As described above, NANOES sources allow a very low sample flowrate and a direct ionisation from buffered aqueous solutions without any volatile solvent added. As the instrument used (see 2.1.2.1) is not equipped with a NANOES source, the existing source had to be modified to obtain a NANOES-like spraying characteristic. In contrast to the source introduced by MATTHIAS WILM and MATTHIAS MANN[93] and distributed by Protana (Odense, Denmark), which requires a modification of the instrument interface, the source developed in this thesis should fit easily into the existing source housing and not require any instrument modification for source exchange. As the instrument is also used for routine investigations which require the standard pneumatically assisted electrospray source, a fast and simple change between both sources was also required.