The First Skimmer

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... 

Figure 8.4 Ion source designs and parameters affecting id/D scrambiing. (a) For Instruments with a heated capillary Ion source design, the capillary temperature, potential difference between transfer capillary and the first skimmer, and the DC voltage of the first multipole Ion guide can contribute to H/D scrambling, (b) For Instruments with a Z-type Ion source, the sample and extraction cone voltage and the velocity of the source F-wave Ion guide (Synapt) can contribute to H/D scrambling...

SP Packs may be effectively multistaged, as shown in Figure 3.31. As shown in Figure 3.32, a two-stage system may consist of an SP Pack, a skim vessel, a second SP Pack, and a second identical skim vessel. One SP Pack and skimmer combination constitutes one stage of coalescence and separation. The second SP Pack coalesces the small oil droplets in the first skimmer s outlet then the second skimmer may remove the larger oil droplets. 

The addition of the SP Pack greatly improves the oil removal in the second skimmer because of coalescence. If the second SP Pack were not in the system, all the large oil droplets would be removed in the first skimmer and the second skimmer would remove little oil. Any number of stages in series may be used in the system. 

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. 

Parallel-plate interceptor. The first form of plate coalescer was the parallel-plate interceptor (PPI). This involved installing a series of plates parallel to the longitudinal axis of an API separator (a horizontal, rectangular cross-section skimmer). When viewed perpendicular to the axis of flow, the plates form a "V so that the oil sheet migrates up the underside of the coalescing plate and to the sides Sediments migrate toward the middle and down to the bottom of the separator where they are removed. 

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. 

After those first attempts to establish analytical applications of electrospray, it took more than ten years for the first bona fide electrospray mass spectrometer to emerge [14]. Yamashita and Fenn published the first electrospray MS experiment in a 1984 paper which was appropriately part of an issue of the Journal of Physical Chemistry dedicated to John Bennett Fenn [15]. They electrosprayed solvents into a bath gas to form a dispersion of ions that was expanded into vacuum in a small supersonic free jet. A portion of the jet was then passed through a skimmer into a vacuum chamber containing a quadrupole mass filter. With this setup, a variety of protonated solvent clusters as well as solvent-ion clusters (Na+, Li+) could be de-... 

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... 

The first vacuum chamber is evacuated down to ca. 0.05 Pa by means of a 1-mVs oil-diffusion pump. The core of the supersonic jet is sampled by a 2-mm-lD skimmer, kept at -20 V, and transported directly into the quadrapole analyser region. The countercurrent bath gas prevents the introduction of non-volatile contaminants into the high-vacuum region. 

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