The mechanism of Spontaneous Desorption Mass Spectrometry (SOMS) has been investigated through the use of extraction electrodes with circular aperture, in place of the more conventional grid system. High emission rates of contaminant and sample specific anions have been recorded from reference materials like CsI and phenylalanine. It is shown that the emission rate is controlled by the existence and the characteristics of the aperture edge such as sharpness or (and) rugosity. Geometrical shape and material composition of this well localized region also influence the emission rate and ion yields. Experimentally, the sample emitting zone has been determined in two ways: (1) from the variation of the emission yield of sample specific ions as a function of the deposit diameter and (2) from the lateral distribution of impacts of these ions after their flight path. Both experiments clearly show that this emitting zone (impact zone of the primaries) has a ring shape related to the circular aperture parameters. SIMION ion trajectory simulations and field strength calculations demonstrate that the primary ions are mostly emitted from the well defined edges (inner and outer) of the electrode. The emission rate is shown to decrease rapidly with time after the application of the extraction potential indicating both a degradation of the edge sharpness or microtips density and a decrease of adsorbates release.