Extending the mass range of the quadrupole ion trap using axial modulation

Abstract

Secondary-ion mass spectrometry, using Cs' ion bombardment of solid and liquid matrices, has been implemented on a quadrupole ion trap. Tetraalkylammonium cations and protonated peptides, generated in an external source using a pulsed primary-ion beam, are injected into the ion trap where they are decelerated and trapped in the course of collisions with the helium bath gas. The contents of the trap are then examined by recording a mass spectrum using the mass-selective instability scan. To improve performance in the examination of these trapped ions, the technique of axial modulation is used to cause resonance ejection of ions from the trap during the mass-analysis step. The supplementary AC voltage applied to the end-caps causes resonant ejection of ions at qr values below those which correspond to the normal stability limit. This is demonstrated to increase the mass limit of a commercial ion trap from 650Da to approximately 1300Da with no loss in resolution or sensitivity, and to more than 20 OOO Da at reduced sensitivity.As both a reactor for gas-phase ion/molecule reactions and as a sensitive mass analyzer, the quadrupole iontrap mass spectrometer has proved to be a versatile tool for the qualitative and quantitative analysis of volatile organic compounds.' The sensitivity of ion traps' and the ability to dissociate ions efficiently in the course of tandem experiment^,^-^ has increased the value of these instruments. Ions are typically generated within the cavity of the cell by electron ionization, chemical i~n i z a t i o n~.~ or photoionization.' However, internal ionization is not the only means of ion-trap operation; ionization by laser desorption with injection of the desorbed ions has also been described. In particular, laser desorption and subsequent injection of both metal ions and organic cations has been reported.'-" Growing interest in the study of biomolecules makes the desorption ionization (DI) methods such as secondary-ion mass spectrometry (SIMS), fast-atom bombardment (FAB) and laser desorption (LD), which are applicable to non-volatile compounds, of increased interest. This paper describes the implementation of desorptionionization techniques with ion traps.To exploit the DI capabilities described here, the mass range of the ion trap must be extended. This can be achieved by using reverse scans,'' by reducing the RF freq~ency'~ or by employing devices of reduced size, as recently r e p~r t e d . '~ An alternative procedure, used here, employs the simple expedient of applying between the end-cap electrodes an AC voltage of a frequency less than one half of the fundamental trapping frequency so permitting a method of mass analysis with an increased mass range. This method called "axial modulation" was first described in a patent by Syka and coworkers3 and was recently reported to provide enhanced resolution for the special case of an applied AC voltage of nearly one half the fundamental trapping frequency (so that the mass range was not extended significantly). l5

Matrix-assisted Laser Desorption Ionization/Quadrupole Ion Trap Mass Spectrometry of Peptides

“…Since only peak differences of 80 Da were observed using the TOF, we assume that the overlap peptide was phosphorylated at only one site. The observation of phosphorylated analogs of b 25 , y 25 , y , y 18 , y 16 -15 , y , y 10 , and y 8 serve to confirm the site of phosphorylation on Ser-249 rather than on Ser-234 or Ser-236. In addition, only Ser-249 is followed by a proline residue, agreeing with the proposed kinase mechanism (15).…”

“…The electron multiplier was then turned on. Data acquisition was accomplished by ramping the amplitude of the rf signal while applying a supplementary signal to the end cap electrodes to resonantly eject ions through the end cap electrode and into the conversion dynode/electron multiplier detection system (18,19). Details appear in the relevant figure legends.…”

Optimized orbitrap HCD for quantitative analysis of phosphopeptides

“…Using this approach, we readily identify activated signaling pathways downstream of oncogenic mutants of Flt-3 kinase in a model system of human myeloid leukemia. [4][5][6], extended mass range capability [7], use of helium buffer gas [3], and automated control of trapped ion populations [8], all combined to catapult trap-based instruments to the forefront of proteomics research. Concomitant advances in CPU and embedded systems provided for rapid and automated control of instrument parameters and yielded heretofore unattainable throughput for peptide sequence analysis via LC-MS/MS.…”

“…A lthough some of the earliest demonstrations of low-energy MS/MS for peptide sequence analysis were performed on triple quadrupole mass spectrometers [1], subsequent technical developments in quadrupole ion trap instrumentation, including improved scan functionality [2,3], injection of externally-formed ions [4][5][6], extended mass range capability [7], use of helium buffer gas [3], and automated control of trapped ion populations [8], all combined to catapult trap-based instruments to the forefront of proteomics research. Concomitant advances in CPU and embedded systems provided for rapid and automated control of instrument parameters and yielded heretofore unattainable throughput for peptide sequence analysis via LC-MS/MS.…”

Quadrupole ion trap mass spectrometry: theory, simulation, recent developments and applications

“…The efficacy of the interface by which ions generated externally are admitted to an ion trap and confined therein depends upon both the efficiency and mass dependence of ion trapping. Methods for injecting ions created externally have been developed for laser desorption ionization, [19][20][21] a Cs secondary ion mass spectrometric (SIMS) source, 22 an atmospheric sampling glow discharge ionization source, 23,24 electrospray ionization (ESI), 25,26 ion spray (pneumatically assisted electrospray) ionization 27 and matrix-assisted laser desorption/ionization (MALDI). [28][29][30][31][32] The three principal approaches to the introduction of externally generated ions to an ion trap are (i) injection through an end-cap electrode or desorption near its internal surface (axial injection), (ii) injection through the ring electrode or desorption near its internal surface (radial injection), and (iii) injection along an asymptote (asymptotic injection).…”