Fourier-transform ion cyclotron resonance

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Fourier transform ion cyclotron resonance
AcronymFTICR
Classification Mass spectrometry
Other techniques
Related Ion trap
Quadrupole ion trap
Penning trap
Orbitrap

Fourier-transform ion cyclotron resonance mass spectrometry is a type of mass analyzer (or mass spectrometer) for determining the mass-to-charge ratio (m/z) of ions based on the cyclotron frequency of the ions in a fixed magnetic field. [1] The ions are trapped in a Penning trap (a magnetic field with electric trapping plates), where they are excited (at their resonant cyclotron frequencies) to a larger cyclotron radius by an oscillating electric field orthogonal to the magnetic field. After the excitation field is removed, the ions are rotating at their cyclotron frequency in phase (as a "packet" of ions). These ions induce a charge (detected as an image current) on a pair of electrodes as the packets of ions pass close to them. The resulting signal is called a free induction decay (FID), transient or interferogram that consists of a superposition of sine waves. The useful signal is extracted from this data by performing a Fourier transform to give a mass spectrum.

Contents

History

FT-ICR was invented by Melvin B. Comisarow [2] and Alan G. Marshall at the University of British Columbia. The first paper appeared in Chemical Physics Letters in 1974. [3] The inspiration was earlier developments in conventional ICR and Fourier-transform nuclear magnetic resonance (FT-NMR) spectrometry. Marshall has continued to develop the technique at The Ohio State University and Florida State University.

Theory

Linear ion trap - Fourier-transform ion cyclotron resonance mass spectrometer (panels around magnet are missing) LTQ-FTICR.jpg
Linear ion trap – Fourier-transform ion cyclotron resonance mass spectrometer (panels around magnet are missing)

The physics of FTICR is similar to that of a cyclotron at least in the first approximation.

In the simplest idealized form, the relationship between the cyclotron frequency and the mass-to-charge ratio is given by

where f = cyclotron frequency, q = ion charge, B = magnetic field strength and m = ion mass.

This is more often represented in angular frequency:

where is the angular cyclotron frequency, which is related to frequency by the definition .

Because of the quadrupolar electrical field used to trap the ions in the axial direction, this relationship is only approximate. The axial electrical trapping results in axial oscillations within the trap with the (angular) frequency

where is a constant similar to the spring constant of a harmonic oscillator and is dependent on applied voltage, trap dimensions and trap geometry.

The electric field and the resulting axial harmonic motion reduces the cyclotron frequency and introduces a second radial motion called magnetron motion that occurs at the magnetron frequency. The cyclotron motion is still the frequency being used, but the relationship above is not exact due to this phenomenon. The natural angular frequencies of motion are

where is the axial trapping frequency due the axial electrical trapping and is the reduced cyclotron (angular) frequency and is the magnetron (angular) frequency. Again, is what is typically measured in FTICR. The meaning of this equation can be understood qualitatively by considering the case where is small, which is generally true. In that case the value of the radical is just slightly less than , and the value of is just slightly less than (the cyclotron frequency has been slightly reduced). For the value of the radical is the same (slightly less than ), but it is being subtracted from , resulting in a small number equal to (i.e. the amount that the cyclotron frequency was reduced by).

Instrumentation

FTICR-MS differs significantly from other mass spectrometry techniques in that the ions are not detected by hitting a detector such as an electron multiplier but only by passing near detection plates. Additionally the masses are not resolved in space or time as with other techniques but only by the ion cyclotron resonance (rotational) frequency that each ion produces as it rotates in a magnetic field. Thus, the different ions are not detected in different places as with sector instruments or at different times as with time-of-flight instruments, but all ions are detected simultaneously during the detection interval. This provides an increase in the observed signal-to-noise ratio owing to the principles of Fellgett's advantage. [1] In FTICR-MS, resolution can be improved either by increasing the strength of the magnet (in teslas) or by increasing the detection duration. [4]

Cells

A cylindrical ICR cell. The walls of the cell are made of copper, and ions enter the cell from the right, transmitted by the octopole ion guides. ICR Cell.jpg
A cylindrical ICR cell. The walls of the cell are made of copper, and ions enter the cell from the right, transmitted by the octopole ion guides.

A review of different cell geometries with their specific electric configurations is available in the literature. [5] However, ICR cells can belong to one of the following two categories: closed cells or open cells.

Several closed ICR cells with different geometries were fabricated and their performance has been characterized. Grids were used as end caps to apply an axial electric field for trapping ions axially (parallel to the magnetic field lines). Ions can be either generated inside the cell or can be injected to the cell from an external ionization source. Nested ICR cells with double pair of grids were also fabricated to trap both positive and negative ions simultaneously.

The most common open cell geometry is a cylinder, which is axially segmented to produce electrodes in the shape of a ring. The central ring electrode is commonly used for applying radial excitation electric field and detection. DC electric voltage is applied on the terminal ring electrodes to trap ions along the magnetic field lines. [6] Open cylindrical cells with ring electrodes of different diameters have also been designed. [7] They proved not only capable in trapping and detecting both ion polarities simultaneously, but also they succeeded to separate positive from negative ions radially. This presented a large discrimination in kinetic ion acceleration between positive and negative ions trapped simultaneously inside the new cell. Several ion axial acceleration schemes were recently written for ion–ion collision studies. [8]

Stored-waveform inverse Fourier transform

Stored-waveform inverse Fourier transform (SWIFT) is a method for the creation of excitation waveforms for FTMS. [9] The time-domain excitation waveform is formed from the inverse Fourier transform of the appropriate frequency-domain excitation spectrum, which is chosen to excite the resonance frequencies of selected ions. The SWIFT procedure can be used to select ions for tandem mass spectrometry experiments.

Applications

Fourier-transform ion cyclotron resonance (FTICR) mass spectrometry is a high-resolution technique that can be used to determine masses with high accuracy. Many applications of FTICR-MS use this mass accuracy to help determine the composition of molecules based on accurate mass. This is possible due to the mass defect of the elements. FTICR-MS is able to achieve higher levels of mass accuracy than other forms of mass spectrometer, in part, because a superconducting magnet is much more stable than radio-frequency (RF) voltage. [10]

Another place that FTICR-MS is useful is in dealing with complex mixtures, such as biomass or waste liquefaction products, [11] [12] since the resolution (narrow peak width) allows the signals of two ions with similar mass-to-charge ratios (m/z) to be detected as distinct ions. [13] [14] [15] This high resolution is also useful in studying large macromolecules such as proteins with multiple charges, which can be produced by electrospray ionization. For example, attomole level of detection of two peptides has been reported. [16] These large molecules contain a distribution of isotopes that produce a series of isotopic peaks. Because the isotopic peaks are close to each other on the m/z axis, due to the multiple charges, the high resolving power of the FTICR is extremely useful. FTICR-MS is very useful in other studies of proteomics as well. It achieves exceptional resolution in both top-down and bottom-up proteomics. Electron-capture dissociation (ECD), collisional-induced dissociation (CID), and infrared multiphoton dissociation (IRMPD) are all utilized to produce fragment spectra in tandem mass spectrometry experiments. [17] Although CID and IRMPD use vibrational excitation to further dissociate peptides by breaking the backbone amide linkages, which are typically low in energy and weak, CID and IRMPD may also cause dissociation of post-translational modifications. ECD, on the other hand, allows specific modifications to be preserved. This is quite useful in analyzing phosphorylation states, O- or N-linked glycosylation, and sulfating. [17]

Related Research Articles

<span class="mw-page-title-main">Ionization</span> Process by which atoms or molecules acquire charge by gaining or losing electrons

Ionization is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule is called an ion. Ionization can result from the loss of an electron after collisions with subatomic particles, collisions with other atoms, molecules and ions, or through the interaction with electromagnetic radiation. Heterolytic bond cleavage and heterolytic substitution reactions can result in the formation of ion pairs. Ionization can occur through radioactive decay by the internal conversion process, in which an excited nucleus transfers its energy to one of the inner-shell electrons causing it to be ejected.

<span class="mw-page-title-main">Mass spectrometry</span> Analytical technique based on determining mass to charge ratio of ions

Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a mass spectrum, a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used in many different fields and is applied to pure samples as well as complex mixtures.

Electron cyclotron resonance (ECR) is a phenomenon observed in plasma physics, condensed matter physics, and accelerator physics. It happens when the frequency of incident radiation coincides with the natural frequency of rotation of electrons in magnetic fields. A free electron in a static and uniform magnetic field will move in a circle due to the Lorentz force. The circular motion may be superimposed with a uniform axial motion, resulting in a helix, or with a uniform motion perpendicular to the field resulting in a cycloid. The angular frequency of this cyclotron motion for a given magnetic field strength B is given by

<span class="mw-page-title-main">Penning trap</span> Device for storing charged particles

A Penning trap is a device for the storage of charged particles using a homogeneous magnetic field and a quadrupole electric field. It is mostly found in the physical sciences and related fields of study as a tool for precision measurements of properties of ions and stable subatomic particles, like for example mass, fission yields and isomeric yield ratios. One initial object of study were the so-called geonium atoms, which represent a way to measure the electron magnetic moment by storing a single electron. These traps have been used in the physical realization of quantum computation and quantum information processing by trapping qubits. Penning traps are in use in many laboratories worldwide, including CERN, to store and investigate anti-particles such as antiprotons. The main advantages of Penning traps are the potentially long storage times and the existence of a multitude of techniques to manipulate and non-destructively detect the stored particles. This makes Penning traps versatile tools for the investigation of stored particles, but also for their selection, preparation or mere storage.

<span class="mw-page-title-main">Synchrocyclotron</span> Special type of cyclic particle accelerator

A synchrocyclotron is a special type of cyclotron, patented by Edwin McMillan in 1952, in which the frequency of the driving RF electric field is varied to compensate for relativistic effects as the particles' velocity begins to approach the speed of light. This is in contrast to the classical cyclotron, where this frequency is constant.

<span class="mw-page-title-main">Polaron</span> Quasiparticle in condensed matter physics

A polaron is a quasiparticle used in condensed matter physics to understand the interactions between electrons and atoms in a solid material. The polaron concept was proposed by Lev Landau in 1933 and Solomon Pekar in 1946 to describe an electron moving in a dielectric crystal where the atoms displace from their equilibrium positions to effectively screen the charge of an electron, known as a phonon cloud. This lowers the electron mobility and increases the electron's effective mass.

<span class="mw-page-title-main">Tandem mass spectrometry</span> Type of mass spectrometry

Tandem mass spectrometry, also known as MS/MS or MS2, is a technique in instrumental analysis where two or more stages of analysis using one or more mass analyzer are performed with an additional reaction step in between these analyses to increase their abilities to analyse chemical samples. A common use of tandem MS is the analysis of biomolecules, such as proteins and peptides.

<span class="mw-page-title-main">Ion trap</span> Device for trapping charged particles

An ion trap is a combination of electric and/or magnetic fields used to capture charged particles — known as ions — often in a system isolated from an external environment. Atomic and molecular ion traps have a number of applications in physics and chemistry such as precision mass spectrometry, improved atomic frequency standards, and quantum computing. In comparison to neutral atom traps, ion traps have deeper trapping potentials that do not depend on the internal electronic structure of a trapped ion. This makes ion traps more suitable for the study of light interactions with single atomic systems. The two most popular types of ion traps are the Penning trap, which forms a potential via a combination of static electric and magnetic fields, and the Paul trap which forms a potential via a combination of static and oscillating electric fields.

<span class="mw-page-title-main">Quadrupole ion trap</span> Type of apparatus for isolating charged particles

In experimental physics, a quadrupole ion trap or paul trap is a type of ion trap that uses dynamic electric fields to trap charged particles. They are also called radio frequency (RF) traps or Paul traps in honor of Wolfgang Paul, who invented the device and shared the Nobel Prize in Physics in 1989 for this work. It is used as a component of a mass spectrometer or a trapped ion quantum computer.

<span class="mw-page-title-main">Electron-capture dissociation</span> Method in mass spectrometry

Electron-capture dissociation (ECD) is a method of fragmenting gas-phase ions for structure elucidation of peptides and proteins in tandem mass spectrometry. It is one of the most widely used techniques for activation and dissociation of mass selected precursor ion in MS/MS. It involves the direct introduction of low-energy electrons to trapped gas-phase ions.

In plasma physics, an electromagnetic electron wave is a wave in a plasma which has a magnetic field component and in which primarily the electrons oscillate.

<span class="mw-page-title-main">Orbitrap</span>

In mass spectrometry, Orbitrap is an ion trap mass analyzer consisting of an outer barrel-like electrode and a coaxial inner spindle-like electrode that traps ions in an orbital motion around the spindle. The image current from the trapped ions is detected and converted to a mass spectrum by first using the Fourier transform of time domain of the harmonic to create a frequency signal which is converted to mass.

Cyclotron resonance describes the interaction of external forces with charged particles experiencing a magnetic field, thus already moving on a circular path. It is named after the cyclotron, a cyclic particle accelerator that utilizes an oscillating electric field tuned to this resonance to add kinetic energy to charged particles.

Alan G. Marshall is an American analytical chemist who has devoted his scientific career to developing a scientific technique known as Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry, which he co-invented.

Ion cyclotron resonance is a phenomenon related to the movement of ions in a magnetic field. It is used for accelerating ions in a cyclotron, and for measuring the masses of an ionized analyte in mass spectrometry, particularly with Fourier transform ion cyclotron resonance mass spectrometers. It can also be used to follow the kinetics of chemical reactions in a dilute gas mixture, provided these involve charged species.

Michael L. Gross is Professor of Chemistry, Medicine, and Immunology, at Washington University in St. Louis. He was formerly Professor of Chemistry at the University of Nebraska-Lincoln from 1968–1994. He is recognized for his contributions to the field of mass spectrometry and ion chemistry. He is credited with the discovery of distonic ions, chemical species containing a radical and an ionic site on different atoms of the same molecule.

<span class="mw-page-title-main">Collision-induced dissociation</span> Mass spectrometry technique to induce fragmentation of selected ions in the gas phase

Collision-induced dissociation (CID), also known as collisionally activated dissociation (CAD), is a mass spectrometry technique to induce fragmentation of selected ions in the gas phase. The selected ions are usually accelerated by applying an electrical potential to increase the ion kinetic energy and then allowed to collide with neutral molecules. In the collision, some of the kinetic energy is converted into internal energy which results in bond breakage and the fragmentation of the molecular ion into smaller fragments. These fragment ions can then be analyzed by tandem mass spectrometry.

Petroleomics is the identification of the totality of the constituents of naturally occurring petroleum and crude oil using high resolution mass spectrometry. In addition to mass determination, petroleomic analysis sorts the chemical compounds into heteroatom class, type. The name is a combination of petroleum and -omics.

References

  1. 1 2 Marshall, A. G.; Hendrickson, C. L.; Jackson, G. S. (1998). "Fourier transform ion cyclotron resonance mass spectrometry: a primer". Mass Spectrom. Rev. 17 (1): 1–35. Bibcode:1998MSRv...17....1M. doi:10.1002/(sici)1098-2787(1998)17:1<1::aid-mas1>3.0.co;2-k. PMID   9768511.
  2. "UBC Chemistry Personnel: Melvin B. Comisarow". University of British Columbia. Retrieved 2009-11-05.
  3. Comisarow, Melvin B. (1974). "Fourier transform ion cyclotron resonance spectroscopy". Chemical Physics Letters. 25 (2): 282–283. Bibcode:1974CPL....25..282C. doi:10.1016/0009-2614(74)89137-2.
  4. Marshall, A. (2002). "Fourier transform ion cyclotron resonance detection: principles and experimental configurations". International Journal of Mass Spectrometry. 215 (1–3): 59–75. Bibcode:2002IJMSp.215...59M. doi:10.1016/S1387-3806(01)00588-7.
  5. Guan, Shenheng; Marshall, Alan G. (1995). "Ion traps for Fourier transform ion cyclotron resonance mass spectrometry: principles and design of geometric and electric configurations". International Journal of Mass Spectrometry and Ion Processes. 146–147: 261–296. Bibcode:1995IJMSI.146..261G. doi:10.1016/0168-1176(95)04190-V.
  6. Marshall, Alan G.; Hendrickson, Christopher L.; Jackson, George S. (1998). "Fourier transform ion cyclotron resonance mass spectrometry: A primer". Mass Spectrometry Reviews. 17 (1): 1–35. Bibcode:1998MSRv...17....1M. doi:10.1002/(SICI)1098-2787(1998)17:1<1::AID-MAS1>3.0.CO;2-K. ISSN   0277-7037. PMID   9768511.
  7. Kanawati, B.; Wanczek, K. P. (2007). "Characterization of a new open cylindrical ion cyclotron resonance cell with unusual geometry". Review of Scientific Instruments. 78 (7): 074102–074102–8. Bibcode:2007RScI...78g4102K. doi:10.1063/1.2751100. PMID   17672776.
  8. Kanawati, B.; Wanczek, K. (2008). "Characterization of a new open cylindrical ICR cell for ion–ion collision studies☆". International Journal of Mass Spectrometry. 269 (1–2): 12–23. Bibcode:2008IJMSp.269...12K. doi:10.1016/j.ijms.2007.09.007.
  9. Cody, R. B.; Hein, R. E.; Goodman, S. D.; Marshall, Alan G. (1987). "Stored waveform inverse fourier transform excitation for obtaining increased parent ion selectivity in collisionally activated dissociation: Preliminary results". Rapid Communications in Mass Spectrometry . 1 (6): 99–102. Bibcode:1987RCMS....1...99C. doi:10.1002/rcm.1290010607.
  10. Shi, S; Drader, Jared J.; Freitas, Michael A.; Hendrickson, Christopher L.; Marshall, Alan G. (2000). "Comparison and interconversion of the two most common frequency-to-mass calibration functions for Fourier transform ion cyclotron resonance mass spectrometry". International Journal of Mass Spectrometry. 195–196: 591–598. Bibcode:2000IJMSp.195..591S. doi:10.1016/S1387-3806(99)00226-2.
  11. Leonardis, Irene; Chiaberge, Stefano; Fiorani, Tiziana; Spera, Silvia; Battistel, Ezio; Bosetti, Aldo; Cesti, Pietro; Reale, Samantha; De Angelis, Francesco (8 November 2012). "Characterization of Bio-oil from Hydrothermal Liquefaction of Organic Waste by NMR Spectroscopy and FTICR Mass Spectrometry". ChemSusChem. 6 (2): 160–167. doi:10.1002/cssc.201200314. PMID   23139164.
  12. Sudasinghe, Nilusha; Cort, John; Hallen, Richard; Olarte, Mariefel; Schmidt, Andrew; Schaub, Tanner (1 December 2014). "Hydrothermal liquefaction oil and hydrotreated product from pine feedstock characterized by heteronuclear two-dimensional NMR spectroscopy and FT-ICR mass spectrometry". Fuel. 137: 60–69. doi: 10.1016/j.fuel.2014.07.069 .
  13. Sleno L.; Volmer D. A.; Marshall A. G. (February 2005). "Assigning product ions from complex MS/MS spectra: the importance of mass uncertainty and resolving power". J. Am. Soc. Mass Spectrom. 16 (2): 183–98. doi:10.1016/j.jasms.2004.10.001. PMID   15694769.
  14. Bossio R. E.; Marshall A. G. (April 2002). "Baseline resolution of isobaric phosphorylated and sulfated peptides and nucleotides by electrospray ionization FTICR ms: another step toward mass spectrometry-based proteomics". Anal. Chem. 74 (7): 1674–9. doi:10.1021/ac0108461. PMID   12033259.
  15. He F.; Hendrickson C. L.; Marshall A. G. (February 2001). "Baseline mass resolution of peptide isobars: a record for molecular mass resolution". Anal. Chem. 73 (3): 647–50. doi:10.1021/ac000973h. PMID   11217775.
  16. Solouki T.; Marto J. A.; White F. M.; Guan S.; Marshall A. G. (November 1995). "Attomole biomolecule mass analysis by matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance". Anal. Chem. 67 (22): 4139–44. doi:10.1021/ac00118a017. PMID   8633766.
  17. 1 2 Scigelova, M.; Hornshaw, M.; Giannakopulos, A.; Makarov, A. (2011). "Fourier Transform Mass Spectrometry". Molecular & Cellular Proteomics. 10 (7): M111.009431. doi: 10.1074/mcp.M111.009431 . ISSN   1535-9476. PMC   3134075 . PMID   21742802.