Fragmentation of ions in a low pressure linear ion trap

Abstract

The efficiency of in-trap fragmentation in a low-pressure linear ion trap (LIT), using dipolar excitation, is dependent upon the choice of both the excitation q and the drive frequency of the quadrupole. In the work presented here, fragmentation efficiencies have been measured as a function of excitation q for drive frequencies of 816 kHz and 1.228 MHz. The experiments were carried out by fragmenting reserpine (609.23¡448.20 Th and 397.21¡365.19 Th transitions) and caffeine (195¡138 Th and 138¡110 Th transitions). The data showed that the onset of efficient fragmentation occurred at a lower Mathieu q for the LIT operated at 1.228 MHz when compared with the LIT operated at 816 kHz. A comparison of the fragmentation efficiency curves as a function of pseudo-potential well depth showed that the onset of fragmentation is independent of the drive frequency. In addition, a comparison of the fragmentation efficiency curves showed that all four of the precursor ions fragmented within a range of four V of pseudo-potential well depth. The choice of an appropriate excitation q can then be determined based upon a minimum pseudo-potential well depth, quadrupole field radius, drive frequency, and the mass of interest. Fragmentation efficiencies were also found to be significantly greater when using the higher drive frequency. [2] enabled the development of the hybrid LIT triple quadrupole mass spectrometer. In this device, the second mass analyzing quadrupole doubles as the LIT. The LIT typically operates at pressures in the 3-5e-5 torr range, which is significantly lower than the mTorr pressure regime typically used for the operation of 3-D ion traps [3,4] and other high-pressure LITs [5,6]. This difference is a key factor in how in-trap fragmentation of ions, through the use of dipolar excitation, is performed [7]. The performance of 3-D ion traps in the mTorr regime have been well characterized and understood with regard to fragmentation efficiency, type of collision gas, excitation q, and a variety of models [8 -13].In the low-pressure LIT the collision frequency is low enough, about tens to a few 100 s between collisions, that the ion experiences several rf cycles between collision events with the background gas. It becomes necessary to take advantage of the presence of higher order fields, occurring naturally with the round rod electrodes used in the construction of the quadrupole, to keep the ions confined radially within the LIT during the fragmentation process. This also necessitates the use of low excitation amplitudes, about several tens of mV amplitudes, and excitation periods about a few tens to a couple of hundred ms. The duration of the excitation periods can be reduced and the excitation amplitudes increased without loss of the precursor ion by increasing the content of the higher order fields. This can be done with the use of additional electrodes along the length of the LIT [14] or by distorting the physical dimensions of the quadrupole electrodes for the purposes of introducing additional higher order fiel...

Quadrupole excitation of ions in linear quadrupole ion traps with added octopole fields

“…Second, the effective potential well depth is greater in the x direction than in the y direction. Collings [17] has shown that at q Ϸ 0.2, small increases in well depth can give relatively large increases in fragmentation efficiency (at least at ca. 3 ϫ 10 Ϫ5 Torr).…”

Multiple Reaction Monitoring with Multistage Fragmentation (MRM3) Detection Enhances Selectivity for LC-MS/MS Analysis of Plasma Free Metanephrines

“…Early MRM 3 studies with quadrupole linear ion trap (Q/LIT) instruments focused on characterization and identification in proteomics (8,9 ) and genomics (10 ). Newer-generation Q/LIT instruments have demonstrated greater quantitative sensitivity along with reduced cycle times owing to decreased excitation times for MRM 3 experiments (11,12 ). These technological advances have made MRM 3 detection more compatible with both the fast liquid chromatography used by modern LC-MS platforms and the sensitivity requirements of quantitative clinical analysis.…”

MS/MS of ions in a low pressure linear ion trap using a pulsed gas

“…T he in-trap fragmentation of ions in a low-pressure (Ͻ5e-5 Torr) linear ion trap (LIT), such as the 4000 QTRAP hybrid linear ion trap triple quadrupole mass spectrometer, require excitation periods typically ranging from 50 to 200 ms. This is due to the low collision rates between the ion and the background nitrogen which are typically about a few collisions every millisecond [1]. Low-pressure LITs rely on the presence of higher order fields to keep ions confined during the excitation process and, as a result, it is necessary to use excitation amplitudes ranging from 10 to 100 mV to reduce the degree of ion ejection.…”

“…This is due to the low collision rates between the ion and the background nitrogen which are typically about a few collisions every millisecond [1]. Low-pressure LITs rely on the presence of higher order fields to keep ions confined during the excitation process and, as a result, it is necessary to use excitation amplitudes ranging from 10 to 100 mV to reduce the degree of ion ejection.…”

“…Increasing the higher order field content allows the use of higher excitation amplitudes [2,3]. The degree of ejection from an LIT can also be reduced by increasing the drive frequency of the LIT [1]. This leads to an increase in the pseudo-potential well depth, which allows for improved radial confinement.…”