Robert T. McIver

Coupling a quadrupole mass spectrometer and a Fourier transform mass spectrometer

Abstract Experiments performed by the authors during the last year have demonstrated the feasibility of a new analytical instrument called a tandem quadrupole-Fourier transform mass spectrometer (QFT-MS). Ions made in the source of a quadrupole mass spectrometer are extracted and focused into a beam. The ion beam is then injected axially into a superconducting electromagnet where the ions are stored in an analyzer cell and detected by Fourier transform mass spectrometry (FT-MS). The goal of this project is to combine the highly developed chromatographic and sample ionization features of a quadrupole mass spectrometer with the versatility and high mass resolution that is available with Fourier transform detection. High mass resolution is possible because differential pumping separates the ion source of the quadrupole mass spectrometer from the analyzer cell of the FT-MS instrument. A novel method is described for efficiently injecting ions axially into the solenoidal magnetic field. The QFT-MS instrument has many features in common with triple quadrupole mass spectrometers but with QFT-MS, much higher mass resolution is possible. For example, a mass resolution of 140 000 is demonstrated for m/z 78 ions produced by collision-induced dissociation of bromobenzene molecular ions.

Experimental determination of the effects of space charge on ion cyclotron resonance frequencies

Abstract In ion cyclotron resonance (ICR) and Fourier transform mass spectrometry (FTMS) the mass-to-charge ratio of an ion is calculated from its measured cyclotron frequency. In principle this can be done very accurately because frequency is a physical parameter that can be measured to sub-p.p.m. accuracy using standard laboratory equipment. In this paper the factors which limit mass measurement accuracy in FTMS and ICR will be discussed in the framework of a theoretical model for the ion space-charge effects derived in the previous paper. The resulting calibration procedure gives accurate mass determinations with errors of less than 1 p.p.m.

Pulsed ion cyclotron resonance mass spectrometer for studying ion–molecule reactions

A pulsed ion cyclotron resonance mass spectrometer utilizes the cyclotron resonance principle for mass analysis of ions trapped at low pressures by electric and magnetic fields. Both mass analysis and ion trapping are accomplished in a one-region device called a trapped ion analyzer cell. A pulsing sequence is described which allows for generation of ions by electron impact, reaction with added gases, and mass analysis of the products of ion-molecule reactions. Methods are described for measuring rate constants and equilibrium constants for ion-molecule reactions. The high ion trapping efficiency and open geometry of the analyzer cell make it well suited for studying the interaction of laser radiation with gaseous ions and may also be useful for high-accuracy isotope ration mass spectrometry.

A diffusion model for nonreactive ion loss in pulsed ion cyclotron resonance experiments

Abstract In a typical pulsed ion cyclotron resonance (ICR) experiment, gaseous ions formed by a pulsed electron beam are trapped by static electric and magnetic fields. The ions are trapped efficiently for several tenths of a second, but collisions with neutral molecules cause them to diffuse gradually to the walls of the analyzer cell. In this paper the diffusion of ions in an ICR trapped-ion analyzer cell is modeled theoretically, and the results are compared with experimental data obtained at a variety of pressures and trapping voltages. The theoretical model assumes that the dominant loss mechanism is diffusion of the ions to the upper and lower plates of the analyzer cell under the influence of an electrostatic trapping field. The theoretical results for the time dependence of the ion concentration in the ICR analyzer cell are in good agreement with experimental time plots. A useful parameter for characterizing ion trapping efficiency is t 1 2 , the time taken by the ion concentration to decrease to one-half of its initial value. The theoretical expression for t 1 2 shows that it is proportional to ( aB 2 )/ PV , where B is the magnetic field strength, a is the distance between the walls of the ICR analyzer cell, P is the pressure in the cell, and V is the electrostatic trapping voltage.

Trajectory calculations for axial injection of ions into a magnetic field : overcoming the magnetic mirror effect with an R.F. quadrupole lens

Abstract The magnetic mirror effect is one of the problems that must be overcome in order to perform Fourier transform mass spectrometry (FT-MS) experiments with an external ion source. Since the ion source is located outside of the magnetic field, about 1 m from the FT-MS analyzer cell, some type of focusing is needed to guide the ions through the fringing fields of the magnet. Several years ago, McIver and co-workers showed that a quadrupole lens operated in the r.f.-only mode was an effective means for accomplishing this. In this paper, the theory of ion injection by a r.f. quadrupole lens is developed and trajectory calculations are performed to determine how the mass range of the quadrupole lens depends on the frequency and low voltage of the signal that is applied to the rods. The calculations show that the quadrupole lens functions like a broadband filter with certain low mass and high mass cut-offs. By selecting the proper operating conditions, the mass range of the injection ions can extend over several thousand mass units and high mass ions (m/z greater than 20 000) can be injected.

Theory for broadband detection of ion cyclotron resonance signals

A complete line shape theory is developed for the transient response of a new type of ion cyclotron resonance (ICR) detector circuit. The detector is basically a balanced capacitance bridge which is sensitive to the abundance of gaseous ions stored in a static magnetic ion trap. For the first time, the equations of motion of ions in the ICR analyzer cell are shown to be coupled to the circuit equations of the detector. Also, the effect of nonreactive ion–molecule collisions on line shapes and on the transient response of the detector are analyzed and shown to allow measurement of ion–molecule collisions frequencies as a function of ion translational energy. One of the most important features of the capacitance bridge detector is its broadband sensitivity to a wide range of ion cyclotron resonance frequencies. This allows a mass spectrum of ions stored in the ICR analyzer cell to be obtained by scanning the frequency ω1 of the irradiating rf electric field at a fixed magnetic field strength. The capacitanc...

Conceptual and experimental basis for rapid scan ion cyclotron resonance spectroscopy

Abstract This is the first paper which describes the conceptual and experimental basis of a rapid scan ion cyclotron resonance (rapid scan ICR) technique for mass analyzing the ions stored in a static magnetic ion trap. At constant magnetic-field strength, a continuous wave excitation frequency is rapidly scanned across the mass spectrum, and the transient response of the coherently cyclotroning ions is detected. Due to the rapid scan rate, the detected signal is greatly distorted, but cross-correlation with the signal of a single line can be used to recover the true mass spectrum. High mass resolution and a scan rate of 66 kHz s−1 are demonstrated. Comparisons are made between rapid scan ICR and Fourier transform ICR.

A Solid‐State Marginal Oscillator for Pulsed Ion Cyclotron Resonance Spectroscopy

A pulsed marginal oscillator has been developed for use in pulsed ion cyclotron resonance experiments. The circuit performs quite well as a means of detecting gaseous ions in a magnetic field when the cyclotron frequency of the ions is equal to the resonant frequency of the oscillator. A pulsed marginal oscillator is different from previous marginal oscillator designs in that a gated amplifier is incorporated in the feedback loop for pulsing the oscillator on and off. This feature is important for delayed detection of ions which are formed as a result of chemical reactions in the gas phase. Oscillation levels from 2 to 500 mV (p‐p) are obtained over a frequency range fram 50 kHz to 2 MHz.

High resolution ion cyclotron resonance spectroscopy

Abstract Methods are described for greatly improving the mass resolution of an ion cyclotron resonance mass spectrometer. The factors affecting mass resolution are examined in detail, and the conditions for maximum resolution are discussed. Only three parameters — magnetic field strength, cyclotron frequency, and analyzer cell trapping voltage — are needed to make accurate, absolute mass measurements. Ions are detected with a solid-state marginal oscillator circuit which has been designed to be stable at very low r.f. levels.

Optimal phase modulation in stored wave form inverse Fourier transform excitation for Fourier transform mass spectrometry. I. Basic algorithm

A new signal processing method has been proposed for generating optimal stored wave form inverse Fourier transform (SWIFT) excitation signals used in Fourier transform mass spectrometry (FTMS or FT‐ICR). The excitation wave forms with desired flat excitation power can be obtained by using the data processing steps which include: (1) smoothing of the specified magnitude spectrum, (2) generation of the optimal phase function, and (3) inverse Fourier transformation. In contrast to previously used procedures, no time domain wave form apodization is necessary. The optimal phase functions can be expressed as an integration of the specified power spectral profiles. This allows one not only to calculate optimal phase functions in discrete data format, but also to obtain an analytical expression (in simple magnitude spectral cases) that is for theoretical studies. A comparison is made of the frequency sweeping or ‘‘chirp’’ excitation and stored wave form inverse Fourier transform (SWIFT) excitation. This shows tha...