Spontaneous desorption : field assisted ion induced desorption mass spectrometry

Abstract

Spontaneous desorption (SD) refers to the emission of ions and electrons from a flat surface biased at a few kV and placed a few mm in front of an extraction grid or aperture at ground. Using SD, time-of-flight mass spectra from organic and inorganic samples can be obtained in a few minutes. However, the desorption mechanism for secondary ions is unclear. SD was studied in an effort to elucidate the mechanism with regard to the initial event. Is the production of ions due to field desorption or sputtering? The requirement of an extraction grid was also examined, along with the effects of other grid configurations, i.e. dual grid, grid-less, coated grids, different percent transmission grids, and electropolished grids. A two step process is proposed for the mechanism. The initial step is the field desorption of ions (most likely polyatomic species) from the edges of the grid. This step is also the rate determining step. The second step involves the sputtering of the sample surface by the primary ions produced from the grid. Coincidence counting techniques in time-of-flight mass spectrometry have been utilized for the detection and subsequent analysis of positive ions produced in SD. The technique has been applied to both organic and inorganic samples. A three-fold enhancement in the signal-to-noise ratio is observed. SD is also compared to two competing surface analytical techniques, SIMS and PDMS. SD is discussed with respect to: a) its surface analysis capabilities and prospects for quantitative analysis, b) mass range and mass resolution, and c) the applicability to different samples. It must also be noted that SD occurs in TOF-SIMS, and PDMS experiments. The SD phenomenon can account for up to 25% of the total number of ion counts for higher sample biases (i.e. 10 to 20 kV).