal Institute of Technology (ETH), Hönggerberg, CH-8093 Zürich, Switzerland Received 10 April 2002; accepted 17 June 2002 Abstract In MALDI, electrons were found to be formed by the photoelectric effect on the metal/organic matrix interface. The thin layer of organic matrix reduces the metal work function, resulting in a high electron yield in a field-free MALDI source. These electrons were detected in a FTICR mass spectrometer indirectly, by the observation of SF 6 - ions, produced by electron capture. The number and velocity distribution of these electrons were estimated. It is shown that the yield of electrons strongly depends on the thickness of the organic sample. In the case of non-metallic substrates or thick matrix layers, much fewer electrons were detected. Electrons confined in a FTICR trap can cool negative ions produced by MALDI by long range Coulomb interaction, thus increasing the resolving power and sensitivity. (Int J Mass Spectrom 220 (2002) 11–19) © 2002 Elsevier Science B.V. All rights reserved. Keywords: MALDI; Electrons; FTICR MS; Ion cooling 1. Introduction The yield of charged species in matrix-assisted laser desorption/ionization (MALDI), is rather low, on the order of 10 -4 [1,2]. In negative mode, a signal for electrons is normally detected in addition to neg- ative ions from the MALDI matrix and the analyte. The origin of these electrons has been unclear. It has, for example, been reasoned that they are produced by photoionization of matrix [3] but photoionization has been shown to be of minor importance in MALDI [4]. The fate of electrons in a MALDI plume is also of interest. For example, are they able to produce neg- ative ions by electron capture? Will they neutralize positive ions thus decreasing the overall useful ions ∗ Corresponding author. E-mail: zenobi@org.chem.ethz.ch yield? Are they responsible, as it has been suggested [3], to reduce multiply charged analyte positive ions to charge state z = 1? The possibility that must be considered is that electrons are simply extracted from the plume by the high acceleration field usu- ally present in MALDI ion sources. Some MALDI experiments, however, are carried out field-free, e.g., in delayed extraction time-of-flight MS, and, in par- ticular, in internal source MALDI Fourier transform ion cyclotron resonance (FTICR) mass spectrometry [5,6]. Our internal source FTICR set-up permitted us to study the origin and effects of electrons in MALDI in more detail. One key idea of this work was to use these electrons for cooling of negative MALDI ions, thus improving sensitivity and resolution. A variety of experimental approaches have been pursued for improving the sensitivity and efficiency of 1387-3806/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII S1387-3806(02)00766-2