Ion lens

From the above, we can see that two cylinders will provide a converging lens. However, they generate outgoing ions at a different electrical potential to the incoming ions. For some 

The decision as to whether to use an aperture or a cylinder lens depends very much on the practical requirements for a particular instrument. Clearly an aperture lens is more compact than a cylindrical lens, and therefore if space is paramount then the likelihood is that an aperture lens would be chosen. On the other hand a cylindrical lens will generally show higher ion transmission and therefore would be the optimum choice for maximum signal. Furthermore, cylinder lenses have fewer aberrations and because of their large apertures they are more tolerant of slight misalignment than would be the case with an aperture lens. 

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The skimmer has a smaller aperture than the sample cone, which creates a pressure of 10 atmospheres in the intermediate region. The ions are conducted through the cones and focused into the quadrupole with a set of ion lenses. Much of the instrument s inherent sensitivity is due to good designs of these ion optics. 

Fig. 11.2. Schematic of a thermospray interface. A cartridge heater B copper block brazed to stainless steel capillary C capillary D copper tube E ion lenses E quadrupole mass analyzer G line to rotary vane pump H ion exit aperture J source heater. Reproduced from Ref. [30] by permission. American Chemical Society, 1983.

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. 

The relative response for equimolar eoneentration of elements of different mass, eorreeted for abundanee. This refleets the variable transmission of ions through the ion lenses and quadrupole. 

A further study by Vela and Caruso [128] evaluated the effects of interface temperature, oven temperature, C02 pressure, mobile phase composition and column length in order to optimise the separation of several tetra and tri organotin compounds. The same interface, described by Shen [127], was used. It was found that the introduction of C02 did not require nebuliser flow-rate and RF power optimisation if the ion lenses were tuned sufficiently. The addition of a polar solvent to the non-polar mobile phase did not yield any improvement in resolution. Longer columns were found to yield broader chromatographic peaks. Absolute detection limits for TBT, tributyltin chloride, triphenyltin chloride and TPT were in the range 0.20-0.80 pg Sn. 

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. 

Generally, dissolved solid concentrations should be kept below 0.2% for ICP-MS. Higher dissolved solid concentrations can lead to deposition of sample on the sampling and skimmer orifices, partial orifice plugging, or deposition of material on ion lenses that degrade sensitivity and medium-term to long-term precision. Furthermore, relatively small concentrations of a heavy element (100 ppm or greater) in a sample can cause a decrease in analyte sensitivity, particularly... 

Figure 1 Schematic diagram of a typical commercial inductively coupled plasma mass spectrometry (ICP-MS) instrument (A) liquid sample, (B) peristaltic pump, (C) nebulizer, (D) spray chamber, (E) argon gas inlets, (F) load coil, (G) sampler cone, (H) skimmer cone, (I) ion lenses, (J) quadrupole, (K) electron multiplier detector, (L) computer.

In ICP-MS (Fig. 112) the ions formed in the ICP are extracted with the aid of a conical water-cooled sampler into the first vacuum stage where a pressure of a few mbar is maintained. A supersonic beam is formed and a number of collision processes take place as well as an adiabatic expansion. A fraction is sampled from this beam through the conical skimmer placed a few cm away from the sampler. Behind the skimmer, ion lenses focus the ion beam now entering a vacuum of 10-5. This was originally done with the aid of oil diffusion pumps or cryopumps, respectively, but very quickly all manufacturers switched to turbomolecular pumps backed by roughing pumps. 

Fig. 18. Diagram of reactive scattering apparatus for the study of non-metal reactions A, scattering chamber B, source chambers C, liquid nitrogen cooled cold shield D, detector E, source bulkheads G, liquid nitrogen trap H, oil diffusion pumps N, free radical source P, nozzle source Q, skimmer E, ion source H, liquid He trap I, ion lenses P, photomultiplier Q, quadrupole rods R, light baffle S, slide valve T, radial electric field pumps (from C. F. Carter et al. 02 by permission of the Chemical...

Quadmpole mass analyzers are found in most commercial ICP-MS instmments, in most GC-MS instmments (Chapter 12) and in many LC-MS instruments (Chapter 13). Quadmpoles are also used in MS-MS systems as mass analyzers and ion lenses. This use will be described in Section 9.2.3.4. 

A series of ion-focusing elements (ion lenses) similar to those developed for double-focusing mass spectrometers have been utilized to introduce the ions into the quadrupole. Also, photon blockers have eliminated interference effects due to the presence of large numbers of photons reaching the detector. Therefore, background signals have been largely ehminated in modem ICP-MS instmments. 

Atmospheric-pressure ionization mass spectrometry (Horning et al., 1973 Carroll et ai, 1974) is a chemical ionization procedure that differs from the standard technique in a number of ways. The reaction chamber (ion source) is at atmospheric pressure and is external to the high-vacuum region of a quadrupole mass analyser. Ions and molecules enter the mass analyser region through a small aperture. The ions are obtained by ion lenses, while... 

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