Zone of silence

Campargue R (1984) Progress in overexpanded supersonic jets and skimmed molecular beams in free zones of silence. J Phys Chem A 88 4466-4474... 

The skimmer cone is another metal cone, the tip of which has an orifice approximately 0.7 mm in diameter, that protrudes into the zone of silence, and is axially in-line with the sampling orifice as shown in Fig. 5.3. The ions from the zone of silence pass through the orifice in the skimmer cone, into a second intermediate vacuum chamber held at < 10 atm, as an ion beam The ion beam can then be focused by means of a series of ion lenses, which deflect the ions along a narrow path and focus them on to the entrance to the mass analyser. 

Behind the sampler a concentric shock wave structure is formed, which surrounds a zone of silence and ends in a shock wave front called the Mach disk. The interface should be such that the skimmer aperture still lies in the zone of silence for an optimal extraction. 

Figure 5.14 Expansion of gas and ions from atmospheric pressure into a vacuum, (a) simple case showing shockwaves (barrel shock and Mach disk) and zone of silence, (b) generation of a beam of gas and ions by saiurling from the silent zone via a skimmer penetrating through the Mach disk, (c) sampling of gas and ions with a skimmer located downstream from the Mach disk, leading to the beam of gas and ions being scattered by passage through the Mach disk. Reproduced from Bruins (1991), Mass Spectrom. Revs. 10, 53, with permission of John Wiley Sons, Ltd.

This region of free expansion is called the zone of silence. Spaced behind the sample cone is a second cone with a smaller orifice in its tip (0.4-0.7 mm), called the skimmer cone. For the skimmer cone to sample the expanding plasma gas effectively, the orifice must be situated... 

Skimming the gas outside the zone of silence produces very low sensitivity. The profile of the skimmer cone tip is sharper than that of the sample cone, because the presence of the skimmer tip in the zone of silence disturbs the gas expansion and in order to maintain optimum ion sampling, this disturhance should he minimisecL The shape of the skimmer and the condition of the skimmer tip both have a profound effect on instrument transmission, mass response, molecular backgrounds, matrix effects and even the subsequent geometry of the ion optics. 

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