Excitation frequencies of ions confined in a quadrupole field with quadrupole excitation

Resonant quadrupole excitation of ions confined in a radio frequency quadrupole field with angular frequency omega by an excitation signal with angular frequency omega has been investigated theoretically. It is shown that the spectrum of excitation frequencies has considerable structure which corresponds to different orders of excitation. The resonance condition for orders K = 1,2,3,... in the general case has been obtained as omega n(K) = (omega/K) magnitude of n + beta, -infinity < n < infinity, where K is the order of the resonance and beta and n determine the unperturbed oscillation frequencies. Resonance curves for ion oscillations with different stability parameters beta = 0.1, 0.5, and 0.9 have been constructed by means of direct numerical solution of the equations of motion. The trajectories of ion motion under resonant excitation of different orders have been investigated. For orders K of two and higher, the ion motion shows a beat character with an overall increase of amplitude with time. The stability diagram for ion motion in a mass filter in the presence of quadrupole excitation has been constructed.     

Improving IRMPD in a quadrupole ion trap

A focused laser is used to make infrared multiphoton photodissociation (IRMPD) more efficient in a quadrupole ion trap mass spectrometer. Efficient (up to 100%) dissociation at the standard operating pressure of 1 × 10⁻³ Torr can be achieved without any supplemental ion activation and with shorter irradiation times. The axial amplitudes of trapped ion clouds are measured using laser tomography. Laser flux on the ion cloud is increased six times by focusing the laser so that the beam waist approximates the ion cloud size. Unmodified peptide ions from 200 Da to 3 kDa are completely dissociated in 2.5–10 ms at a bath gas pressure of 3.3 × 10⁻⁴ Torr and in 3–25 ms at 1.0 × 10⁻³ Torr. Sequential dissociation of product ions is increased by focusing the laser and by operating at an increased bath gas pressure to minimize the size of the ion cloud.     

Energy-resolved collisional activation of dimethyl phosphonate and dimethyl phosphite ions in a quadrupole ion trap and a triple quadrupole mass spectrometer

Collision-activated dissociation spectra of dimethyl phosphonate and dimethyl phosphite ions were measured as a function of the amplitude of a supplementary AC voltage applied across the end-caps of an ion-trap mass spectrometer. These spectra yield breakdown graphs which bear a close resemblance to those obtained by varying collision energy in a triple-quadrupole mass spectrometer operating under multiple-collision conditions. Variation in the time of excitation at the resonance frequency provides an alternative route to breakdown graphs. The results demonstrate that energy deposition occurs via multiple activating collisions in the ion trap. Maximum energy deposition observed is somewhat smaller under normal operating conditions in the ion trap than in the triple-quadrupole mass spectrometer.     

Charge exchange using a double quadrupole mass spectrometer

Charge exchange mass spectra obtained on a double quadrupole (QQ) mass spectrometer are compared with those obtained by other methods. The effects of reagent ion recombination energies and of axial ion translational energy on these spectra are followed.     

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.     

A novel quadrupole,quistor, quadrupole tandem mass spectrometer

A quadrupole, quistor, quadrupole tandem mass spectrometer allowing selected ion/selected molecule reactions was built. The quistor will be used as a reaction chamber for the study of organic ion-molecule reactions. Ions are generated in a differentially pumped ion source, quadrupole mass selected and injected into the quistor. The ions are trapped in the quistor by the combined action of a deceleration lens, the presence of helium buffer gas and the quistor RF field. As a first test of the performance of the instrument, the reaction rate constant of the reaction of O₂⁺˙ with methane was measured to be 5 × 10^?12 cm³s^?1. This is in good agreement with literature values, indicating that the ions are near thermal equilibrium after a few milliseconds of trapping time.     

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.     

Estimation of effective ion temperatures in a quadrupole ion trap

A modified Finnigan LCQ quadrupole ion trap has been used to determine the equilibrium constant of the complexation reaction of thiophenolate with 2,2,2-trifluoroethanol. The process is particularly useful as a thermometer reaction because it has an exceptionally large temperature dependence. Using literature values for the thermochemistry, an effective ion temperature of 310 ± 20 K is indicated for the ion trap. This value is much lower than some earlier estimates for ion traps, but is consistent with a recent theoretical analysis and some previous interpretations of experimental data. The results suggest that quadrupole ion traps are suitable for studying gas phase reactions under nearly thermal conditions.     

High resolution on a quadrupole ion trap mass spectrometer

By using a modified ion trap mass spectrometer, resolution in excess of 30,000 (FWHM) at m I z 502 is demonstrated. The method of increasing resolution in the ion trap mass spectrometer operated in the mass-selective instability mode depends on decreasing the rate of scanning the primary radio frequency amplitude as well as using resonance ejection at the appropriate frequency and amplitude. A theoretical basis for the method is introduced.     

Nuclear quadrupole resonance in a single crystal of dimethyltin dichloride

The Zeeman NQR spectrum of a single crystal of dimethyltin dichloride has been reinvestigated in view of an improved determination of the crystal structure. The study shows the existence of two zero-splitting loci for the chlorine atoms. The quadrupole coupling constant was found to be ?30.92 MHz with an asymmetry parameter of η = 0.32. The principal EFG Z-axes are coincidental with the SnCl bonds, the Y-axes parallel to the crystallographic b-axis and the X-axes lie in the cb-plane. Using the Townes Dailey theory the fraction of π-bonding was calculated to be 0.24 with π_y-π_x = ?0.06. The imbalance in the π-bonding is rationalized on the basis of d-p overlap between the chlorine p_y-orbitals and the tin d-orbitals.     

Homotopy analysis method to study a quadrupole mass filter

The homotopy analysis method (HAM) is applied to study the behavior of a hyperbolic rods of quadrupole mass filter and a sinusoidal potential form V_ac cos(Ωt). Numerical computation method of a 20th‐order HAM is employed to compare the physical properties of the confined ions with fifth‐order Runge–Kutta method. Also, comparison is made for the first stability region, the ion trajectories in real time, the polar plots, and the ion trajectory in x ? y plan. The results show that the two methods are fairly similar; therefore, the HAM method has potential application to solve linear and nonlinear equations of the charge particle confinement in quadrupole field. Copyright © 2012 John Wiley & Sons, Ltd.     

Field-modulated selective ion storage in a quadrupole ion trap

A new method of selective ion storage in a quadrupole ion trap is described. Broadband waveforms were applied to the endcaps of an ion trap to eject unwanted ions by resonance excitation, which enhanced the storage of selected target ions. A unique trapping field amplitude modulation technique allowed the use of waveforms with fewer frequency components. The requirements and methods of calculations for frequency-optimized wave-forms are discussed. Advantages of this method include the reduction of target ion loss that results from collision-activated dissociation. In other applications, equivalent performance, relative to methods that use nonmodulated trapping fields combined with waveforms that have a higher frequency density, was shown.     

Equilibrium space charge distribution in a quadrupole ion trap

A simple model provides a basis for evaluating the ion spatial distribution in a uadrupole (Paul) ion trap and its effect on the total potential (trap potential plus space charge 3 acting on ions in the trap. By combining the pseudopotential approximation introduced by Dehmelt 25 years ago with the assumption of thermal equilibrium (leading to a Boltzmann ion energy distribution), the resulting ion spatial distribution (for ions of a single mass-to-charge ratio) depends only on total number of ions, trap pseudopotential, and temperature. (The equilibrium assumption is justified by the high helium bath gas pressure at which analytical quadrupole ion traps are typically operated.) The electric potential generated by the ion space charge may be generated from Poisson’s equation subject to a Boltzmann ion energy distribution. However, because the ion distribution depends in turn on the total potential, solving for the potential and the ion distribution is no longer a simple boundary condition differential equation problem; an iterative procedure is required to obtain a self-consistent result. For the particularly convenient operating condition, (a _z = -8qU/m? ₀ ² Ω², and q _z =-4qV m? ₀ ² Ω², where U and V are direct current and radiofrequency (frequency, ω) voltages applied to the trap, m/q is ion mass-to-charge ratio, and ?₀ is the radius of the ring electrode at the z=0 midplane], both the pseudopotential and the ion distribution become spherically symmetric. The resulting one-dimensional problem may be solved by a simple optimization procedure. The present model accounts qualitatively for the dependence of total potential and ion distribution on number of ions (higher ion density or lower temperature flattens the total potential and widens the spatial distribution of ions) and pseudopotential (higher pseudopotential increases ion density near the center of the trap without widening the ion spatial distribution).     

Observation of a quadrupole plasmon mode for a colloidal solution of gold nanoprisms

The synthesis and optical properties of single crystalline gold nanoprisms have been investigated. A three-step mediated seed growth process in an aqueous solution generated gold nanoprisms with a relatively homogeneous size distribution. The purity of these nanostructures has allowed us to observe a weak quadrupole resonance in addition to a strong dipole resonance associated with these novel structures. The experimental optical spectra agree with discrete dipole approximation calculations that have been modeled from the dimensions of gold nanoprisms produced in this synthesis.     

Nuclear quadrupole resonance of bromothiophenes

Br⁷⁹ NQR frequencies of a number of bromothiophene derivatives have been measured at 77°K. The frequencies correlate with Hammett δ values and this correlation demonstrates the importance of the inductive effect. The influences of more than one substituent on the thiophene ring are additive.