Exchange-Collision Technique for the rf Spectroscopy of Stored Ions

Abstract

A description is given of a technique, whose application to He + has previously been briefly reported, whereby the rf spectrum of field-confined paramagnetic ions in ultrahigh vacuum is observed through spin-dependent collision processes with a spin-polarized beam of neutral particles. A rf electric quadrupole ion trap is used, and a description of the ion motion, based on the adiabatic approximation, is given, including the effect of randomizing elastic collisions with neutral background particles. With particular reference to the (He 3 ) + -Cs system, the rate equations for the magnetic sublevel populations for an ion with /=},/==} are derived under the simultaneous action of spin exchange and spin-dependent charge exchange with an alkali atom. According to these equations, the relative intensities observed in the AF=0 transitions of (He 3 ) 4 " indicate that a Cs spin polarization of 0.5 was achieved in the optically pumped atomic beam. The He + polarization approached that of the Cs atoms. With on-off modulation of the Cs polarization, a total 2% change in the He + lifetime was observed, with a signal-to-noise ratio of 4, in an interaction period having a duration of 0.8 sec, the optimum value for the observed 0.4-sec lifetime against Cs-induced ion loss. In the absence of the beam, the lifetime was 8 sec at a residual pressure of 3 X10"" 8 Torr. The AF = 0 lines obtained with long integration times had a signal-to-noise ratio which indicated that the AF= ±1 transitions should be observable, as has since been demonstrated.

Resonant excitation in a low-pressure linear ion trap

“…In the work presented here, ions were excited resonantly with the RF amplitude, V RF Ϸ 150 V, corresponding to q Ϸ 0.21 for m/z 609. This setting offered a good compromise between a radial potential well, sufficiently deep to reduce losses from scattering [16,17], while retaining an acceptable mass range of fragment ions. That is, the lowest stable mass is determined by the ratio of the q, which corresponds to the auxiliary frequency, and the low-mass cut-off at q ϭ 0.908.…”

Studying mass spectrometric behaviors of {Au6Ag2(C)[PPh2(4‐CH3‐Py)]6}(BF4)4 and {Au8[(PPh3)2O]3(PPh3)2}(NO3)2 by electrospray time‐of‐flight mass spectrometry and electrospray ion trap mass spectrometry

“…Since the flight path of ions in a reflectron TOF-MS is only 2 meters, it is obvious that the probability of bimolecular collision is extremely low under the high vacuum condition in TOF-MS. IT-MS, on the other hand, requires a buffer gas pressure (such as helium, nitrogen or argon) for colliding the ions to remove the excess kinetic energy and to ensure the ions are confined at the center of the trap. [22,23] This is known as the "collision cooling" process, which ensures the instrument maintains a high-mass resolution and sensitivity. Because of the different gas pressure and bimolecular collision degree in the two mass analyzers, it is safe to reach the conclusion that metal clusters have a higher chance of remaining intact in TOF-MS than IT-MS, which means TOF is a "softer" analyzer.…”

“…[21] Besides, mass analyzers with certain operating principles also make a contribution to matter decomposition in MS analysis. Take the ion trap (IT) analyzer as an example, to ensure the ions are confined at the center of the IT, a "collision cooling" process [22,23] is induced to remove the excess kinetic energy of ions by colliding with buffer gas molecules (e.g., helium), which may create more fragments. While no collision is needed in a time-of-flight (TOF) analyzer and it is believed that there is a good chance to capture intact metal clusters in TOF.…”

On the time scale of internal energy relaxation of AP-MALDI and nano-ESI Ions in a quadrupole ion trap

“…During isolation, the ion cloud will experience several different trapping environments, most notably a gain in kinetic energy as the q z value approaches a resonant ejection point, and a decrease in the depth of the pseudopotential trapping well as q z is reduced to the value used for CID. The depth of the pseudopotential well is given in eq 7 [17,18].…”