Energetics and efficiencies of collision‐induced dissociation achieved during the mass acquisition scan in a quadrupole ion trap

Abstract

Using n-butylbenzene as a test molecule, evidence is provided that fast, efficient or highly energetic collision-induced dissociation (CID) can be achieved during the mass acquisition ramp of a commercially available quadrupole ion trap (QIT) mass spectrometer. The method of excitation is very similar to axial modulation for mass range extension except that lower amplitude waveforms are used to excite the precursor ions within the trap instead of ejecting them from the trap. ITSIM simulations verify that fast kinetic excitation followed by kinetic-to-internal energy transfer occurs on the rapid time-scale required for the recapture and mass analysis of product ions during the mass acquisition ramp. CID efficiencies larger than 50% can be obtained using this new approach and ratios of Th 91/92 of n-butylbenzene fragment ions as large as 9 are possible, albeit at significantly reduced efficiencies. These very large ratios indicate an internal energy above 7 eV for the precursor ions indicating that fragmentation of larger ions could also be possible using this new approach. The main benefits of the new method are that no extra time is required for fragmentation or cooling and that on-resonance conditions are guaranteed because the ions' secular frequencies are swept through the fixed frequency of excitation. Also presented are the effects of experimental variables such as excitation frequency, excitation amplitude and scan rate on the CID efficiencies and energetics. Copyright # 2005 John Wiley & Sons, Ltd.Quadrupole ion traps (QITs) are widely recognized for their flexibility and sensitivity in accomplishing tandem mass spectrometry (MS/MS), which is an invaluable tool for the quantitative, qualitative and mechanistic interrogation of gas-phase ions.1,2 The ability to perform MS/MS is important because it is a central technology for proteomics 3,4 and applications such as drug/metabolite monitoring and forensic sciences. 1,5,6 Most applications of MS/MS rely on the ability to obtain reproducible and reliable fragmentation spectra from selected precursor ions in order to match database entries or to make other dependable conclusions based on the fragmentation spectra. However, faster and more complex separation technologies are being developed that demand MS/MS spectra to be obtained more quickly and on smaller sample sizes than is favorable for reliable on-resonance excitation to be performed. These new demands require fresh approaches for achieving MS/MS while, if possible, not adding to the complexity or cost of the instrumentation.On-resonance excitation using supplemental alternating current (ac)-waveforms applied to the end-cap electrodes has been the most widely used approach for achieving collisional activation of selected precursor ions. This method of fragmentation is available on almost all modern commercially available instruments and has been extremely well characterized. For example, the performance of on-resonance excitation has been found to be dependent on a number of experimental variables includi...

Dynamic collision-induced dissociation (DCID) in a quadrupole ion trap using a two-frequency excitation waveform: I. Effects of excitation frequency and phase angle

“…As°previously°reported° [30],°our°group°used°Visual Basic ® 6.0 (Redmond, WA, USA) software command programs provided by Thermo to generate an arbitrary waveform that can be applied at any period of QIT operation. In the present work, a two-frequency sumof-sines°waveform° [4]°was°applied°to°the°end-cap electrodes in a dipolar fashion during mass acquisition.…”

“…In the excitation method proposed herein, a second frequency is used in an effort to provide additional excitation of the precursor ions, without changing the scan rate or timescale of the experiment. In this multiple-frequency excitation approach, the ions' secular frequencies are swept through the fixed frequencies of excitation as in the previously reported study [30]. The superimposition of the two sinusoidal frequencies results in an excitation waveform with a beating pattern.…”

“…Previous studies investigated the influence of scan direction [25][26][27], resonance ejection frequency [27], and axial modulation amplitudes [28] on the mass spectra of fragile ions during the mass instability scan. Although unintentional fragmentation of precursor ions during axial modulation has generally been shown to be detrimental, it was previously shown [29,30] that deliberate fragmentation of precursor ions during mass acquisition can be used to provide reliable fragmentation spectra of the precursor ions. For example, product ions can be recaptured and detected at their corresponding m/z values as long as their m/z value is above the low mass cutoff (LMCO) at the time of fragmentation.…”

“…Our group recently demonstrated that collision-induced dissociation (CID) efficiencies as large as 60% and internal energies up to 7 eV can be achieved with the application of a single-frequency excitation waveform applied to the end-cap electrodes during mass acquisition, in addition to the axial modulation frequency [30]. As the ions are swept through the stability diagram by gradually increasing the radio-frequency (rf) amplitude on the ring electrode, each ion comes into resonance with the applied fixed-excitation frequency at a specific time during the scan.…”

“…The on-resonance condition-as in the case of axial modulation-causes an increase in the oscillation amplitude of the ions and results in energetic collisions with the bath gas. It was observed [30] that at low excitation amplitudes, precursor ions gain only a small amount of internal energy and fragment with less energy and poorer efficiencies. At large excitation amplitudes, the ions were accelerated beyond the Dehmelt pseudopotential well depth, resulting in ion ejection and poorer CID efficiencies.…”