"Fast excitation" CID in a quadrupole ion trap mass spectrometer

Collision-induced dissociation (CID) in a quadrupole ion trap mass spectrometer is usually performed by applying a small amplitude excitation voltage at the same secular frequency as the ion of interest. Here we disclose studies examining the use of large amplitude voltage excitations (applied for short periods of time) to cause fragmentation of the ions of interest. This process has been examined using leucine enkephalin as the model compound and the motion of the ions within the ion trap simulated using ITSIM. The resulting fragmentation information obtained is identical with that observed by conventional resonance excitation CID. "Fast excitation" CID deposits (as determined by the intensity ratio of the a(4)/b(4) ion of leucine enkephalin) approximately the same amount of internal energy into an ion as conventional resonance excitation CID where the excitation signal is applied for much longer periods of time. The major difference between the two excitation techniques is the higher rate of excitation (gain in kinetic energy) between successive collisions with helium atoms with "fast excitation" CID as opposed to the conventional resonance excitation CID. With conventional resonance excitation CID ions fragment while the excitation voltage is still being applied whereas for "fast excitation" CID a higher proportion of the ions fragment in the ion cooling time following the excitation pulse. The fragmentation of the (M + 17H)(17+) of horse heart myoglobin is also shown to illustrate the application of "fast excitation" CID to proteins.     

Resonant excitation in a low-pressure linear ion trap

It has been shown that through the process of resonant excitation the fragmentation of ions confined in a low-pressure (<0.05 mTorr) linear ion trap (LIT) can be accomplished while maintaining both high fragmentation efficiency and high resolution of excitation. The ion reserpine, 609.23 Da, has been fragmented with efficiencies greater than 90% while a higher mass ion, a homogeneously substituted triazatriphosphorine of mass 2721.89 Da, has been fragmented with 48% efficiency. This was accomplished by extended resonant excitation by low-amplitude auxiliary RF signals. Computer modelling of ion trajectories and analysis of the trapping potentials have demonstrated that a reduction in neutralization of ions on the rods (or losses on the rods) and increased fragmentation is a consequence of higher order terms in the potential introduced by the round-rod geometry of the LIT.     

Resonant ac dipolar excitation for ion motion control in the Orbitrap mass analyzer

A dipolar ac signal applied to the split outer electrode of an Orbitrap mass spectrometer at the axial resonance frequency causes excitation of ion axial motion and either eventual ion ejection from the trap, if applied in phase with ion motion, or de-excitation, if applied 180 degrees out of phase. Both de-excitation and excitation may be achieved mass-selectively. The extent of ion axial de-excitation depends on the ac amplitude and on the number of cycles applied; sufficient de-excitation can be accomplished such that the ion signal cannot be observed above baseline noise. After de-excitation, the ions remain trapped and in rapid orbital (but not axial) motion, which allows them to be re-excited coherently by application of a second ac waveform allowing the signal again to be observed. Both broad-band and narrow-band waveforms have been used to de-excite and to re-excite ion motion. Using narrow-band waveforms, selective de-excitation and re-excitation can be performed with unit mass selection, leaving an adjacent 13C isotopic peak unaffected. The origin and potential applications of these new capabilities is delineated.     

A library of atmospheric pressure ionization daughter ion mass spectra based on wideband excitation in an ion trap mass spectrometer

A searchable library of MS/MS spectra obtained using a quadrupole ion trap mass spectrometer and electrospray or atmospheric pressure chemical ionization is presented. The application of wideband excitation (activation) and normalized collision energy leads to highly reproducible mass spectra which are searched using the NIST algorithm. Flow injection and LC/MS/MS applications of this powerful technique in the biomedical (diastereoisomeric steroids, morphine glucuronides, isovalerylcarnitine) and environmental (pirimicarb and desmethyl-pirimicarb) areas are described.     

Sodium ion attachment reactions in an ion trap mass spectrometer

The capabilities of ion traps to perform attachment reactions with alkali cations using classical scanning sequences have been exploited here with an ion trap mass spectrometer equipped with an external ion source to generate the reagent Na(+) ions. Kinetic studies have shown that, as expected, the attachment efficiency is very high, near-collision efficiency, and illustrate how the present method is particularly well suited for ion trap mass spectrometers. The large adaptability of the experimental conditions suggests that a wide range of organic molecules, characterized by a large alkali ion affinity, could be readily detected even at low levels. Applications of sodium ion attachment reactions are illustrated by the detection and characterization of explosives and some of their correlated pyrolytic degradation products. Detection -limits for phthalate compounds are shown to reach the low ng range of injected samples, without any noticeable difficulties in the full scan mode of acquiring mass spectra. Copyright 2000 John Wiley & Sons, Ltd.     

Fragmentation of phosphopeptides in an ion trap mass spectrometer

A systematic study of the fragmentation pattern of phosphopeptides in an electrospray (ESI) ion trap mass spectrometer is presented. We show that phosphotyrosine- and phosphothreonine-containing peptides show complicated fragmentation patterns. These phosphopeptides were observed to lose the phosphate moiety in the form of H₃PO₄ and/or HPO₃, but were also detected with no loss of the phosphate group. The tendency to lose the phosphate moiety depends strongly on the charge state. Thus, the highest observed charge state tends to retain the phosphate moiety with extensive fragmentation along the peptide backbone. We also show that phosphoserine-containing peptides have relatively simple fragmentation patterns of losing H₃PO₄. This loss is independent of the charge state. We suggest strategies for the accurate identification of phosphorylation sites using the ion trap mass spectrometer.     

Double resonance ejection in a micro ion trap mass spectrometer

Ion ejection from a cylindrical micro ion trap by resonance excitation of the secular motion is observed to be strongly dependent on the frequency of the secular motion at resonance. Both the intensity of the ion signal and the mass resolution of the resulting mass spectrum are increased when the ion secular frequency is approximately that of a nonlinear resonance of the trap. The resonances are attributed to electrical as well as geometrical considerations.     

Solid phase micro-extraction in a miniature ion trap mass spectrometer

Fiber introduction mass spectrometry (FIMS), a variation of solid-phase microextraction (SPME) and membrane introduction mass spectrometry (MIMS), is employed with a miniature mass spectrometer. The inlet system, constructed of commercially available vacuum parts, allows the direct introduction of the SPME needle vacuum chamber into the mass spectrometer. Thermal desorption of the analyte from the poly(dimethylsiloxane) (PDMS) coated fiber was achieved with a built in nichrome heater, followed by electron ionization of the analytes internal to the cylindrical ion trap (CIT). The system has been tested with several volatile organic compounds (VOC) in air and to analyze the headspace over aqueous solutions, with limits of detection in the low ppb range. The signal rise (10-90%) and fall (90-10%) times for the system ranged from 0.1 to 1 s (rise) and 1.2 to 6 s (fall) using heated desorption. In addition, this method has been applied to quantitation of toluene in benzene, toluene, xylene (BTX) mixtures in water and gasoline. This simple and rapid analysis method, coupled to a portable mass spectrometer, has been shown to provide a robust, simple, rapid, reproducible, accurate and sensitive (low ppb range) fieldable approach to the effective in situ analysis of VOC in various matrices.     

Ion motion in the rectangular wave quadrupole field and digital operation mode of a quadrupole ion trap mass spectrometer

A quadrupolar electric field driven by a rectangular wave voltage can be used for mass-selective storage and analysis. The ion motion in such an electric field is derived, and the stability of ions is presented in the a-q diagram that is commonly used for sinusoidal wave quadrupole mass spectrometry in association with the solution of the Mathieu equation. The pseudo-potential well is discussed in an approximation that leads to the relation of secular frequency to operating parameters. A scheme for a digital ion trap mass spectrometer is described, based on this theory. An ion optics simulation was performed to check the theory of resonant ejection, and to prove the feasibility of the mass scan method for a practical ion trap of such geometry.     

High amplitude short time excitation: A method to form and detect low mass product ions in a quadrupole ion trap mass spectrometer

Collision induced dissociation (CID) in a quadrupole ion trap mass spectrometer using the conventional 30 ms activation time is compared with high amplitude short time excitation (HASTE) CID using 2 ms and 1 ms activation times. As a result of the shorter activation times, dissociation of the parent ions using the HASTE CID technique requires resonance excitation voltages greater than conventional CID. After activation, the rf trapping voltage is lowered to allow product ions below the low mass cut-off to be trapped. The HASTE CID spectra are notably different from those obtained using conventional CID and can include product ions below the low mass cut-off for the parent ions of interest. The MS/MS efficiencies of HASTE CID are not significantly different when compared with the conventional 30 ms CID. Similar results were obtained with a two-dimensional (linear) ion trap and a three-dimensional ion trap.     

Duty cycle enhancement of an orthogonal acceleration TOF mass spectrometer using an axially-resonant excitation linear ion trap

We report a new technique to enhance detection duty cycle of an orthogonal-acceleration time-of-flight mass spectrometer (oaTOF) over a broad mass range. To this end, we used an axially-resonant-excitation linear ion trap, which ejects ions axially and mass selectively into a non-mass-selective linear ion trap in front of the TOF pusher. A delay between the ejection timing of the non-mass-selective LIT and the push timing of the oaTOF was swept mass-synchronously with the axially-resonant-excitation linear ion trap, so that ions are detected with duty cycle larger than 60% over a wide mass range from m/z 174.1 to 1922.0, which is 3 to 10 times better than conventional oaTOF.     

Characteristics of stability boundary and frequency in nonlinear ion trap mass spectrometer

In this article, the Poincare-Lighthill-Kuo (PLK) method is used to derive an analytical expression on the stability boundary and the ion trajectory. A multipole superposition model mainly including octopole component is adopted to represent the inhomogeneities of the field. In this method, both the motional displacement and secular frequency of ions have been expanded to asymptotic series by the scale of nonlinear term ε, which represents a weak octopole field. By solving the zero and first-order approximate equations, it is found that a frequency shift exists between the ideal and nonlinear conditions. The motional frequency of ions in nonlinear ion trap depends on not only Mathieu parameters, a and q, but also the percentage of the nonlinear field and the initial amplitude of ions. In the same trap, ions have the same mass-to-charge ratio (m/z) but they have different initial amplitudes or velocities. Consequently, they will be ejected at different time through after a mass-selective instability scan. The influences on the mass resolution in quadrupole ion trap, which is coupled with positive or negative octopole fields, have been discussed respectively.     

Surface-induced dissociation of molecular ions in a quadrupole ion trap mass spectrometer

A method is reported by which surface-induced dissociation is used to activate ions stored in a quadrupole ion trap mass spectrometer. The method employs a short (< 5 μs), fast-rising (< 20-ns rise time), high voltage direct current (dc) pulse, which is applied to the endcaps of a standard Paul-type quadrupole ion trap. This is in contrast to the application of an alternating current (ac) signal normally used to resonantly excite and dissociate ions in the trap. The effect of the dc pulse is to cause the ions rapidly to become unstable in the radial direction and subsequently to collide with the ring electrode. Sufficient internal energy is acquired in this collision to cause high energy fragmentations of relatively intractable molecular ions such as pyrene and benzene. The dissociations of limonene are used to demonstrate that high energy demand processes increase in relative importance in the dc pulse experiment compared with the usual resonance excitation method used to cause activation. The fragments are scanned out of the ion trap using the conventional mass-selective instability scan mode. Simulations of ion motion in the trap provide evidence that surface collisions occur at kinetic energies in the range of tens to several hundred electronvolts. The experiments also demonstrate that production of fragment ions is sensitive to the phase of the main radiofrequency drive voltage at the point when the dc is initiated.     

Peptide photodissociation at 157 nm in a linear ion trap mass spectrometer

The photodissociation by 157 nm light of singly- and doubly-charged peptide ions containing C- or N-terminal arginine residues was studied in a linear ion trap mass spectrometer. Singly-charged peptides yielded primarily x- and a-type ions, depending on the location of the arginine residue, along with some related side-chain fragments. These results are consistent with our previous work using a tandem time-of-flight (TOF) instrument with a vacuum matrix-assisted laser desorption/ionization (MALDI) source. Thus, the different internal energies of precursor ions in the two experiments seem to have little effect on their photofragmentation. For doubly-charged peptides, the dominant fragments observed in both photodissociation and collisionally induced dissociation (CID) experiments are b- and y-type ions. Preliminary experiments demonstrating fragmentation of multiply-charged ubiquitin ions by 157 nm photodissociation are also presented.     

Analytical approach for description of ion motion in quadrupole mass spectrometer

Implementation of the analytical method of the solution of the Mathieu equation in conjunction with the algebraic presentation of Mathieu functions is discussed in this work. This approach is used for the analytical expression of fundamental properties of the quadrupole field such as ion trajectory stability and transmission. Extensive comparison with the matrix method is presented with demonstration of the fundamental advantages of the analytical method. However, contrary to the matrix method, the analytical method is limited to the cos trapping waveforms.     

Co-occurrence of boundary and resonance ejection in a multiplexed rectilinear ion trap mass spectrometer

A method is reported for evaluating ion trap mass analyzers by selection of operating conditions under which both boundary and resonance ejection peaks occur in a single mass scan. The choice of frequency and amplitude of the auxiliary waveform applied for resonance ejection can be such as to produce a resonance ejection mass spectrum with unit resolution or, under selected conditions, signals attributable to both boundary and resonance ejection in a single mass scan. The contrasting mass resolution associated with these two ejection processes is evident in these data. The co-occurrence of the two ejection phenomena is ascribed to the effects of higher-order fields; it is more marked in some rectilinear ion traps (RITs) than in other nominally identical devices, leading to the possibility of using it to compare individual mass analyzers in multiplexed instruments. The method is used to compare multiple ion traps driven by the same RF signal in a fully-multiplexed mass spectrometer, composed of parallel ion source/mass analyzer/detector channels each housed in one quadrant of a specialized vacuum chamber.