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...
This study describes the application of a two-frequency excitation waveform to the end-cap electrodes of a quadrupole ion trap (QIT) during the mass acquisition period to deliberately fragment selected precursor ions. This approach obviates the need for a discrete excitation period and guarantees on-resonant excitation conditions without any requirement for resonant tuning; it is therefore faster than the conventional approach to collision-induced dissociation (CID) in QITs. The molecular ion of n-butylbenzene is used as thermometer molecule to determine the energetics of the new excitation procedure. The excitation waveform, consisting of two closely spaced sinusoidal frequencies, has an interference pattern that displays nodes and crests in the time domain. The energetics (determined by the product ion ratios of 91/92 Th) and CID efficiencies are highly dependent on the excitation amplitude, the relative position of the excitation frequencies in the Mathieu stability diagram, and whether the ions come into resonance during a node or crest of the excitation waveform. Under highly energetic conditions, ratios of 91/92 as large as 15 can be obtained at concomitant CID efficiencies of 10%, indicating internal energies in excess of 10 eV at the time of fragmentation. These extremely high internal energies far exceed the energetics achievable using conventional on-resonance excitation in QITs, indicating that the collisional heating rate is very fast in the new approach. Under less energetic conditions CID efficiencies as high as 70% are possible, which compares favorably with results obtained by conventional on-resonance excitation. Correlation analyses are used to determine the conditions that simultaneously optimize energetic and efficient fragmentation conditions. (J Am Soc Mass Spectrom 2007, 18, 749 -761) © 2007 American Society for Mass Spectrometry T he ever-increasing need for reliable mass spectrometric data drives researchers to develop increasingly sophisticated instruments that allow fast, cost-effective, and reproducible analysis of a wide variety of samples. Quadrupole ion traps (QITs) are of significant interest in the achievement of this goal because of their flexibility and excellent sensitivity. The ability to isolate precursor ions [1][2][3], effect fragmentation [4 -6], and to detect trapped ions in a variety of methods [7][8][9] makes QITs a very useful tool in many applications [10,11]. The design of faster, more sensitive ion traps that allow coupling with fast separation methods and overcome the necessity for resonance tuning is a major issue for QIT developers. The present study describes a small step toward the long-term goal of achieving fast, simple, and reliable tandem mass spectrometry (MS/MS).The use of QITs in tandem mass spectrometric experiments is made possible through on-resonance excitation [12], also known as axial modulation. Excitation is usually performed by matching the frequency of
Dynamic CID of selected precursor ions is achieved by the application of a two-frequency excitation waveform to the end-cap electrodes during the mass instability scan of a quadrupole ion trap (QIT) mass spectrometer. This new method permits a shorter scanning time when compared with conventional on-resonance CID. When the excitation waveform consists of two closely-spaced frequencies, the relative phase-relationship of the two frequencies plays a critical role in the fragmentation dynamics. However, at wider frequency spacings (Ͼ10 kHz), these phase effects are diminished, while maintaining the efficacy of closely-spaced excitation frequencies. The fragmentation efficiencies and energetics of n-butylbenzene and tetra-alanine are studied under different experimental conditions and the results are compared at various scan rate parameters between 0.1 and 1.0 ms/Th. Although faster scan rates reduce the analysis time, the maximum observed fragmentation efficiencies rarely exceed 30%, compared with values in excess of 50% achieved at slower scan rates. The internal energies calculated from the simulations of n-butylbenzene at fast scan rates are ϳ4 eV for most experimental conditions, while at slow scan rates, internal energies above 5.5 eV are observed for a wide range of conditions. Extensive ITSIM simulations support the observation that slowing the scan rate has a similar effect on fragmentation as widening the frequency spacing between the two excitation frequencies. Both approaches generally enhance CID efficiencies and make fragmentation less dependent upon the relative phase angle between the excitation waveform and the ion motion. . In this approach, a supplementary AC voltage is applied to the end-cap electrodes as the ions are held at a fixed trapping potential. If the applied excitation frequency matches the secular frequency of the precursor ion, the on-resonance condition results in an increase of the ion kinetic energy. Energetic collisions with the bath gas then occur and the kinetic energy of the ions is converted to internal energy. This, in turn, leads to cleavage of the chemical bonds and fragmentation takes place. One of the drawbacks of applying this method is the resonance ejection of the ions when, due to the increased kinetic energy, their motion becomes unstable and the ions are ejected without being mass analyzed [2]. Although it is quite straightforward to calculate the approximate secular frequency of an ion under a specific set of experimental conditions, the only way to ensure on-resonance excitation is through lengthy empirical calibration procedures. Consequently, there has been an intense search to find new, improved ways to perform and optimize tandem mass spectrometry in QITs.A widely used approach to improve the performance of the QITs is the application of custom-tailored waveforms to the end-cap electrodes. Goeringer and coworkers tackled the problem of selective ion accumulation by applying filtered noise fields (FNF) [3], while Doroshenko and Cotter described the means of inj...
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