Ions, collisional cooling

IRMPD is a valuable tool for gas phase ion studies and analytical applications which often do not necessarily require the full power of an extremely expensive FT-ICR instmment. Therefore, attempts have been made to apply IRMPD on three-dimensional (Fig. 9.33) [140,141] and linear quadrupole ion traps [142-144]. Due to the slow heating of ions, collisional cooling by the buffer gas required for quadmpole ion trap operation is counteracting the accumulation of ion internal energy. To circumvent this problem a dynamic pressure operation mode has been developed for IRMPD in QITs where initial ion storage and precursor ion selection are mn at standard QIT pressure (3 x 10" mbar) while the buffer gas flow is... 

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.

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. 

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... 

Optional ion Cooler Guide allows collisional cooling of large, noncovalently-bound complexes, significantly increasing the (b) sensitivity for complexes of 0.5 MDa and above. 

The observed bands are more than twenty times broader than the bandwidth of the laser radiation (-5-15 cm i). The increased width could be caused by vibrational hot-bands, but these should be largely eliminated in the spectra because the ions are collisionally cooled prior to interaction with the laser pulse. However, the zero-point motion of the central proton on the extremely flat PES likely extends over an unusually large area, leading to the exploration of a much larger part of... 

The ion trap can be viewed as a three-dimensional quadrupole and consists of a ring electrode and two end caps (Figure 1.19). Ions are stored inside the trap by biasing the ring electrode with an rf voltage of low amplitude and grounding the end caps. Repulsive forces befween the trapped ions increase the velocities and amplitudes of their motion, which could lead to tiieir ejection from the trap. This is prevented by introducing a He bath gas in the trap (10" Torr), so that the ions are collisionally cooled and drift toward the trap center. 

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