Collisional focusing

Douglas DJ, French JB (1992) Collisional focusing effects in radio frequency quadropoles. J Am Soc Mass Spectrom 3 398-407... 

Douglas, D. J., and French, J. B. (1992). Collisional focusing effects in radio frequency quadruples. J. Am. Soc. Mass Spectrom. 3 398-408. 

Turner et al. [114] described an ICP-MS with a hexapole transfer lens (Fig. 3.12a) in a tube that allows the pressure to be maintained. The cell was initially reported to contain He, although it is likely that H2 or H20 vapor was also in the cell and is now purposely added to the cell [115]. Ar2+, ArCl+, ArO+, and Ar+ signals were reduced relative to Se+, As+, Fe+, and Ca+, respectively. Detection limits for Fe, Ca, K, Se, and As near 10 parts per trillion (ppt) have been reported [115]. Recently, Beaty and Liezers [116] also described a collision-reaction cell at a pressure of 30 mtorr that reduced the ion kinetic energy spread as well as continuum ICP-MS background to less than 1 count/sec. Previously, Douglas and French had described the use of an rf-only quadrupole for collisional focusing and reduction of the ion kinetic energy spread [117]. 

Collision/reaction cells are typically operated at the lowest possible kinetic energy for several reasons. First, the benefits of collisional focusing are realized at low... 

Ion trap capacities A simple model for the comparison of ion trap capacities based on the volumes of the trapped ion clouds subjected to collisional focusing in each instrument yields... 

Anrcjl) are the occupied volumes of the same instruments, respectively. For collisionally focused ion clouds at a common rf potential and radial frequency, rA = = fc = 1.0 mm based on ion tomography experiments, the ratio Nzd,a N2D,b N3d = 95 22 1. This ratio suggests an ion capacity for the linear ion trap two orders of magnitude greater than that of the 3D ion trap. 

One result of the collisional focusing caused by the low pressure of helium is to initially restrict ions to orbits in the trap that have maximum displacements from the centre of about 0.1 rg ( 1 mm) or less (i.e. well away from the electrodes) this effect directly reduces ion losses arising from initial positions and velocities of the ions, and thus also the detection limit. A less direct but still significant effect of the coUisional focusing arises because, during the mass selective instability scan, ions of a given mJz spend most of their lifetime in the center of the trap where the theoretical field is zero (see discussion of Equation [6.28]) so that field imperfections, caused by mechanical errors in the fabrication of the electrode structure, are at a minimum. 

In practice, for quadrupole collision cells operated with typical values of rg and Vg, the low mass cut off problem is not often a major concern. In such cells that are operated with a sufficiently high pressure of collision gas to be in the collisional focusing regime (Section 6.4.3), the loss to the quadrupole rods of ions with q > 0.908 and mjz < ijnlz) (predicted for collision free conditions) is countered to a large degree by the collisional focusing of ions on the main axis of the device. 

KED process has allowed the analytes to be efficiently separated from their respective polyatomic interfering ions. The additional benefit of using helium is that it is inert and even if it is not being used in an interference reduction mode, it can have a beneficial effect on the other elements in a multielement run by increasing sensitivity via the process of collisional focusing. This makes the use of helium and KED very useful for both quantitative and semiquantitative multielement analysis using one set of tuning conditions. 

Based on a quadrupole ICP-MS, another technique, termed collision/reaction cell, has frequently been used to reduce polyatomic ion interferences. " These techniques provide simple, efficient, and low-cost methods in the face of many difficult interference problems. In these methods, ions to be analyzed first enter a radio-frequency-only multipole (e.g. a quadrupole, hexapole, or octapole), in which the analytes react with the collision/reaction gas, which is usually oxygen, ammonia, xenon, or methane, " to remove polyatomic interference or generate a new analyte ion of mjz showing less interference. The RP-only multipole does not separate ions like a traditional quadrupole, but it has profound influence on collisional focusing of ions, both of the energy and spatial distributions. An example of removing the polyatomic interference is shown below, which uses ammonia gas to reduce any "" Ar" " interference in the measurement of " Ca ... 

Siu, K. W. M. Ion mobiUty spectrometer with radial collisional focusing. Anal. Chem. 2005, 77, 266-275. 

Chemushevich, I. V., Tolmachev, A. V., Krutchinsky, A. N., Spicer, V. L., Ens, W., and Standing, K. G. (1996) Collisional focusing of ions in the electrospray TOE mass spectrometer. In The 44th ASMS Conference on Mass Spectrometry and Allied Topics, Portland, Oregon. 

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