Richard L. Hunter

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.

An elongated trapped-ion cell for ion cyclotron resonance mass spectrometry with a superconducting magnet

Abstract A one-region trapped-ion analyzer cell that is elongated in the dimension parallel to the magnetic field is described and shown to be well suited for ion cyclotron resonance (ICR) experiments using a solenoidal-geometry superconducting magnet. An analysis of the static electric fields inside the elongated cell shows that the resonance frequency of the ions is far less affected by the trapping voltages than with a cubic cell or the original trapped-ion cell. The main advantages of the elongated cell are improved mass measurement accuracy and increased dynamic range.

A capacitance bridge circuit for broadband detection of ion cyclotron resonance signals

Abstract A capacitance bridge circuit has been developed for detecting ions stored in an ion cyclotron resonance (ICR) analyzer cell. The circuit functions by accelerating ions at their cyclotron frequency and detecting the resulting ion image current which is induced by the coherent motion of the ions. The main feature of the capacitance bridge circuit is that a mass spectrum can be obtained at constant magnetic field. This not only improves the speed and flexibility of the ICR experiments, but also allows superconducting magnets to be used for greatly improved sensitivity and mass resolution. In our laboratory the capacitance bridge is used in place of a marginal oscillator for pulsed ICR studies of gas-phase ion chemistry and for rapid-scan ICR studies of low-volatility compounds. This paper presents a simplified theory of operation for the capacitance bridge detector and a description of the circuitry which we have developed. Performance tests have been conducted to establish a range of useful operating parameters for the detector. Using an electromagnet at 1.2 tesla, the minimum detectable signal (1 : 1 S/N) is ~7000 ions, and linear response is achieved up to the space-charge limit of ~350 000 ions.

Multiphoton ionization detected by fourier-transform mass spectrometry

Abstract Ions produced by multiphoton ionization (MPI) of naphthalene, fluoranthene and triphenylene have been detected by Fourier-transform mass spectrometry (FT MS). Paret ions are produced very efficiently at 250 and 222 nm with pulse energies as low as 1 mJ. With FT MS a complete, high-resolution mass spectrum is obtained for each laser pulse.

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.

Theory of impulse excitation for Fourier transform mass spectrometry

Abstract Most Fourier transform mass spectrometers utilize a rapid radio frequency sweep, or “chirp”, to accelerate and detect ions. Impulse excitation is different because the ions are accelerated by a very short, high voltage d.c. pulse. In this paper, an analytical theory of impulse excitation is presented. Ion trajectories are plotted to illustrate the effects of pulse width, pulse height and “risetime” and “falltime” of the pulse on the ion motion. To minimize mass discrimination, the risetime of the impulse signal must be very fast (50–100 ns) and the pulse width must be much shorter than the period of the cyclotron orbit of an ion.

FTMS method for high resolution matrix-assisted laser desorption

Abstract A Fourier transform mass spectrometer with an external ion source has been modified for use with matrix-assisted laser desorption. High trapping potentials on the FTMS analyzer cell decelerate and trap the laser-produced ions, and a pulsed argon buffer gas coosl them prior to detection. For gramicidin S, only one laser pulse is needed to produce ass spectra with a high signal-to-noise ratio and a mass resolution of 1 100 000 (FWHM). Several other oligopeptides and small proteins have been analyzed, including bovine insulin that was detected at a mass resolution of 90 000. These results represent the highest mass resolution ever demonstrated for ions made by MALDI.

Ultrahigh-resolution fourier transform mass spectrometry of biomolecules above m/z 5 000

Abstract Ions with a mass-to-charge ratio (m/z) greater than 5 000 have been made by matrix-assisted laser desorption/ionization (MALDI) and detected at high mass resolving power by Fourier transform mass spectrometry (FTMS). The FTMS instrument used for the investigations has a MALDI source mounted outside of the magnetic field in a separate, differentially-pumped chamber. Ions were extracted from the source and transported efficiently to the FTMS analyzer cell by a r.f.-only quadrupole ion guide. To optimize the mass resolution for high mass ions, the operating parameters of the instrument were varied systematically. It was found that the parameters for formation of ions in the MALDI source, such as laser irradiance and amount of sample ablated, do not affect the mass resolution significantly. The biggest effects resulted from changing the ion detection parameters in the FTMS analyzer cell, in particular the trapping voltage, excitation r.f. level, background pressure, and number of ions stored. With optimal tuning, the external ion source FTMS method gave a mass resolving power of M/ΔM1/2 = 830 000 for human insulin at m/z 5 807 and 81 000 for cytochrome C at m/z 12 360.