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

Abstract

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 collision-induced dissociation (DCID) in a quadrupole ion trap using a two-frequency excitation waveform: II. Effects of frequency spacing and scan rate

“…When the two frequencies are applied further apart, the term will be larger, indicating a faster beat pattern in the time domain of the interference pattern of the excitation waveform. It was previously shown that the fragmentation of the selected precursor ions depends on the time at which they come into resonance with the interference pattern of the excitation waveform, and on the excitation amplitude [14]. When the excitation waveform goes through a minimum, or a node, at the time of excitation, reduced kinetic energy deposition is expected and the fragmentation is less efficient.…”

“…The variables of the experiment were previously described [14]. The simulations were conducted for frequency spacings between 171 and 176 kHz in increments of 0.05 kHz and at phase angles from 0°to 360°by increments of 20°for each frequency spacing.…”

“…Each plot shows the results of more than 18,000 different experimental conditions in a sequence-looped format [14]. The ability to perform experiments in such vast number is offered by the flexible programming permitted by the Visual Basic platform.…”

“…We have presented the effect of several experimental parameters, such as excitation amplitude and frequency [14] and frequency spacing and scan rate (present report) on the fragmentation energetics and efficiencies when a one- [12,13] or two-frequency excitation waveform is used to fragment selected precursor ions during mass acquisition. It is evident that several other parameters influence the energy deposition process (bath gas pressure, space charge effects) and these parameters will require future investigation.…”

“…Single-frequency DCID has been successfully applied for the fragmentation of biomolecules such as leucine enkephalin, insulin chain A and substance P [12]. Previous studies have focused on characterizing the fragmentation energetics and efficiencies when a single frequency is applied to the end-cap electrodes during mass acquisition [13], and understanding the waveform-ion interaction for two-frequency DCID [14]. The effect of excitation amplitude and q z q q has been described in a companion paper to this [14]; the effect of frequency spacing and mass scanning rate are presented here.…”