ABSTRACT
The addition of high pressures of helium (ca 1 × 10-3 Torr) to quadrupole ion traps is a practice that is unique to this type of mass analyzer. The fact that a mass spectrometer can operate at all in this pressure regime was surprising to the mass spectrometry (MS) community. Most mass spectrometers operate at background gas pressures at least three and up to eight orders of magnitude lower (1 × 10-6 – 1 × 10−11 Torr) in order to minimize the scattering effects caused by collisions of the ions with background gases. The effects of collisions have been observed since the early days in quadrupole ion trap mass spectrometer (QITMS) development. Increased background gas pressure was rst seen to stabilize arrays of charged iron and aluminum
17.1 Introduction ................................................................................................ 739 17.1.1 Helium Bath Gas and the Quadrupole Ion Trap Mass
Spectrometer (QITMS) ................................................................. 739 17.1.2 Why Is Collisional Cooling Important? ....................................... 741 17.1.3 Internal Energy in MS/MS .......................................................... 741
17.2 Theory ......................................................................................................... 745 17.3 Collisions .................................................................................................... 746
17.3.1 Collisional Cooling Rate Constants ............................................. 748 17.3.2 Probing Collisional Cooling Experimentally ............................... 751 17.3.3 Collisional Cooling Rate Constants as a Function of Ion Size .... 758
17.4 Conclusions ................................................................................................. 762 Acknowledgments .................................................................................................. 763 References ............................................................................................................. 763
particles in a QITMS [1],and was later shown to have a profound effect on both the number of trapped ions and their storage lifetimes [2-5]. The effect of added neutral gas pressure on the peak width and height of N 2 + ions analyzed with the mass-selective storage technique was investigated, and the peak height was found to maximize at around 1 × 10-4 Torr, while peak width remained largely constant [6]. Simulation work showed that charge-exchange collisions caused ions to concentrate to the center of the trap, decreasing kinetic energy and increasing extraction efciency [7-9]. Nonetheless, most QITMS systems were in fact operated in the 10-5 Torr range for many years. In the early 1980s, when the QITMS was being developed as a gas chromatography detector by Finnigan MAT (now Thermo Fisher Scientic), experiments to determine the effect of the helium carrier gas on mass spectra obtained with the mass-selective instability scan mode yielded the surprising discovery that increasing background pressure into the milliTorr range actually enhanced simultaneously the sensitivity and the resolution [10,11]. Both effects have been rationalized on the basis of kinetic damping of the ion motion to the center of the ion trap. This damping increases trapping efciency and causes the ions to experience a more ideal and homogeneous quadrupolar eld, away from the higher-order eld perturbations caused by the exit and entrance holes in the end-cap electrodes and electrode truncation. Kinetic damping proved later to be indispensable for achieving high dissociation efciencies with infrared multiphoton dissociation (IRMPD), see below, collision-induced dissociation (CID) in mass spectrometry/mass spectrometry (MS/MS) [12], and for trapping ions formed externally to the ion trapping volume via techniques such as electrospray ionization (ESI) [13] and matrix-assisted laser induced desorption ionization (MALDI) [14-16]. Specic mention must be made to several previous simulation studies of trapping externally formed ions in a QITMS [17-20]; however, these works were mostly not focused on the role of kinetic damping through collisions but instead elaborated on the optimum RF-phase relation and timing for external injection of ions.*
As well as the damping of the ion kinetic energy, a second process caused by a high helium gas pressure is the dissipation of ion internal energy.† Calculations give the frequency of helium/ion collisions to be of the order of 20 per millisecond [21], with the typical ion residence time in the QITMS being several tens to hundreds of milliseconds. In fact, during normal ‘storage’ conditions in a QITMS, one can assume generally that ions have been quasi-thermalized, such that the distribution of ion internal energies can be described with Boltzmann’s equation at the temperature of the background gas [22-24]. These hundreds to thousands of helium/ion collisions that occur before an ion is mass analyzed can have appreciable effects on MS and MS/MS spectra. The goal of this chapter is to give the reader a solid foundation for understanding collisional cooling: what it is, why it is important to consider, and how it has been measured theoretically and experimentally.
