Characterization of a high-pressure quadrupole collision cell for low-energy collision-indneed dissociation

A RF-only quadrupole collision cell of new design has been evaluated for use in tandem mass spectrometry experiments as a component of a triple quadrupole mass spectrometer. The new design permits operation at values of collision gas thickness higher by 1 order of magnitude than those used in most cells of this type. When operated at sufficiently high collision gas pressures, the transmission efficiency for precursor ions increases with increasing pressure, often to values greater than those observed in the absence of collision gas. Simultaneously, the attainable resolving power for fragment ions across the entire mass-to-charge ratio range, even for multiply charged precursors, also increases to the point where isomers of a quadruply charged fragment are resolved. The performance of the cell, judged in terms of yields and resolution of fragment ions, has been investigated as a function of the nature and pressure of collision gas, the kinetic energy of the precursor ions that enter the cell, and of the size and charge state of the precursors. The enhanced performance is explicable in terms of a marked deceleration of all ions that emerge from the cell to very low energies, probably a few tens of millielectronvolts, so that the cell effectively acts as an ion source for the second mass filter (fragment ion analyzer) to provide a spectrum of ions of fixed axial energy. The high transmission efficiency appears to arise from a collisional focusing effect analogous to that exploited in three-dimensional RF ion traps. The low axial energies imply that ion transit times through the cell are sufficiently long (several milliseconds) that, in precursor ion experiments where the first mass filter is scanned, a hysteresis effect is observed. This implies that in this operating mode compromises must be sought between scan speed and quality of peak shape. Examples are given of spectra obtained under realistic operating conditions that employ flow injection of samples.     

Collisional focusing effects in radio frequency quadrupoles

The transmission of ions through a conventional two-dimensional radiofrequency-only (rf) quadrupole has been studied for comparatively high operating pressures between 5 × 10^?4 and 1 × 10^?2 torr. Measurements of signals from mass-resolved analyte ions and total ion currents show that, provided the initial injection ion energy is low (1–30 eV), the ion transmission observed through a small aperture at the exit of the rf quadrupole first increases as the gas pressure increases, reaching a maximum at ? 8 × 10^?3 torr before decreasing at higher pressures. This is in direct contrast to the expectations of classical scattering. This “collisional focusing” appears to be analogous to effects seen in three-dimensional ion traps. The collisional focusing increases with the mass of the ion (not mass-to-charge ratio) for masses up to at least 16,950 u. The collisional focusing of the ions is found to be accompanied by significant losses of axial kinetic energy. A Monte Carlo simulation of the energy loss process is reported that can provide agreement with the observed losses for reasonable collision cross-sections. The results suggest that operation of rf quadrupoles at relatively high pressure may find practical application in sampling ions from high (e.g., atmospheric) pressure ion sources.     

The high-pressure segmented quadrupole collision cell (HP-SQCC) as an ion molecule reactor

A high-pressure 20-segment quadrupole collision cell (HP-SQCC), which replaces a collision cell in a modified triple-quadrupole mass spectrometer is investigated in this work as an ion-molecule reactor with an inherent heat source. The highest working pressure achievable is 20 mTorr. The 20 quadrupole segments permit superimposition of linear axial electric field over the conventional quadrupole field in the radial direction. The axial and radial fields are employed to control ion temperature. Heat is transferred to the reactants through ion frictional heating. The HP-SQCC utilizes a combination of several physicochemical phenomena and an attempt is made to examine a range of ion-molecule reactions. Due to a sufficiently large number of reactive collisions, the reactor is used to promote sequential exothermic ion-molecule reactions. To characterize the performance of the HP-SQCC, the various ion-molecule reactions between the fragment ions of ferrocene (Cp(2)Fe), cobaltocene (Cp(2)Co) and nitrogen, oxygen, water and carbon monoxide are investigated.     

Ion trap mass analysis at high pressure: A theoretical view

The mass-selective manipulation of ions at elevated pressure, including mass analysis, ion isolation, or excitation, is of great interest for the development of mass spectrometry instrumentation, particularly for systems in which ion traps are employed as mass analyzers or storage devices. While experimental exploration of high-pressure mass analysis is limited by various difficulties, such as ion detection or electrical discharge at high-pressure, theoretical methods have been developed in this work to study ion/neutral collision effects within quadrupole ion traps and to explore their performance at pressures up to 1 Torr. Ion trapping, isolation, excitation, and resonance ejection were investigated over a wide pressure range. The theoretically calculated data were compared with available experimental data for pressures up to 50 mTorr, allowing the prediction of ion trap performance at pressures more than 10 times higher.     

Miniaturized EI/Q/oa TOF mass spectrometer

A miniaturized orthogonal time-of-flight mass spectrometer with an electron impact ionization ion source and a rf quadrupole ion guide has been developed. A mass resolving power of m/deltam = 5500 has been obtained in a 0.4 m instrument. The addition of helium at pressures of about 4.0 mtorr into the ion source showed collisional focusing taking place in the rf quadrupole. An automated gas chromatograph designed for air monitoring applications has been coupled to the time-of-flight mass analyzer and tested for the detection of simulants of chemical-warfare agents.     

Review paper: Design of a mass and ion kinetic energy spectrometer for organic chemical work

The design of a mass spectrometer for the determination of the structural formulae of organic compounds is discussed. The ion-optical characteristics of electric and magnetic sectors and also of quadrupole mass analysers are considered and the additional information that can be gleaned when such components are combined in various ways is listed. The advantages of using collision cells for inducing fragmentation of selected ion species are listed including those that result when the collision cell is floated at an electrical potential different from that of the incident ion beam. Important performance characteristics are the resolution with which a particular ion may be selected and the resolution with which daughter ions formed from it can be separated. It is concluded, that for instruments comprising three analysing units together with the appropriate collision cells, the most versatile combinations and those with the highest performance are an arrangement consisting of a magnetic sector followed by an electric sector, this being followed either by a further magnetic sector or a quadrupole. The properties of these two systems are compared in detail.     

A segmented radiofrequency-only quadrupole collision cell for measurements of ion collision cross section on a triple quadrupole mass spectrometer

The gas collision cell of a triple quadrupole mass spectrometer has been modified to consist of ten short quadrupole rod segments that allow an axial field to be applied to the cell in order to make measurements of ion mobility. The radiofrequency (rf)-quadrupole field provides effective radial confinement that greatly reduces diffusional losses at low pressure. The mobilities of mass-selected ions from an ionspray source have been measured at a pressure of 8 × 10^?3 torr at electric fields of 0. 1 to 3 V/cm, and used to calculate the collision cross sections of the ions. The measured cross sections compare well with those measured by other techniques.     

New possibilities and prospects for the development of quadrupole transit time mass spectrometers (monopole,tripole, mass filter)

Potentials of a new operating mode of quadrupole transit time mass spectrometers (monopole, tripole, mass filter) are considered. This new mode was named the three-dimensional focusing mode. The results of numerical simulations of the shape of mass peaks of quadrupole transit time mass spectrometers are presented. The prospects for the drastic improvement of the resolution and sensitivity of these devices in the new mode are shown and substantiated.     

Spherical FAIMS: comparison of curved electrode geometries

High-field asymmetric waveform ion mobility spectrometry (FAIMS) can operate at atmospheric pressure to separate gas-phase ions on the basis of a difference in the mobility of an ion at high fields relative to its mobility at low field strengths. Several novel cell geometries have been proposed in addition to the commercially available planar and cylindrical designs. Nevertheless, there is still much to explore about three-dimensional (3-D) curved cell geometries (spherical and hemispherical) and comparison to two-dimensional (2-D) curved geometries (cylindrical). The geometry of a FAIMS cell is one of the essential features affecting the transmission, resolution, and resolving power of FAIMS. Electric fields in a spherical design allow advantages such as virtual potential wells that can induce atmospheric-pressure near-trapping conditions and help reduce ion losses. Curvature of electrodes enables the ions to remain focused near the gap median, which help to improve sensitivity and ion trapping at higher pressures. Here we detail the design and characterization of a novel FAIMS cell having spherical electrode geometry and compare it to hemispherical and cylindrical cells. These FAIMS cells were interfaced with a quadrupole ion trap mass spectrometer in this study. Several structural classes of common explosives were employed to evaluate the separation power of these geometries. FAIMS spectra were generated by scanning the compensation voltage (CV) while operating the mass spectrometer in total ion mode. The identification of ions was accomplished through mass spectra acquired at fixed values of CVs. The performance of FAIMS using cylindrical, hemispherical, and spherical cells was compared and trends identified. For all trials, the best transmission was obtained by the spherical FAIMS cell while hemispherical FAIMS provided the best resolution and resolving power.     

Increased Fragmentation Efficiency of Ions in a Low Pressure Linear Ion Trap with an Added dc Octopole Field

In-trap fragmentation of ions in a hybrid linear ion trap triple quadrupole mass spectrometer occurs at pressures about 5e-5 torr. At these low pressures, efficient fragmentation of heavy ions (such as the singly charged homogenously substituted triazatriphosphorine of mass 2721.89 Da) can take a long time because of the relatively low collision frequency with the background gas and the high internal energy content required to produce fragmentation. Increasing the amplitude used for dipolar excitation leads to loss of the ion upon the quadrupole rods. In the work presented here, the addition of a dc octopolar field to a linear ion trap is described. The dc octopolar field was created by the addition of four auxiliary electrodes situated between the quadrupole rods at a distance of 10 mm from the axis. The inclusion of the dc octopolar field was shown to cause the ions’ frequency of motion to shift out of phase with the excitation signal at high radial amplitudes. This resulted in beat-like trajectories with periods of excitation and de-excitation as the ions’ frequency of motion shifted in and out of phase with the excitation signal. This led to a reduction in the loss of ions on the quadrupole rods during the excitation process. The result is an increased fragmentation efficiency relative to the fragmentation efficiency obtained when using an LIT constructed of round rods only. The inclusion of the dc octopolar field allowed the ion to be fragmented more efficiently in a relatively short excitation period.     

Multipurpose Dissociation Cell for Enhanced ETD of Intact Protein Species

We describe and characterize an improved implementation of ETD on a modified hybrid linear ion trap-Orbitrap instrument. Instead of performing ETD in the mass-analyzing quadrupole linear ion trap (A-QLT), the instrument collision cell was modified to enable ETD. We partitioned the collision cell into a multi-section rf ion storage and transfer device to enable injection and simultaneous separate storage of precursor and reagent ions. Application of a secondary (axial) confinement voltage to the cell end lens electrodes enables charge-sign independent trapping for ion–ion reactions. The approximately 2-fold higher quadrupole field frequency of this cell relative to that of the A-QLT enables higher reagent ion densities and correspondingly faster ETD reactions, and, with the collision cell’s longer axial dimensions, larger populations of precursor ions may be reacted. The higher ion capacity of the collision cell permits the accumulation and reaction of multiple full loads of precursor ions from the A-QLT followed by FT Orbitrap m/z analysis of the ETD product ions. This extends the intra-scan dynamic range by increasing the maximum number of product ions in a single MS/MS event. For analyses of large peptide/small protein precursor cations, this reduces or eliminates the need for spectral averaging to achieve acceptable ETD product ion signal-to-noise levels. Using larger ion populations, we demonstrate improvements in protein sequence coverage and aggregate protein identifications in LC-MS/MS analysis of intact protein species as compared to the standard ETD implementation.

Characterization of the Ion Beam Focusing in a Mass Spectrometer Using an IonCCD™ Detector

A position sensitive pixel-based detector array, referred to as the IonCCD, has been employed to characterize the ion optics and ion beam focusing in a custom built mass spectrometer designed for soft and reactive landing of mass-selected ions onto surfaces. The IonCCD was placed at several stages along the path of the ion beam to determine the focusing capabilities of the various ion optics, which include an electrodynamic ion funnel, two radiofrequency (rf)-only collision quadrupoles, a mass resolving quadrupole, a quadrupole bender, and two einzel lens assemblies. The focusing capabilities of the rf-only collision quadrupoles and einzel lenses are demonstrated by large decreases in the diameter of the ion beam. In contrast, the mass resolving quadrupole is shown to significantly defocus the mass-selected ion beam resulting in an expansion of the measured ion beam diameter. Combined with SIMION simulations, we demonstrate that the IonCCD can identify minor errors in the alignment of charged-particle optics that result in erratic trajectories and significant deflections of the ion beam. This information may be used to facilitate the design, assembly, and maintenance of custom-built mass spectrometry instrumentation.     

Applications of collision dynamics in quadrupole mass spectrometry

Some applications of collision dynamics in the field of quadrupole mass spectrometry are presented. Previous data on the collision induced dissociation of ions in triple quadrupole mass spectrometers is reviewed. A new method to calculate the internal energy distribution of activated ions directly from the increase in the cross section for dissociation with center of mass energy is presented. This method, although approximate, demonstrates explicitly the high efficiency of transfer of translational to internal energy of organic ions. It is argued that at eV center of mass energies, collisions between protein ions and neutrals such as Ar are expected to be highly inelastic. The discovery and application of collisional cooling in radio frequency quadrupoles is reviewed. Some previously unpresented data on fragment ion energies in triple quadrupole tandem mass spectrometry are shown that demonstrate directly the loss of kinetic energy of fragment ions in the cooling process. The development of the energy loss method to measure collision cross sections of protein ions in triple quadrupole instruments is reviewed along with a new discussion of the effects of inelastic collisions in these experiments and related ion mobility experiments.     

Positive-ion mass spectra and collision-induced dissociation of some 2,3-didehydro amino acids

The positive-ion mass spectra of a number of didehydro amino acids, ionized by electron impact and/or thermospray, and collision-induced dissociation spectra taken at collision energies of a few electron volts and keV have been performed on multiple quadrupole and reversed geometry sector instruments. Observed differences in the mass spectra and in the fragmentation patterns are explained in terms of different isomeric structures, different internal excitation energies and different ion transit times between the ion source and the collision cell. Molecular ions of unhydrated amino acids are efficiently formed both by electron impact and thermospray, whilst molecular ions of the hydrated compounds are formed more efficiently by the latter technique. The present investigation demonstrates that the use of different ionization techniques combined with mass spectrometry/mass spectrometry measurements at different collision energies yields a wealth of information relevant to structural characterization of this important class of molecules.     

Application of high-performance liquid chromatography–tandem mass spectrometry with a quadrupole/linear ion trap instrument for the analysis of pesticide residues in olive oil

This article describes the development of an enhanced liquid chromatography-mass spectrometry (LC-MS) method for the analysis of pesticides in olive oil. One hundred pesticides belonging to different classes and that are currently used in agriculture have been included in this method. The LC-MS method was developed using a hybrid quadrupole/linear ion trap (QqQ(LIT)) analyzer. Key features of this technique are the rapid scan acquisition times, high specificity and high sensitivity it enables when the multiple reaction monitoring (MRM) mode or the linear ion-trap operational mode is employed. The application of 5 ms dwell times using a linearly accelerating (LINAC) high-pressure collision cell enabled the analysis of a high number of pesticides, with enough data points acquired for optimal peak definition in MRM operation mode and for satisfactory quantitative determinations to be made. The method quantifies over a linear dynamic range of LOQs (0.03-10 microg kg(-1)) up to 500 microg kg(-1). Matrix effects were evaluated by comparing the slopes of matrix-matched and solvent-based calibration curves. Weak suppression or enhancement of signals was observed (<15% for most-80-of the pesticides). A study to assess the identification criteria based on the MRM ratio was carried out by comparing the variations observed in standard vs matrix (in terms of coefficient of variation, CV%) and within the linear range of concentrations studied. The CV was lower than 15% when the response observed in solvent was compared to that in olive oil. The limit of detection was < or =10 microg kg(-1) for five of the selected pesticides, < or =5 microg kg(-1) for 14, and < or =1 microg kg(-1) for 81 pesticides. For pesticides where additional structural information was necessary for confirmatory purposes-in particular at low concentrations, since the second transition could not be detected-survey scans for enhanced product ion (EPI) and MS3 were developed.     

Nonlinear resonance effects during ion storage in a quadrupole ion trap

Contributions of higher-order fields to the quadrupolar storage field produce nonlinear resonances in the quadrupole ion trap. Storing ions with secular frequencies corresponding to these nonlinear resonances allows adsorption of power from the higher-order fields. This results in increased axial and radial amplitudes which can cause ion ejection and collision-induced dissociation (CID). Experiments employing long storage times and/or high ion populations, such as chemical ionization, ion-molecule reaction studies, and resonance excitation CID, can be particularly susceptible to nonlinear resonance effects. The effects of higher-order fields on stored ions are presented and the influence of instrumental parameters such as radiofrequency and direct current voltage (q_z and a_z values), ion population, and storage time are discussed.     

Pulsed helium introduction into a quadrupole ion trap for reduced collisional quenching during infrared multiphoton dissociation of electrosprayed ions

Previous infrared multiphoton dissociation (IRMPD) experiments utilizing a quadrupole ion trap mass spectrometer yielded limited photodissociation efficiencies. Helium buffer gas continuously infused into the analyzer region at pressures of typically 1 x 10(-3) Torr to improve ion trap performance can collisionally quench photoexcited ions during the IRMPD process. Photodissociation experiments have indicated that uncorrected pressures below 2 x 10(-5) Torr are necessary to avoid collisional deactivation of photoexcited ions. This paper describes IRMPD in the quadrupole ion trap at reduced pressures utilizing a dual-pulsed introduction of helium buffer gas incorporated into the ion trap scan function. The pulsed introduction of helium buffer gas before ion injection allows the efficient trapping of ions injected from an electrospray source and the removal of helium before laser irradiation. A second pulse of helium directly before ion detection improves the intensity of the ion signal. The use of this dual-pulsed inlet of helium for improved IRMPD is demonstrated with the carbohydrate antibiotics neomycin and erythromycin. Copyright 2000 John Wiley & Sons, Ltd.     

Scanwave: A new approach to enhancing spectral data on a tandem quadrupole mass spectrometer

A new type of mass analyzer is described, which allows lowresolution axial ion ejection to be obtained from a traveling wave based, stacked ring collision cell. Linking this ejection temporally with the scanning of the second quadrupole of a tandem quadrupole mass spectrometer provides an improvement in sampling duty cycle, which results in significant signal intensity improvements for scanning acquisitions such as product ion spectra. A near 100% storage efficiency is enabled by a split cell design, which allows ion fragmentation and accumulation to be performed in one section of the collision cell whilst previously accumulated ions are simultaneously ejected from the rear of the cell. These characteristics combine to give an m/z-dependent signal gain of 7–20X over a conventional scanning quadrupole for a 1000 Th scan. The ability to swap very rapidly from a non-enhanced mode of operation to an enhanced mode whilst retaining the existing sensitivity, speed, and functionality of a conventional tandem quadrupole mass spectrometer is also described.     

Dipolar DC induced collisional activation of non‐dissociated electron‐transfer products

The application of electron transfer and dipolar direct current induced collisional activation (ET‐DDC) for enhanced sequence coverage of peptide/protein cations is described. A DDC potential is applied across one pair of opposing rods in the high‐pressure collision cell of a hybrid quadrupole/time‐of‐flight tandem mass spectrometer (QqTOF) to induce collisional activation, in conjunction with electron transfer reactions. As a broadband technique, DDC can be employed for the simultaneous collisional activation of all the first‐generation charge‐reduced precursor ions (eg, electron transfer no‐dissociation or ETnoD products) from electron transfer reactions over a relatively broad mass‐to‐charge range. A systematic study of ET‐DDC induced collision activation on peptide/protein cations revealed an increase in the variety (and abundances) of sequence informative fragment ions, mainly c‐ and z‐type fragment ions, relative to products derived directly via electron transfer dissociation (ETD). Compared with ETD, which has low dissociation efficiency for low‐charge‐state precursor ions, ET‐DDC also showed marked improvement, providing a sequence coverage of 80% to 85% for all the charge states of ubiquitin. Overall, this method provides a simple means for the broadband collisional activation of ETnoD ions in the same collision cell in which they are generated for improved structural characterization of polypeptide and protein cations subjected to ETD.     

The use of static pressures of heavy gases within a quadrupole ion trap

The performance of quadrupole ion traps using argon or air as the buffer gas was evaluated and compared to the standard helium only operation. In all cases a pure buffer gas, not mixtures of gases, was investigated. Experiments were performed on a Bruker Esquire ion trap, a Finnigan LCQ, and a Finnigan ITMS for comparison. The heavier gases were found to have some advantages, particularly in the areas of sensitivity and collision-induced dissociation efficiency; however, there is a significant resolution loss due to dissociation and/or scattering of ions. Additionally, the heavier gases were found to affect ion activation and deactivation during MS/MS, influencing the product ion intensities observed. Finally, the specific quadrupole ion trap design and the ion ejection parameters were found to be crucial in the quality of the spectra obtained in the presence of heavy gases. Operation with static pressures of heavy gases can be beneficial under certain design and operating conditions of the quadrupole ion trap.