Skimmer lens

Figure 5. Front end of triple a quadrupole mass spectrometer with an ion source and reaction chamber (see Figure 4) mounted in front of an evacuated space containing skimmer cone CB, AC only quadrupole lens Qo, and first quadrupole Qi. The second and third quadrupoles, Q2 and Q3, are not shown. CR denotes cryopumping surfaces. IQ is an interquad lens.

Several effects, like instrumental mass bias, isobaric interferences, instrumental background, contamination of the solution introduction system and the sampler and skimmer cone and lens... 

From these spectra, the (skimmer + ion-source + magnetic analyzer + lens transport) efficiency was roughly estimated to be better than 1-0 %. 

Figure 10 Ion optics (a) Einzel lens/Bessel box similar to optics used in Sciex ELAN 250,500, and 5000 ICP-MS instruments, (b) Multiple lens stack similar to those used in VG instruments, (c) Optics similar to those used in HP 4500. (d) Offset ion lens system developed by Hu and Houk [108,109] and used in Thermo ICP-MS. (e) Ion optics used in Seiko instrument with the quadmpole mounted perpendicular to the sampler-skimmer axis, (f) Single-lens-based ion optics similar to the Perkin-Elmer ELAN 6000 system.

Analyte ions of different mass travel through the skimmer at about the same velocity because the gas is neutral overall so gas flow is determined predominantly by the neutral Ar atoms. As a result, the ion kinetic energy increases with increasing mass over a range from about 3.3 to 7.3 eV [97,98]. The focusing of ion lenses is dependent on ion kinetic energy. Therefore, the optimal lens voltages are mass-dependent. 

Micromass has developed a potentially powerful new technique that eliminates many of these molecular interferences and also removes ions with an energy that differs from that of the analyte, such as components of the Ar support gas. This has a dramatic effect on the performance of the instrument. The technique deploys a hexapole ion lens (Szabo, 1986) located behind the skimmer cone and surrounded by a gas cell (Fig. 8.5). The hexapole uses a hexagonal array of rods between which a 400-V rf field is applied, confining the ions of interest to stable trajectories be-... 

Micromass has applied this new hexapole technology to a fast scanning magnetic sector multiple-collector instrument (Fig. 8.6). The source is at ground potential so most of the lens system and analyzer float at —6 kV. The ions are extracted into the hexapole through a sample cone with a 1.1-mm orifice, a 0.8-mm orifice skimmer cone, and finally a 2-mm orifice transfer lens held at —400 V. The hexapole is inclined to prevent line-of-sight transmission and damage to the detectors from the source. A lens system transfers the ions into the mass analyzer. 

A 3D ion trap with an external ESI source (Finnigan LCQ). Ions produced by the ESI source are focused through a skimmer and octapole lenses to the ion trap. The gating lens is used to limit the number of ions injected in the trap, to avoid space charge effects (see further). 

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