Quadrupole Ion Store Research Articles

Infrared multiple photon dissociation in the quadrupole ion trap via a multipass optical arrangement

The design of a novel multipass optical arrangement for use with infrared multiple photon dissociation (IRMPD) in the quadrupole ion trap is presented. This design circumvents previous problems of limited IR laser power, small IR absorption cross sections for many molecules, and the limited ion statistics of trapping and detection of ions for IRMPD in the quadrupole ion trap. In contrast to previous designs that utilized the quadrupole ion store, the quadrupole ion trap was operated in the mass selective instability mode with concurrent resonance ejection. The instrumental design consisted of a modified ring electrode with three spherical concave mirrors mounted on the inner surface of the ring. This modified design allowed for eight laser passes across the radial plane of the ring electrode. IRMPD of protonated bis(2-methoxyethyl)ether (diglyme) was used to characterize the performance of the multipass ring electrode. Two consecutive reactions for the IRMPD of protonated diglyme were observed with a lower energy channel predominant at less than 0.6 J (irradiation times from 1 to 30 ms) and a second channel predominant at energies greater than 0.6 J (irradiation times > 30 ms). Other studies presented include a discussion of the dissociation kinetics of protonated diglyme, the use of a pulsed valve for increased trapping efficiency of parent ion populations, and the effects of laser wavelength and of ion residence time in the radial plane of the ring electrode on photodissociation efficiency.

Quadrupole store (quistor) mass spectrometry

A new quadrupole ion store mass spectrometer has been constructed and tested. The system uses a new scanning method which incorporates both mass selective storage and mass selective ejection. The features and advantages of the system are presented and discussed.

Experimentally determined proton affinities of 4-methyl-3-penten-2-one, 2-propyl ethanoate, and 4-hydroxy-4-methyl-2-pentanone in the gas phase

Proton affinities have been determined for 4-methyl-3-penten-2-one, 2-propyl ethanoate, and 4-hydroxy-4-methyl-2-pentanone in the gas phase at 333 K. A quadrupole ion store (QUISTOR) was employed to study mass spectrometrically the equilibrium between a species of known proton affinity and one of the above compounds; equilibrium between protonated species was monitored over an ion storage duration of 100 ms. The values of the proton affinities were found to be 870.5 ± 0.8 kJ mol−1 for 4-methyl-3-penten-2-one (mesityl oxide); 842.7 ± 0.6 kJ mol−1 for 2-propyl ethanoate; and 831.6 ± 0.8 kJ mol−1 for 4-hydroxy-4-methyl-2-pentanone (diacetone alcohol).

Studies of infrared multiphoton dissociation rates of protonated aliphatic alcohol dimers

The rates of photodissociation induced by absorption of infrared radiation of protonated dimers of 2-propanol and its deuterated analogues are reported. Protonated alcohol dimer ions created in a quadrupole ion store (QUISTOR) were irradiated in isolation and as a function of parent gas pressure and added collision gas (N2). The relative weights and nature of ionic photoproducts were determined by mass spectrometry. The effect upon photolysis of collisions both prior to and during irradiation were examined. Photodissociation rates were defined by σD, which is the measured photodissociation rate constant normalized to the laser intensity. Nascent dimer ions which can possess up to 130 kJ mol−1 internal energy dissociated readily with σD of 15.8 × 10−2 cm2 extrapolated to collision-free conditions. Photodissociation products from nascent dimer ions were produced via two major reaction channels. As the number of collisions suffered by the nascent dimer ion prior to irradiation was increased, the value of σD diminished and a single reaction channel became more prevalent. After some 270 collisions the ions appear to be collisionally relaxed prior to photolysis. The value of σD of 13.1 × 10−21 cm2 extrapolated to collision-free conditions was obtained. The products from photodissociation of relaxed ions were obtained from a single reaction channel. An increase in σD values obtained with added nitrogen collision gas indicated a blockage or "bottleneck" in multiphoton absorption which is attributed to rotational population depletion. Three deuterated analogues of 2-propanol showed similar behaviour to that of 2-propanol with the exception that deuteron-bound dimers formed from 2-propanol (OD) and perdeutero-2-propanol failed to show rotational population depletion. The apparent lack of a "bottleneck" in multiphoton absorption by deuteron bound dimers is attributed to the diminished spacings between rotational levels of these species compared with the rotational level spacings for proton-bound dimers. Proton-bound ethanol dimers were selected for the examination of temperature effects upon photodissociation rates. While values for σD, extrapolated to collision-free conditions, of 15.6 × 10−21 cm2 at 321 K (in agreement with the 2-propanol results obtained solely at this temperature) and 8.55 × 10−21 cm2 at 293 K were obtained, the precise interpretation of such a marked temperature effect is not immediately obvious.

Optimization of ion trapping characteristics for studies of ion photodissociation

In order to characterize unambiguously photochemical energy absorption and subsequent intramolecular relaxation dissociation, collision-free conditions are required. Isolated molecule photochemistry may be studied in a quadrupole ion store (QUISTOR) under near collision-free conditions. Photodissociation of primary ions, H2+ and CH4+, has been described and we have reported on the slow multiphoton induced dissociation of protonated dimers of 2-propanol in the infrared region. Ions may be trapped in a QUISTOR in either a total storage mode wherein ions of different masses are stored simultaneously, or a mass selective mode wherein single ion species are isolated. Utilization of the QUISTOR as an ion/molecule reactor in the total ion storage mode facilitates the formation of secondary ions chosen for study, their isolation, and irradiation. Determination of the ranges of ion masses which may be stored in each mode is described. In the total storage mode, ion mass-to-charge ratios to m/e 455 have been observed. Selective storage to m/e 219 is reported, though with poor resolution. The effects of variation in trapping parameters including drive frequency and physical dimensions of the device are discussed.

Multiphoton dissociation of gaseous ions in a quadrupole ion store (QUISTOR)

Multiphoton dissociation of gas-phase ions derived from iso -propanol and deuterated analogues is reported. Low intensity CW CO 2 laser radiation at 944 cm −1, chopped and directed radially through the ring electrode of a QUISTOR, effected extensive dissociation of protonated dimers of iso -propanol at a pressure of 5.3mPa. The relative weights of three photodissociation pathways were determined.

Multiphoton dissociation of ions derived from 2-propanol in a QUISTOR with low-power CW infrared laser radiation

We here report multiphoton dissociation of gas-phase ions derived from 2-propanol, using low-intensity infrared CO 2 laser radiation at 944 cm −1 A quadrupole ion store (QUISTOR) was used to store ions, which were then irradiated for up to 100 ms at a laser intensity of ∼80 W cm −2. The unfocused laser beam was allowed to pass radially through the ring electrode of the QUISTOR. At 4 × 10 −5 torr, the protonated dimer of 2-propanol was photodissociated to the extent of ∼25% in 50 ms, which constitutes evidence strongly supportive of collisional focusing of ions at the center of the QUISTOR. An assumed first-order dissociative rate constant was measured using a pulsed mode wherein the accumulation of protonated dimer was followed by resonance ejection of the precursor ion species, protonated 2-propanol, with concomitant laser irradiation.

The quadrupole ion store (QUISTOR). Part xii. The trapping of ions injected from an external source: A description in terms of phase-space'dynamics

The methods of phase-space dynamics have been employed to predict the trapping efficiency for ions injected into the QUISTOR from an external source. Broad agreement with previously published work has been obtained and, of the three geometric arrangements which have been considered, the most effective appears to be radial injection through the ring electrode.

The quadrupole ion store (QUISTOR). Part XI. The model of ion motion in a pseudo-potential well: An appraisal in terms of phase-space dynamics

An expression for the maximum kinetic energy of motion of ions within a radio-frequency quadrupole electric field has been derived from the Mathieu equation and, at a particular phase-angle, is shown to be identical to the expression for the same quantity derived from the model of ion motion in a pseudo-potential well. A phase-space treatment of the well model has been developed and employed to provide semi-quantitative estimates of the validity of the model, and certain shortcomings in its application to the three-dimensional motion of ions within the QUISTOR are discussed.

The quadrupole ion store (QUISTOR). Part X. Space charge and ion stability. B. On the theoretical distribution and density of stored charge in RF quadrupole fields

The method of phase-space dynamics has been extended to provide equations for the ion density distributions theoretically expected in the cases of one-, two- and three-dimensional radio-frequency quadrupole electric fields. The method, which ignores space charge, has been employed to predict the ion distribution within the mass filter and within the ion trap under specified conditions, and has been found to agree in each case with published experimental results where these are available. For the ion trap, predictions of oscillations in the trapped ion plasma have been obtained, and a model which closely replicates the “total pressure” curve of the QUISTOR has been developed.

Radio-frequency mass selective excitation and resonant ejection of ions in a three-dimensional quadrupole ion trap

The three−dimensional quadrupole ion store (QUISTOR) has been employed as a reactor for the study of ion/molecule reactions using external mass analysis, and as an ion trap for the study of stored ion dynamics using either external mass or in situ resonant absorption. Excitation of the axial frequency component of ion motion is accomplished by application of small (∠1 V) RF potentials to an end−cap of the QUISTOR. The frequencies at which ion resonance is observed differ from those predicted by theory and are dependent upon trapped ion density. Thus, the frequency shift may be related to both the space charge within the QUISTOR and perturbation of the stability diagram. the simultaneous application of in situ resonance absorption and external mass analysis has been utilized in three separate studies. (1) In ionized argon, Ar+ has been ejected by resonance absorption and the rate constant for the subsequent partial charge reaction of Ar2+ with argon has been determined to be 4.8±1.6×10−12 cm3 molecule−1s−1. (2) In ionized 2−propanol at ∠0.1 Pa and 4 ms after the ionizing pulse, we have found the ambient space charge experienced simultaneously by nine ion species to be sensibly constant. (3) The coupling of precursor to product ions in ion/molecule reactions of the 2−propanol system has been investigated: this particular application closely resembles the double resonance technique in ion cyclotron resonance mass spectrometry.

The quadrupole ion store (quistor) part IX. space-charge and ion stability. a theoretical background and experimental results

Stability diagrams for a series of ionic species trapped within the QUISTOR are obtained under various experimental conditions. The extent to which the boundaries of the diagrams are displaced from their theoretical positions is discussed in terms of existing models for the influence of space charge upon ion stability. Whilst space-charge within the trapped ion cloud does appear to exert an effect there is evidence of some, as yet unexplained, additional factor causing destabilisation.

The quadrupole ion store (QUISTOR) part VIII. The theoretical estimation of ion kinetic energies: A comparative survey of the field

Theoretical models for estimating the velocities and kinetic energies of ions trapped in the quadrupole ion store (QUISTOR) are considered. Numerical computations based on each model are tabulated and compared; those resulting from phase-space dynamics are plotted and tabulated and formulae for kinetic energies fitted to a simple law.

The application of resonant ion ejection to quadrupole ion storage mass spectrometry: a study of ion/molecule reactions in the QUISTOR

Resonant ejection of ion species from the quadrupole ion store (QUISTOR) is shown to reveal the coupling of reactant ions with product ions in ion/molecule reactions studied with the QUISTOR – quadrupole mass filter combination. The technique involves the use of selective ejection of a chosen ionic precursor and the simultaneous observation of the perturbations in the concentration of product ions. The technique is demonstrated in studies of the ion chemistry and reaction kinetics of two known chemical systems.

Gas phase ion–molecule reactions of dimethylsulphoxide

The gas phase ion-chemistry of dimethylsulphoxide (DMSO) and deuterated dimethylsulphoxide (DMSO-d6) has been examined using a quadrupole ion store (QUISTOR) as an ion–molecule reaction chamber. The QUISTOR results are compared with those obtained by ion trapping and high pressure mass spectrometry as reported by other workers. The performance of the QUISTOR demonstrates the versatility of the technique for ion–molecule reaction studies with variation of ambient pressure and duration of ion storage.

Ketene: Ion chemistry and proton affinity

The gas phase ion-chemistry of ketene has been examined using a Quadrupole Ion Store (QUISTOR) as an ion-molecule reaction chamber. The ion chemistry of ketene is remarkably similar to that of the primary aliphatic alcohols and mercaptans and is characterized by associative and eliminative reactions of ions with a labile proton attached to the electronegative atom. The rate coefficients for the disappearance of CH 2 −+ and CO −+ in ketene were found to be 3.8 ± 1 × 10 −10 and 3.2 ± 0.5 × 10 −11 cm 3 molecule −1 s −1, respectively. The proton affinity of ketene as determined by the bracketing technique was estimated to lie between the proton affinities of dimethyl ether and 2-propanol at a value of 807 ± 8 kJ mol −1.

A new mode of operation for the three-dimensional quadrupole ion store (QUISTOR): The selective ion reactor

Experimental verification is presented of a new mode of operation proposed recently for the QUISTOR-quadrupole combination. The new mode entails the application of the d.c. component of φ 0 to the end-cap electrodes and the r.f. component to the ring electrode. Spatial separation of the voltages combined with a pulsed d.c. potential permits operation of the ion storage source in a mass selective mode with relaxation to a total ion storage or total pressure mode. When operated in this mode the QUISTOR may act as a specific ion reactor.

Effect of charge exchange reactions on the motion of ions in three-dimensional quadrupole electric fields

In order to explain some apparently anomalous charge exchange results in argon, observed using a three-dimensional quadrupole ion store (QUISTOR) as a reaction chamber, the motion of ions in such a device has been investigated using Monte-Carlo methods, with the ions allowed to undergo charge exchange collisions. The results suggest that the ions migrate towards the centre of the device with a corresponding increase in extraction efficiency and a decrease in the ion kinetic energy.

Quadrupole ion store (quistor). Part 1.—Ion–molecule reactions in methane, water and ammonia

The rate constants for ion–molecule reactions occurring in carbon dioxide + hydrogen mixtures, methane, water and ammonia have been determined employing the novel technique of trapping ions within a quadrupole ion store (Quistor) attached to a quadrupole mass filter. The results are consistent with literature values and with tentative calculations, both indicating a mean primary ion kinetic energy in the range 1 to 3 eV.

The quadrupole ion store (quistor) as a novel source for a mass spectrometer

The quadrupole ion store (quistor) as a novel source for a mass spectrometer