Collisional cooling

Figure 3. Positive ion spectrum showing the reaction of V+ with gaseous Sg. Spectrum (a) shows the reaction products when the V+ ions are collisionally cooled before reaction with Sg. Spectra (b) and (c) show the reaction products when the isolated V+ ions are increasingly accelerated by a rf pulse.

The reactions of small cluster cations of copper and silver, Cu and Ag (n = 1-5), with methanol, ethanol, the two isomers of propanol, and the four isomers of butanol have been studied in a FT-ICR mass spectrometer (200). The ions were produced by FAB and exited through a small hole that aided the clustering process. Once in the cell, the ions were collisionally cooled with argon and allowed to react with the alcohols (3-100 x 10 6 Pa) for periods up to 60 s. The Cu4 ion was produced but was of insufficient abundance for reactivity studies. 

Tolmachev, A.V. Udseth, HR. Smith, R.D. Radial Stratification of Ions As a Function of Mass to Charge Ratio in Collisional Cooling Radio Frequency Multipoles Used As Ion Guides or Ion Traps. Rapid Commun. Mass Spectrom. 2000,74, 1907-1913. 

Fig. 1.28 Schematic of a quadrupole-time of flight instrument. Quadrupole qO is used for collisional cooling and ion focusing.

Regardless of these short-ranged cohesive forces, the formation and stability of particle clusters in a fluidized bed appears to be a multistep process [27], Some shear (as in two particles grazing each other) may be needed to promote collisional cooling, but less than that perhaps in the dense emnlsion of a fluidized bed. Perhaps the lower particle concentration in a babble provides the environment where clnster stability is promoted for the smaller particles. Collisional stresses in the emnlsion may be too high and the cohesive forces may be too low to have long-lasting particle clusters. Indeed, the only evidence of particle clnsters in fluidized beds offered here is that the clusters are located near the bubbles. 

Although the pulse duration of the valve was 2 ms, the high pressure of the reagent gas had a rise time of about 200 ms and was pumped away by a high speed 5 in. diffusion pump in approximately 400 ms. Swept double resonance pulses were then used to isolate the ion of interest, which was subsequently trapped for 3 6 seconds (determined by the ion s cross section for photodissociation) either in the presence or absence of radiation. For each ion, two sets of,photodissociation spectra were taken, one at 2 x 10 ° torr argon, to permit collisional cooling, and another at a background pressure of 10 torr [28]. In all cases, data from the collisionally cooled ions are presented. 

This is sometimes referred to as delayed extraction. A second technique is to use a reflectron. More recently, orthogonal introduction of ions has been introduced, in analogy to the orthogonal introduction of ions discussed above. The prOTOF 2000 marketed by Perkin Elmer Sciex is an instru-ment using orthogonal introduction. The prOTOF 2000 also uses collisional cooling in the interface between ion source and mass analyzer. 

High-pressure and —temperature Raman data were used to construct a phase diagram for N20.531 IR spectra have been reported for small N20 clusters, as particles formed by collisional cooling.532... 

The two-dimensional linear ion trap (2D-LIT) is a logical development of the Qjnass fdter, described above, in that by the imposition of appropriate potentials at the entrance and exit of the Qjs, ions with a range of m/z values can be trapped within the axial quadrupolar field (see March and Todd69 for a detailed theory of operation and history of development). In common with the QJT, the LIT operates at relatively high pressure (10 3 torr) with a helium buffer gas. The buffer gas collisionally cools the ions and also acts as a collision gas for MS/MS experiments.70,71... 

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