Christopher L. Hendrickson

FOURIER TRANSFORM ION CYCLOTRON RESONANCE MASS SPECTROMETRY: A PRIMER

This review offers an introduction to the principles and generic applications of FT-ICR mass spectrometry, directed to readers with no prior experience with the technique. We are able to explain the fundamental FT-ICR phenomena from a simplified theoretical treatment of ion behavior in idealized magnetic and electric fields. The effects of trapping voltage, trap size and shape, and other nonidealities are manifested mainly as perturbations that preserve the idealized ion behavior modified by appropriate numerical correction factors. Topics include: effect of ion mass, charge, magnetic field, and trapping voltage on ion cyclotron frequency; excitation and detection of ICR signals; mass calibration; mass resolving power and mass accuracy; upper mass limit(s); dynamic range; detection limit, strategies for mass and energy selection for MSn; ion axialization, cooling, and remeasurement; and means for guiding externally formed ions into the ion trap. The relation of FT-ICR MS to other types of Fourier transform spectroscopy and to the Paul (quadrupole) ion trap is described. The article concludes with selected applications, an appendix listing accurate fundamental constants needed for ultrahigh-precision analysis, and an annotated list of selected reviews and primary source publications that describe in further detail various FT-ICR MS techniques and applications.

External Accumulation of Ions for Enhanced Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Electrospray ionization (ESI) in combination with Fourier transform ion cyclotron resonance (FTICR) mass spectrometry provides for mass analysis of biological molecules with unrivaled mass accuracy, resolving power and sensitivity. However, ESI FTICR MS performance with on-line separation techniques such as liquid chromatography (LC) and capillary electrophoresis has to date been limited primarily by pulsed gas assisted accumulation and the incompatibility of the associated pump-down time with the frequent ion beam sampling requirement of on-line chromatographic separation. Here we describe numerous analytical advantages that accrue by trapping ions at high pressure in the first rf-only octupole of a dual octupole ion injection system before ion transfer to the ion trap in the center of the magnet for high performance mass analysis at low pressure. The new configuration improves the duty cycle for analysis of continuously generated ions, and is thus ideally suited for on-line chromatographic applications. LC/ESI FTICR MS is demonstrated on a mixture of 500 fmol of each of three peptides. Additional improvements include a fivefold increase in signal-to-noise ratio and resolving power compared to prior methods on our instrument.

Free electron laser-Fourier transform ion cyclotron resonance mass spectrometry facility for obtaining infrared multiphoton dissociation spectra of gaseous ions

A Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer has been installed at a free electron laser (FEL) facility to obtain infrared absorption spectra of gas phase ions by infrared multiple photon dissociation (IRMPD). The FEL provides continuously tunable infrared radiation over a broad range of the infrared spectrum, and the FT-ICR mass spectrometer, utilizing a 4.7Tesla superconducting magnet, permits facile formation, isolation, trapping, and high-mass resolution detection of a wide range of ion classes. A description of the instrumentation and experimental parameters for these experiments is presented along with preliminary IRMPD spectra of the singly-charged chromium-bound dimer of diethyl ether (Cr(C4H10O)2+) and the fluorene molecular ion (C13H10+). Also presented is a brief comparison of the fluorene cation spectrum obtained by the FT-ICR-FEL with an earlier spectrum recorded using a quadrupole ion trap (QIT).

High-Resolution Mass Spectrometers

Over the past decade, mass spectrometry has been revolutionized by access to instruments of increasingly high mass-resolving power. For small molecules up to approximately 400 Da (e.g., drugs, metabolites, and various natural organic mixtures ranging from foods to petroleum), it is possible to determine elemental compositions (C(c)H(h)N(n)O(o)S(s)P(p)...) of thousands of chemical components simultaneously from accurate mass measurements (the same can be done up to 1000 Da if additional information is included). At higher mass, it becomes possible to identify proteins (including posttranslational modifications) from proteolytic peptides, as well as lipids, glycoconjugates, and other biological components. At even higher mass ( approximately 100,000 Da or higher), it is possible to characterize posttranslational modifications of intact proteins and to map the binding surfaces of large biomolecule complexes. Here we review the principles and techniques of the highest-resolution analytical mass spectrometers (time-of-flight and Fourier transform ion cyclotron resonance and orbitrap mass analyzers) and describe some representative high-resolution applications.

Electrospray Ionization Fourier Transform Ion Cyclotron Resonance at 9.4 T

We present the first results from a new electrospray ionization Fourier transform ion cyclotron resonance mass spectrometer operated at a magnetic field of 9.4 T (i.e. > or = 2.4 T higher than for any prior FTICR instrument). The 9.4 T instrument provides substantially improved performance for large molecules (> or = 50% increase in mass resolving power) and complex mixtures (> or = 100% increase in dynamic range) compared to lower-field (< or = 6 T) instruments. The higher magnetic field makes possible larger trapped-ion population without introduction of significant space--charge effects such as spectral peak shift and/or distortion, and coalescence of closely-spaced resonances. For bovine ubiquitin (8.6 kDa) we observe accurate relative isotopic abundances at a signal-to-noise ratio greater than 1000:1, whereas a complete nozzle-skimmer dissociation electrospray ionization (ESI) FTICR mass spectrum of bovine carbonic anhydrase (29 kDa) is achieved from a single scan with a signal-to-noise ratio of more than 250:1. Finally, we are able to obtain mass resolving power, m/delta m > 200,000, routinely for porcine serum albumin (67 kDa). The present performance guides further modifications of the instrument, which should lead to significant further improvements.

Comparison and interconversion of the two most common frequency-to-mass calibration functions for Fourier transform ion cyclotron resonance mass spectrometry

In a perfect three-dimensional axial quadrupolar electrostatic potential field, Ledford et al. showed that the frequency-to-mass calibration relation m/z = AL/v + BL/v2is valid for ions of any mass-to-charge ratio, m/z < (m/z)critical = eB02a2/4Vtrapα, in which v is the “reduced” (observed) ion cyclotron frequency, e is the electronic (elementary) charge, z is the number of elementary charges per ion, B0 is magnetic field induction, a is a characteristic trap dimension, vtrap is the potential applied to each trap endcap, α is a constant determined by the trap geometrical configuration, and AL and BL are constants that are determined by fitting experimental ion cyclotron resonance (ICR) frequencies for ions of at least two known masses in a Fourier transform ICR (FT-ICR) mass spectrum. In the further limit that m/z ≪ (m/z)critical, Francl et al. obtained a different frequency-to-mass relation m/z = AF/(BF+ v). Here, we rederive both frequency-to-mass relations to derive a simple conversion between ALand BL, versus AFand BF(e.g. for comparing calibrated FT-ICR mass spectral data from different vendors). For accurate mass measurement, the conversion introduces a small error (a few parts per billion) that can usually be neglected. More important, by applying both calibration equations to the same experimental time-domain data, we find that mass accuracy resulting from the two calibration functions (or their interconversion) is indistinguishable, because Ledford et al.’s validity criterion, m/z < 0.001 (m/z)critical, is generally satisfied for modern high-field instruments with optimized cell geometry. Interestingly, a small difference may result when different forms of the same calibration function are employed, presumably due to different roundoff errors in the calculation.

High-performance mass spectrometry: Fourier transform ion cyclotron resonance at 14.5 Tesla.

We describe the design and current performance of a 14.5 T hybrid linear quadrupole ion trap Fourier transform ion cyclotron resonance mass spectrometer. Ion masses are routinely determined at 4-fold better mass accuracy and 2-fold higher resolving power than similar 7 T systems at the same scan rate. The combination of high magnetic field and strict control of the number of trapped ions results in external calibration broadband mass accuracy typically less than 300 ppb rms, and a resolving power of 200,000 (m/Delta m50% at m/z 400) is achieved at greater than 1 mass spectrum per second. Novel ion storage optics and methodology increase the maximum number of ions that can be delivered to the FTICR cell, thereby improving dynamic range for tandem mass spectrometry and complex mixture applications.

Fourier transform ion cyclotron resonance detection: principles and experimental configurations

Fourier transform ion cyclotron resonance mass spectrometry is based on image current detection of coherently excited ion cyclotron motion. The detected signal magnitude and peak shape may be understood from idealized behavior: single ion, zero-pressure, spatially uniform magnetic field, three-dimensional axial quadrupolar electrostatic trapping potential, and spatially uniform resonant alternating electric field. In practice, deviation from any of the above conditions will shift, distort, split, and/or coalesce FT-ICR mass spectral peaks. Fortunately, such peak distortions may typically be avoided by appropriate experimental design and/or greatly minimized by internal frequency-to-m/z calibration. Various aspects of modern FT-ICR detection (hardware and software) are discussed.

High Resolution Mass Spectrometry

■ CONTENTS Mass Resolution, Mass Resolving Power 708 Mass Resolution and Accuracy 708 Time-of-Flight Mass Analyzers 708 Orthogonal Acceleration (see ref 13) 709 Reflectron/Multipass TOF 709 Recent Advances in TOF Mass Analyzers 709 Selected Applications 710 Fourier Transform Mass Analyzers 710 Common Features of Fourier Transform Mass Analyzers 710 Ion Accumulation and Detection 711 Advances in Fourier Transform Mass Analyzers 711 Selected Applications 713 Author Information 715 Corresponding Author 715 Biographies 715 Acknowledgments 715 References 715

Improved ion extraction from a linear octopole ion trap: SIMION analysis and experimental demonstration

Externally generated ions are accumulated in a linear octopole ion trap before injection into our 9.4 T Fourier transform ion cyclotron resonance (FT-ICR) mass analyzer. Such instrumental configuration has previously been shown to provide improved sensitivity, scan rate, and duty cycle relative to accumulated trapping in the ICR cell. However, inefficient ion ejection from the octopole currently limits both detection limit and scan rate. SIMION 7.0 analysis predicts that a dc axial electric field inside the linear octopole ion trap expedites and synchronizes the efficient extraction of the octopole-accumulated ions. Further SIMION analysis optimizes the ion ejection properties of each of three electrode configurations designed to produce a near-linear axial potential gradient. More efficient extraction and transfer of accumulated ions spanning a wide m/z range promises to reduce detection limit and increase front-end sampling rate (e.g., to increase front-end resolution for separation techniques coupled with FT-ICR mass analysis). Addition of the axial field improves experimental signal-to-noise ratio by more than an order of magnitude.