First skimmer, 78 from

The flow of droplets is directed through a small orifice (Skimmer 1 Figure 12.1) and across a small region that is kept under vacuum by rotary pumps. In this region, approximately 90% of solvent and injected helium is removed from the incipient particle beam. Because the rate of diffusion of a substance is inversely proportional to its molecular mass, the lighter helium and solvent molecules diffuse away from the beam and are pumped away. The heavier solute molecules diffuse more slowly and pass through the first skimmer before they have time to leave the beam the solute is accompanied by residual solvent and helium. 

The beam from the first skimmer is directed toward a second one (Figure 12.1), again across an evacuated region where almost all of the residual solvent and helium are pumped away to leave a... 

The particle beam — after linear passage from the evacuation chamber nozzle, through the first and second skimmers, and into the end of the ion source — finally passes through a heated grid immediately before ionization. The heated grid has the effect of breaking up most of the residual small clusters, so residual solvent evaporates and a beam of solute molecules enters the ionization chamber. 

In the first instrument, sensitivity and limits of detection were constrained by the total ion beam current measured after the extraction region of the TOF-MS [42]. Although pulser circuitry limited the repetition rate to 7.1 kHz and a duty factor of 3.5%, detection limits ranged from 0.03 to 3 ppb using ultrasonic nebuliza-tion [16]. Mahoney et al. later reported on improvements to the same orthogonal ICP-TOF-MS [29]. The use of a commercial skimmer cone and quadrupole doublet in the extraction optics led to an increase in primary ion current from 2 nA to 50 nA [29]. This, along with electronic improvements allowing a 16-kHz spectral-repetition rate, dropped limits of detection to 1-10 ppt when ultrasonic nebuliza-tion was utilized. 

The extraction lens, held at a negative voltage, attracts the positive ions as they emerge from the skimmer cone. The negative ions are repelled and the neutral species diffuse away. The accelerated positive ions are focused by the ion-lens stack and then enter the quadrupole region. Positioned between the extraction lens and the first ion... 

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