Dipole Excitation: A New Method for Mass Analysis with a Quadrupole Mass Filter

Dipole and quadrupole resonance excitation in linear quadrupoles

In the development of commercial quadrupole mass spectrometers, there is an interest in improving the performance characteristics such as transmission, resolution, and mass range. In particular, parametric and dipolar resonance excitation of trapping ions are used for linear quadrupole mass filters. Theoretical methods and numerical simulation of ion trajectories were applied for study of ion-optical properties. The review is devoted to description of different excitation methods to improve QMF performance and consists of three parts. The first part presents the results of a linear ion trap simulation for various operating conditions and excitation methods. The second part considers the effects of dipole excitation (DE) on the performance of the quadrupole mass filter. The last part analyzes the formation of stability islands by different methods of quadrupole excitation. To date conditions of mass separation in quadrupole mass filters with sin wave supply were described for stability islands of the first and third stability regions formed by quadrupole and DE. By complicating the electronics such methods allow to overcome the destructive influence of electric field distortions and obtain a resolving power and ion transmission efficiency comparable with commercial devices. At quadrupole resonance excitation by a two-frequency signal, it is possible to reduce the length of electrodes three times without losses in resolution and transmission, which reduces the cost of rod set production with micrometer accuracy. Dipole resonance excitation allows controlling the shape of the mass peak by changing amplitude and phase of the auxiliary AC signal. The main factors affecting the resolving power of a linear ion trap are described theoretically. The numerical modeling results are confirmed by experiment.

Modeling dipolar excitation for quadrupole mass filter

The results of modeling AC and DC dipole excitation of ion oscillations in a quadrupole mass filter are presented. The simulation is done by numerical integration of the ion motion equations, ions' initial coordinates and velocities are distributed normally. For AC dipole excitation the instability bands on the (a, q) stability diagram follow along the isolines βx/2 and βy/2, creating regular dips on the transmission contour. We show that AC excitation at frequency Ω/2 makes it possible to control the resolution of the mass filter by either changing the AC amplitude or phase. Instability bands can be used for mass selective excitation in a linear ion trap. Options for the joint use of DC and AC dipole excitation to form the mass peak shape are considered.

Stable X-islands of quadrupole mass filter

Quadrupole mass filters are normally operated as narrow band pass filters by appropriate choices of rf and DC voltages corresponding to Mathieu a and q values near the apex of the first stability region. We add an auxiliary quadrupole excitation potential to the main drive voltage. As a result, stability islands appear on the (a,q) plane. The method of the islands mapping on the (a,q) plane is discussed in detail. The DC electric field's effect responsible for removing "shadowing" islands has been studied using a combination of analytic theory and computer modeling. We call a narrow stability region elongated along an iso-βx line an X-island. Two such stability islands were found where operation is possible without mass spectra interference when the DC potential is used. Those islands are formed by quadrupole potentials with relative excitation frequencies ν=β,ν=1±β and ν=2±β, where β << 1. Many other stability X-islands are presented in detail and illustrated by their transmission contours. This data is necessary for experimental testing of the separation mode at those islands. The study demonstrates how to use X-islands to achieve relatively high resolution 4000-12,000 at the transmission of about 28-15%, respectively.

Quadrupole mass filter operation with dipole direct current and quadrupole radiofrequency excitation

RATIONALE

For mass analysis, a quadrupole mass filter (QMF) usually operates at the upper tip of the first stability region. We introduce a new mode of QMF operation with dipole direct current (dc) and with auxiliary quadrupole excitation. Before experimental investigation of this mode, we have carried out numerical simulations of this process.

METHODS

Based on the analytical description of the equations of ion motion and mapping of stability islands, ion trajectory calculations are used to calculate peak shapes or mass filter transmission contours. Ions are given Gaussian distributions of initial positions in x and y, and thermal initial velocity distributions. The effects of the dipole dc in the y direction and auxiliary quadrupole excitation in the x and y directions are modeled.

RESULTS

We find that, with dipole dc excitation, the working area of the first stability region decreases and a new y stability boundary follows a β_y line. This allows control of the resolution with the removal of the low-mass tail of a peak thereby improving the isotopic abundance sensitivity by about two orders of magnitude. The operation of a QMF with high resolution (about 5000) and high transmission (20-25%) and with a relatively short sorting time of ions n = 150 radiofrequency (rf) cycles based on the use of dipole dc and quadrupole excitation is shown.

CONCLUSIONS

A new mode of QMF operation with dipole direct current (dc) and with an auxiliary quadrupole is discussed. Using dc excitation allows control of the resolution with the removal of the low-mass tail of a peak. The method of operation of a quadrupole mass filter with high resolution (about 5000) and high transmission (20-25%) and a relatively short sorting time (150 rf cycles) based on the use of dipole dc and on quadrupole excitation is shown.

Ion Isolation in a Linear Ion Trap Using Dual Resonance Frequencies

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. Graphical Abstract ᅟ.