Douglas E. Goeringer

SPECIAL FEATURE:TUTORIAL Slow Heating Methods in Tandem Mass Spectrometry

Several approaches to ion activation in tandem mass spectrometry have been developed in recent years for use in ion trapping instruments that allow for conditions to be reached wherein rates of ion activation and deactivation are comparable. These approaches are defined as slow heating methods and include continuous-wave laser infrared multiphoton dissociation, dissociation driven by blackbody radiation, quadrupole ion trap collisional activation and sustained off-resonance irradiation in ion cyclotron resonance mass spectrometry. In the limiting case in which ion activation and deactivation rates are equal, a steady-state parent ion internal energy distribution is achieved and the kinetics of dissociation can be interpreted in analogy with thermal dissociation. This discussion describes the thermal analogy and the limiting conditions of rapid energy exchange and slow energy exchange along with the possible ramifications for dissociation rates and product ion spectra. The figures of merit that the various slow heating methods share as a class of activation methods are also discussed. The purpose of this perspective is to provide a frame-of-reference from which slow heating methods can be considered. Such methods are seeing increasing use as the number of ion trapping instruments grows and have shown remarkable success with dissociation of high-mass ions.

Selective ion isolation/rejection over a broad mass range in the quadrupole ion trap

Techniques are presented for mass-selective ion manipulation over a wide mass range in a three-dimensional quadrupole. The methods use an auxiliary, low-amplitude radio-frequency signal applied to the endcap electrodes. This signal is either held at a single frequency as the fundamental radio-frequency trapping amplitude is ramped or swept over a frequency range while the fundamental radio-frequency trapping amplitude is held at a fixed level. Ion isolation and ejection are demonstrated for ions formed within the ion trap using electron ionization and for ions injected into the ion trap formed either by an air-sustained glow discharge or by electrospray. Mass-selective ion ejection is used to reduce matrix-ion-induced space charge during ion injection, thereby producing signal enhancement for the detection of 2, 4, 6-trinitrotoluene in air. Mass-selective isolation of ions with mass-to-charge ratios above the normal operating range (m / z 650) for the ion trap is also demonstrated after injection of myoglobin ions formed via electrospray.

Evolution of ion internal energy during collisional excitation in the Paul ion trap: A stochastic approach

A first‐order model is developed for collisional activation as effected via resonance excitation and helium buffer gas in the Paul ion trap. For an ion population at steady‐state under specified experimental conditions, the kinetic theory of ion transport in gases is first used to calculate an effective temperature shown to be identical to the internal temperature for molecular ions in an atomic gas. The evolution of the ion internal energy is then followed by a random walk simulation designed to be representative of the actual collisional energy transfer process, except ion losses due to dissociation and reactive processes during collisional activation are excluded. During the simulation, inelastic ion‐neutral collisions increase the average ion internal energy via small energy changes (both positive and negative) until a steady‐state condition is reached in which excitation and deexcitation processes are balanced. Histogramming the simulated data reveals a Boltzmann‐type internal energy distribution who...

High explosives vapor detection by glow discharge ion trap mass spectrometry

The combination of atmospheric sampling glow discharge ionization with quadrupole ion trap mass spectrometry for the detection of traces of high explosives is described. Atmospheric sampling glow discharge provides a simple, rugged, and efficient means for anion formation while the quadrupole ion trap provides for efficient tandem mass spectrometry. Mass-selective ion accumulation and non-specific ion activation methods can be used to overcome deleterious effects arising from ion/ion interactions. Such interactions constitute the major potential technical barrier to the use of the ion trap for real-time monitoring of targeted compounds in uncontrolled and highly variable matrices. Tailored waveforms can be used to effect both mass-selective ion accumulation and ion activation. Concatenated tailored waveforms allow for both functions in a single experiment, thereby providing the capability for monitoring several targeted species simultaneously.

Laser desorption mass spectrometry and MS/MS with a three-dimensional quadrupole ion trap

Abstract An ion trap mass spectrometer (ITMS) has been modified to enable laser desorption mass spectrometry to be performed. The effects of the ion trap r.f. amplitude and helium bath gas pressure during desorption, along with the time delay between the laser pulse and the acquisition of the mass spectrum have been investigated. The ability to perform multiple stages of mass spectrometry (MS/MS) on laser-desorbed ions is also demonstrated.

Ion-ion reactions in the gas phase: Proton transfer reactions of protonated pyridine with multiply charged oligonucleotide anions

Isolated triply and doubly charged anions of the single-stranded deoxynucleotide 5′-d(AAAA)-3′ were allowed to undergo ion-ion proton transfer reactions with protonated pyridine cations within a quadrupole ion trap mass spectrometer. Sufficiently high ion number densities and spatial overlap of the oppositely charged ion clouds could be achieved to yield readily measurable rates. Three general observations were made: (1) the ion-ion reaction rate constants were estimated to be 10− (7 − 8) cm3 ion−1 s−1; (2) the ion-ion reaction rates were found to be dependent on the reactant ion number density, which could be controlled by both the reactant ion number and the pseudopotential well depth, and (3) very little fragmentation, if any, was observed, as might normally be expected with highly exothermic proton transfer reactions.

Thermal dissociation in the quadrupole ion trap: ions derived from leucine enkephalin

Abstract Rates of dissociation of protonated leucine enkephalin and the b 4 + fragment ion derived from protonated leucine enkephalin have been measured as a function of helium bath gas temperature in a quadrupole ion trap. Dissociation rates were observed to be insensitive to the amplitude of the trapping voltage over the range of values studied. This observation, along with theoretical arguments based on predicted levels of “rf heating,” indicates that any internal excitation of the ions due to ion trap storage is minimal. The bath gas temperature can therefore be used to characterize the internal temperatures of the ions. This approximation is expected to be most valid for high mass ions and low mass bath gases, such as helium. Activation parameters were obtained from Arrhenius plots of the rate data, and master equation modeling of the activation, deactivation, and dissociation processes was performed to provide an indication as to how closely these ions approached high-pressure limit behavior. Protonated leucine enkephalin more closely approached the high-pressure limit than the b 4 + ion due to its larger size and the fact that the activation parameters were derived from somewhat lower dissociation rates. These studies suggest that the quadrupole ion trap operated in the presence of a light, heated bath gas can be used to obtain Arrhenius activation parameters from the dissociation kinetics of relatively high mass ions.

Relaxation of internally excited high-mass ions simulated under typical quadrupole ion trap storage conditions

Collisional relaxation of internally excited high-mass (>1 kDa) ions has been simulated under typical quadrupole ion trap storage conditions. Two models have been employed that are expected to bracket the range of cooling rates that prevail for such ions present in ∼1 mTorr of room temperature helium. A diffuse scattering model that assumes that the helium target atom thermally equilibrates with the high-mass ion upon collision is expected to yield a maximum cooling rate. A random walk algorithm using the exponential model for inefficient colliders and a relatively small average energy down-step size provides an estimate for the lowest cooling rates that might be expected. The two models give cooling rates that differ by about a factor of three and fall within the range of 200–2000 s−1 for the ions and energies considered in the simulations. Unimolecular dissociation rates have also been determined for the same model ions. Random walk simulations employing collisional cooling and dissociation clearly show how a rapid input of internal energy, as with the absorption of an ultraviolet photon, can either result in dissociation of a large fraction of the ions or can lead to an insignificant degree of dissociation, depending largely on the unimolecular dissociation rate of the ion.

Ion internal temperature and ion trap collisional activation: protonated leucine enkephalin

Abstract Protonated leucine enkephalin has been used as a prototypical high-mass ion to yield a quantitative estimate of the relationship between the amplitude of the resonance excitation voltage used in an ion trap collisional activation experiment, and the internal temperature to which an ion can be elevated over the bath gas temperature. The approach involves the measurement of the ion dissociation rate as a function of resonance excitation voltage, and the correlation of dissociation rate with ion internal temperature. The relatively high ion trap dissociation rates observed under typical resonance excitation conditions preclude the direct application of the Arrhenius equation to derive internal temperatures. An empirical determination of the relationship between ion internal temperature and dissociation rate over the rate range of interest here was made via the systematic variation of bath gas temperature. The data suggest a very nearly linear relationship between ion internal temperature and resonance excitation voltage, at least under conditions in which ion ejection is minimal. It is shown that protonated leucine enkephalin ions can be elevated by about 357 K over the bath gas temperature using a monopolar resonance excitation voltage of 540 mV p − p(qz = 0.163) without significant ion ejection. It is also demonstrated that ion internal temperature can be readily increased by increasing the bath gas temperature, by accelerating the ions in the presence of a room temperature bath gas (i.e. conventional ion trap collisional activation), or by a combination of the two approaches.

Kinetics of Collision-induced Dissociation in the Paul Trap: a First-order Model

A first-order model describing the kinetics of collision-induced dissociation (CID) of polyatomic ions as effected via single-frequency resonance excitation in the Paul ion trap is presented. A mathematical expression for the dissociation rate constant associated with the model is developed from the kinetic theory of ion transport in gases, the forced, damped harmonic oscillator model for ion-trap resonance excitation, and thermal kinetic theory. Ion-trap CID is also simulated using a random walk sequence, corresponding to the processes believed to occur during collisional activation, which allows ion internal energy changes due to inelastic ion/helium collisions to be followed as a function of time; dissociation kinetics are included in the simulation by terminating the random walk when the ion internal energy exceeds the minimum required for fragmentation. Validity of the mathematical analysis and random walk simulation for the model is confirmed by comparison with experimental phenomenology associated with CID kinetics in the Paul trap. The first-order model in its present form, along with refinements resulting from further detailed comparison with experimental data, should therefore be a useful tool in expanding our understanding of polyatomic ion energetics in the Paul trap.