“…Regarding the electron source, prominent examples are electrospray ionization (typically for liquid samples) (Wittmer et al 1994, Chen et al 1996, Wu et al 2000, Harris et al 2008, Yamagaki and Sato 2009, optical sources (high-intensity light sources resp. laser, X-ray) (Matsaev et al 2002, Sielemann et al 2002, Oberh ü ttinger et al 2009 ), corona discharges (Tabrizchi et al 2000, Schmidt et al 2001, Khayamian et al 2003, Han et al 2007, Mulugeta et al 2010, Tabrizchi and Ilbeigi 2010, and electron guns (Gunzer et al 2010a ). These sources were also examined in order to have non-radioactive alternatives because of the technical, organizational and related financial complications that accompany the application of radioactive substances.…”