Collisional cooling rates of infrared excited ions are measured in a quadrupole ion trap (QIT) mass spectrometer at different combinations of temperature and pressure. Measurements are carried out by monitoring fragmentation efficiency of leucine enkephalin as a function of irradiation time by an infrared laser after a short excitation and incrementally increasing cooling periods. Cooling rates are observed to be directly related to bath gas pressure and inversely related to bath gas temperature. The cooling rate at typical ion trap operating pressure (1 mTorr) and temperature (room T) is faster than can be measured. At elevated temperature and the lowest pressure used for the studies, the rate of collisional cooling becomes negligible compared to the rate of radiative cooling. 相似文献
Ion isolation in a linear ion trap is demonstrated using dual resonance frequencies, which are applied simultaneously. One frequency is used to eject ions of a broad m/z range higher in m/z than the target ion, and the second frequency is set to eject a range of ions lower in m/z. The combination of the two thus results in ion isolation. Despite the simplicity of the method, even ions of low intensity may be isolated since signal attenuation is less than an order of magnitude in most cases. The performance of dual frequency isolation is demonstrated by isolating individual isotopes of brominated compounds.
Infrared multiphoton photodissociation (IRMPD) in a quadrupole ion trap is not selective for a parent ion. Product ions are
decreased in abundance by continuous sequential dissociation and may be lost below the low mass cut-off. The IRMPD process
is made selective by resonantly exciting trapped ions into an axially offset laser path. Product ions form and collisionally
relax out of the laser path to accumulate in the center of the trap. The technique, termed selective broadband (SB) IRMPD,
limits sequential dissociation to preserve first generation product ion abundance. The abundances of larger product ions are
maximized by completely dissociating the parent ion, but continuous sequential dissociation does not form small product ions
below the low mass cut-off associated with conventional IRMPD. Smaller product ions are further increased in abundance in
another tandem mass spectrum by performing sequential stages of SB-IRMPD, adjusting the trapping rf amplitude to dissociate
larger product ions at the same qz range. Thermal assistance is used to perform SB-IRMPD at higher bath gas pressures for increased sensitivity. 相似文献
The fragmentation patterns obtained by ultraviolet photodissociation (UVPD) and collision-induced dissociation (CID) in a
quadrupole ion trap mass spectrometer were compared for peptides modified at their C-termini and at acidic amino acids. Attachment
of Alexa Fluor 350 or 7-amino-4-methyl-coumarin chromophores at the C-terminal and acidic residues enhances the UV absorptivity
of the peptides and all fragment ions that retain the chromophore, such as the y ions that contain the chromophore-modified
C-terminus. Whereas CID results in the formation of the typical array of mainly y-type and a/b-type fragment ions, UVPD produces
predominantly a/b-type ions with greatly reduced abundances of y ions. Immonium ions, mostly ones from aromatic or basic amino
acids, are also observed in the low m/z range upon UVPD. UVPD of peptides containing two chromophore moieties (with one at the C-terminus and another at an acidic
residue) results in even more efficient photodissociation at the expense of the annihilation of almost all diagnostic b and
y ions containing the chromophore. 相似文献
A new, variable-temperature mass spectrometer system is described. By applying polyimide heating tape to the end-cap electrodes of a Bruker (Bremen, Germany) Esquire ion trap, it is possible to vary the effective temperature of the system between 40 and 100°C. The modification does not impact the operation of the ion trap and the heater can be used for extended periods without degradation of the system. The accuracy of the ion trap temperatures was assessed by examining two gas-phase equilibrium processes with known thermochemistry. In each case, the variable-temperature ion trap provided data that were in good accord with literature data, indicating the effective temperature in the ion trap environment was being successfully modulated by the changes in the set-point temperatures on the end-cap electrodes. The new design offers a convenient and effective way to convert commercial ion trap mass spectrometers into variable-temperature instruments.
Laser-induced fluorescence is used to visualize populations of gaseous ions stored in a quadrupole ion trap (QIT) mass spectrometer. Presented images include the first fluorescence image of molecular ions collected under conditions typically used in mass spectrometry experiments. Under these “normal” mass spectrometry conditions, the radial (r) and axial (z) full-width at half maxima (FWHM) of the detected ion cloud are 615 and 214 μm, respectively, corresponding to ~6 % of r0 and ~3 % of z0 for the QIT used. The effects on the shape and size of the ion cloud caused by varying the pressure of helium bath gas, the number of trapped ions, and the Mathieu parameter qz are visualized and discussed. When a “tickle voltage” is applied to the exit end-cap electrode, as is done in collisionally activated dissociation, a significant elongation in the axial, but not the radial, dimension of the ion cloud is apparent. Finally, using spectroscopically distinguishable fluorophores of two different m/z values, images are presented that illustrate stratification of the ion cloud; ions of lower m/z (higher qz) are located in the center of the trapping region, effectively excluding higher m/z (lower qz) ions, which form a surrounding layer. Fluorescence images such as those presented here provide a useful reference for better understanding the collective behavior of ions in radio frequency (rf) trapping devices and how phenomena such as collisions and space-charge affect ion distribution.
The effects on ion motion caused by the application of a resonance AC dipole voltage to the end-cap electrodes of the quadrupole ion trap are described. An excimer laser is used to photodissociate benzoyl ions, and its triggering is phase locked to the AC voltage to follow the motion of the ion cloud as a function of the phase angle of the AC signal. Resonantly excited ions maintain a coherent motion in the presence of He buffer gas, which dissipates energy from the ions via collisions. Maximum ion displacements, which depend upon the potential well depth (qz value), occur twice each AC cycle. Axial components of ion velocities are determined by differentiating the displacements of the distributions with respect to time. The experimental data show that these velocities are maximized when the ion cloud passes through zero axial displacement, and they compare favorably with results calculated using a simple harmonic oscillator model. Axial components of ion kinetic energies are low (<5 eV) under the chosen experimental conditions. At low values of q2 (≈ 0.2), the width of the ion distribution increases as the ion cloud approaches the center of the trap and decreases as it approaches the end-cap electrodes. This effect is created by compaction of the ion trajectories when ion velocities are decreased, 相似文献
A method of fragmenting ions over a wide range of m/z values while balancing energy deposition into the precursor ion and available product ion mass range is demonstrated. In the method, which we refer to as “multigenerational collision-induced dissociation”, the radiofrequency (rf) amplitude is first increased to bring the lowest m/z of the precursor ion of interest to just below the boundary of the Mathieu stability diagram (q = 0.908). A supplementary AC signal at a fixed Mathieu q in the range 0.2–0.35 (chosen to balance precursor ion potential well depth with available product ion mass range) is then used for ion excitation as the rf amplitude is scanned downward, thus fragmenting the precursor ion population from high to low m/z. The method is shown to generate high intensities of product ions compared with other broadband CID methods while retaining low mass ions during the fragmentation step, resulting in extensive fragment ion coverage for various components of complex mixtures. Because ions are fragmented from high to low m/z, space charge effects are minimized and multiple discrete generations of product ions are produced, thereby giving rise to “multigenerational” product ion mass spectra.
Examination of the collisional cooling effect of the buffer gases on ion trapping and detection in an ion trap mass spectrometer has been undertaken by the SIMION 3D program. Computation for the kinetic energy of ions under various conditions was used to account for the effects of collisional cooling of ions. Several parameters that may affect the collisional cooling effects of ions are evaluated including the existence and the variation of pressure of the buffer gas; the temperature of the ion trap; the size of the inner radius of the ion trap electrodes; the mass to charge ratio of ions; the alternative buffer gases and the qz. values which establish the ion trap trapping environment. 相似文献
The extent of internal energy deposition into ions upon storage, radial ejection, and detection using a linear quadrupole ion trap mass spectrometer is investigated as a function of ion size (m/z 59 to 810) using seven ion-molecule thermometer reactions that have well characterized reaction entropies and enthalpies. The average effective temperatures of the reactants and products of the ion-molecule reactions, which were obtained from ion-molecule equilibrium measurements, range from 295 to 350 K and do not depend significantly on the number of trapped ions, m/z value, ion trap qz value, reaction enthalpy/entropy, or the number of vibrational degrees of freedom for the seven reactions investigated. The average of the effective temperature values obtained for all seven thermometer reactions is 318?±?23 K, which indicates that linear quadrupole ion trap mass spectrometers can be used to study the structure(s) and reactivity of ions at near ambient temperature.
Means to allow for the application of a dipolar DC pulse to the end-cap electrodes of a three-dimensional (3-D) quadrupole
ion trap for as short as a millisecond to as long as hundreds of milliseconds are described. The implementation of dipolar
DC does not compromise the ability to apply AC waveforms to the end-cap electrodes at other times in the experiment. Dipolar
DC provides a nonresonant means for ion acceleration by displacing ions from the center of the ion trap where they experience
stronger rf electric fields, which increases the extent of micro-motion. The evolution of the product ion spectrum to higher
generation products with time, as shown using protonated leucine enkephalin as a model protonated peptide, illustrates the
broad-band nature of the activation. Dipolar DC activation is also shown to be effective as an ion heating approach in mimicking
high amplitude short time excitation (HASTE)/pulsed Q dissociation (PQD) resonance excitation experiments that are intended
to enhance the likelihood for observing low m/z products in ion trap tandem mass spectrometry. 相似文献
This study investigated the determination of Au in rock samples using collision cell quadrupole inductively coupled plasma
mass spectrometry (ICP-MS). It is essential to remove various interferents using a collision cell because polyatomic ions
such as 181Ta16O+ and 180Hf16O1H+ can interfere with the direct determination of monoisotopic 197Au when using ICP-MS. The addition of oxygen as a reaction gas removed isobaric interferents by transforming TaO+ and HfOH+ to TaO2+, TaO3+, and HfO2H+, HfO3H+, respectively, in the cell without significant Au+ loss. The ion kinetic energy effect (IKEE) due to the potential difference between the plasma and the hexapole affected the
reactions in the cell. Au and interfering ions were very sensitive to cell bias voltage (Vc) at constant plasma potential
(Vp) and quadrupole bias voltage (Vq). Under the condition of hot plasma, the transmission of ions was promoted, and the maximum
Au signal intensity was 50% greater than under normal conditions. At Vc > 7 V, TaO+ ions were removed to background level. Optimized conditions for real sample analysis were obtained by introducing He as an
additional collision gas in hot plasma. TaO+ ions were removed to background level at He flow rates above 0.6 mL min−1, and the Au signal remained high. The detection limit (three times the standard deviation of the blank) of this method was
3.06 pg g−1. The results for reference materials (STM-1 and DGPM-1) and spiked samples showed good agreement between specified and measured
concentrations. 相似文献
UV–vis photodissociation action spectroscopy is becoming increasingly prevalent because of advances in, and commercial availability of, ion trapping technologies and tunable laser sources. This study outlines in detail an instrumental arrangement, combining a commercial ion-trap mass spectrometer and tunable nanosecond pulsed laser source, for performing fully automated photodissociation action spectroscopy on gas-phase ions. The components of the instrumentation are outlined, including the optical and electronic interfacing, in addition to the control software for automating the experiment and performing online analysis of the spectra. To demonstrate the utility of this ensemble, the photodissociation action spectra of 4-chloroanilinium, 4-bromoanilinium, and 4-iodoanilinium cations are presented and discussed. Multiple photoproducts are detected in each case and the photoproduct yields are followed as a function of laser wavelength. It is shown that the wavelength-dependent partitioning of the halide loss, H loss, and NH3 loss channels can be broadly rationalized in terms of the relative carbon-halide bond dissociation energies and processes of energy redistribution. The photodissociation action spectrum of (phenyl)Ag2+ is compared with a literature spectrum as a further benchmark.
Abstract The use of a direct liquid introduction probe with a short guard column as the method of sample introduction is explored. This technique is an alternative to the conventional direct probe method. The method is rapid, involatile compounds can be analyzed, and volatile compounds are not lost in the vacuum lock. Screening for trichlorophenol in urine, by observing the loss of [M-HCOCI]+, is used to test the technique. The advantages and disadvantages of split and splitless direct liquid introduction probes and column concentration are discussed. Detection limits in the low nanograms were observed, and samples may be analyzed every 30 seconds. 相似文献
Flavonoids are one of the largest classes of plant secondary metabolites serving a variety of functions in plants and associating with a number of health benefits for humans. Typically, they are co-identified with many other secondary metabolites using untargeted metabolomics. The limited data quality of untargeted workflow calls for a shift from the breadth-first to the depth-first screening strategy when a specific biosynthetic pathway is focused on. Here we introduce a generic multiple reaction monitoring (MRM)-based approach for flavonoids profiling in plants using a hybrid triple quadrupole linear ion trap (QTrap) mass spectrometer. The approach includes four steps: (1) preliminary profiling of major aglycones by multiple ion monitoring triggered enhanced product ion scan (MIM-EPI); (2) glycones profiling by precursor ion triggered EPI scan (PI-EPI) of major aglycones; (3) comprehensive aglycones profiling by combining MIM-EPI and neutral loss triggered EPI scan (NL-EPI) of major glycone; (4) in-depth flavonoids profiling by MRM-EPI with elaborated MRM transitions. Particularly, incorporation of the NH3 loss and sugar elimination proved to be very informative and confirmative for flavonoids screening. This approach was applied for profiling flavonoids in Astragali radix (Huangqi), a famous herb widely used for medicinal and nutritional purposes in China. In total, 421 flavonoids were tentatively characterized, among which less than 40 have been previously reported in this medicinal plant. This MRM-based approach provides versatility and sensitivity that required for flavonoids profiling in plants and serves as a useful tool for plant metabolomics.
