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Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry☆
Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry☆ EW Robinson, Pacific Northwest National Laboratory, Richland, WA, United States ã 2017 Elsevier Ltd. All rights reserved.
FT-ICR MALDI MS
Abbreviations ESI ETD FID
Electrospray ionization Electron transfer dissociation Free induction decay
Introduction Melvin B. Comisarow and Alan G. Marshall published a series of papers in 1974 first describing and reporting results of Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS). FT-ICR MS to date has the highest mass measurement accuracy and resolution of any mass spectrometry platform. In recent years, Orbitrap mass spectrometers have narrowed the performance gap considerably relative to other mass spectrometers. This is in part due to the many similarities of Orbitrap and FT-ICR mass spectrometers and Fouriertransform mass spectrometry (FTMS) as terminology has come to refer to both of these high-resolution methods. Occasionally, it has been stated that FT-ICR MS was always going to be the highest performing mass spectrometer. Such statements have been made for a variety of justifications, including that fundamentally FT-ICR MS is about measuring frequencies, a measurement that can be performed with unparalleled precision and accuracy, and FT-ICR MS is inherently a multichannel detection method, meaning all possible ion frequencies are detected simultaneously. There is no scanning across an ion mass-to-charge ratio (m/z) range; all ions are detected at the same time. Thus, the superior FT-ICR MS analytical performance results from the physics and analytical method foundation of the technique.
Fourier-transform ion cyclotron resonance Matrix-assisted laser desorption ionization Mass spectrometry
Under suitable conditions of magnetic field and ion mass, charge, and velocity, the trajectory of the ion is bent into a circle, or cyclotron orbital. The rate at which the ion circles around, or the ion cyclotron frequency, is determined by the magnetic field, ion charge, and ion mass. Once the ion begins the circular cyclotron motion, barring external/additional forces, the ion will remain on the page indefinitely. In practice, this type of spatial confinement is known as ion trapping. In most FT-ICR MS applications, the ions actually enter the magnetic field in the same direction of the magnetic field, or in terms of our previous example, into the page. The ion will still undergo cyclotron motion, due to the components of ion motion in the plane perpendicular to the magnetic field, for example, the plane of the page. The combination of ion motion into the page and cyclotron motion results in the ion precessing around its axis of motion, into the page, at the ion cyclotron frequency which is proportional to the magnetic field and ion mass to charge ratio, m/z. An additional electric field is required to confine the ions in the z-axis, as the magnetic field confines the ions only in the xy-plane. Fig. 2 illustrates one type of ICR trapping cell geometry and
Theory Fundamentally, FT-ICR MS measures the frequency of ion motion in a magnetic field, known as cyclotron motion. Movement of ions in a magnetic field generates a radial component of force, which causes a deflection in the direction of ion motion. Assuming a magnetic field going into this page, the trajectory of a charged ion moving across the page will bend due to forces arising from the magnetic field interacting with the charged particle (Fig. 1). The force bending the ion motion is known as the Lorentz force. As the ion is still moving, the Lorentz force continues to bend the direction of ion travel. ☆ Change History: April 2016. DW Koppenaal updated text throughout. Errol W. Robinson, Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS), In Encyclopedia of Spectroscopy and Spectrometry (Second Edition), edited by John C. Lindon, Academic Press, Oxford, 2010, Pages 714-719
Encyclopedia of Spectroscopy and Spectrometry, Third Edition
Fig. 1 By convention the magnetic field is defined to align with the z-axis. An ion moving through a magnetic field experiences a force, called the Lorentz force, which bends the trajectory of the ion. Under suitable conditions the trajectory of the ion will bend into a circle. Note, as illustrated, that the velocity, or kinetic energy, of the ion does not change the cyclotron frequency.
http://dx.doi.org/10.1016/B978-0-12-409547-2.12676-9
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Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry
the use of trapping plates to provide the required electric field to trap ions. Typically, when the ions enter the magnetic field, the resulting ion cyclotron motion is a relatively tight circle. If multiple ions enter the magnetic field, even if the ions have the same mass and charge, the cyclotron motion of the ions will not be correlated; that is, the ions will not be in-phase. Owing to these two factors, the cyclotron frequency of the ions cannot be measured. What is needed is coherent ion motion, that is, inphase, in a cyclotron orbital with a relatively large radius. Both of these needs are satisfied by uniformly increasing the ion kinetic energy, a process known as excitation. As the frequency of ion motion is independent of ion energy, increasing the ion kinetic energy increases the ion cyclotron orbital radius without changing the ion cyclotron frequency (Fig. 1). In practice, ions are coherently excited by applying an RF electric field, or excitation waveform, between two metal plates (Fig. 2). If the applied excitation waveform has a constant magnitude, independent of frequency, then the
ions will all be excited to the same cyclotron orbital radius. The result of this excitation is that ions with the same m/z circle around as a coherent packet with an increased cyclotron orbital radius. The ion cyclotron frequency of the excited coherent ion packet is determined by measuring the image current that the packet of ions induces ina metal plate as the ions circle past the plate (Fig. 2). This image current is recorded as a function of time and is known as the transient or free induction decay (FID). A Fourier-transform of this recorded time series of image current corresponds to the number of ions rotating past the plate as a function of frequency. Simple calibration functions then convert the frequencies to values of m/z. The resolution of the peaks in FT-ICR mass spectra is directly related to the length of time for which the image current is recorded after ion excitation. The frequency at which the image current must be sampled is determined by the minimum m/z (fastest cyclotron frequency). This is based on known sampling constraints to correctly determine frequency.
Instrumentation Significant improvements in mass spectrometry instrumentation have occurred since the first commercial FT-ICR MS was introduced by Nicolet Instruments in 1980. An FT-ICR mass spectrometer consists of an intricate and precise collection of components, which will be discussed as in functional subsystems (Figs. 3 and 4). Improvements in one area can have a cumulative improvement effect or present additional challenges. For example, improvements in magnetic field shielding have allowed for the use of turbomolecular vacuum pumps, whereas overall high magnetic fields have introduced
Fig. 2 The top illustration is a schematic of a cylindrical ICR trapping cell. Note the cylindrical trapping plates are used to confine ions in the z-axis (the magnetic field confines ions in the xy-plane). The middle drawing is a rendering of the PNNL compensated cylindrical ICR trapping cell with segmented capacitive coupled trapping (rendering courtesy of James Ewing, EMSL). The bottom photo is the same PNNL compensated ICR trapping cell rendered above. The photo reveals the cell wiring, insulation, and titanium support structure that holds the cell plates in position. Typically, only nonmagnetic, ultrahigh vacuum compatible components are used in ICR trapping cell construction.
Fig. 3 A typical commercial 12 T FT-ICR mass spectrometer utilizing active shielding and a refrigerated cryogenic system to reduce the physical and magnetic footprints and the overall system cost. (photo courtesy of Bruker Daltonics and used with permission)
Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry
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Fig. 4 Diagrammed here are some of the major components typically used to generate and transmit ions through the FT-ICR mass spectrometer. Many alternative designs have been implemented with some variations noted in the text. Electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) are illustrated though many other ionization methods have been successfully implemented with FT-ICR MS. The illustration also demonstrates the use of an ion funnel, though historically many systems have utilized a nozzle skimmer. After the ion funnel, ions travel through a series of differentially pumped regions until they pass through a selection quadrupole and are trapped in an external accumulation multipole. The selection and accumulation regions can be operated in several modes, some of which are further illustrated in Fig. 6. Ion trajectories during injection to the ICR trapping cell from the external accumulation multipole are typically guided using either RF multipole ion guides or high-voltage ion optics to maximize ion transfer into the ICR trapping cell. After excitation of the trapped ions, the induced image current on the detection plates is first amplified and then passes through a series of signal processing steps, resulting in mass spectra.
additional challenges in ion transfer. Overall, significant progress continues to be made in every generation of FT-ICR MS platforms. Some improvements have led to new analytical methods that have found broad application in many areas of mass spectrometry. Electron capture dissociation (ECD) developed exclusively in FT-ICR MS led to electron transfer dissociation (ETD), which has been more broadly applied in MS, as just one example. All FT-ICR mass spectrometers require a homogenous magnetic field in which to trap ions and generate the ion cyclotron motion of the ions (Fig. 3). As many of the fundamental properties of FT-ICR MS directly relate to the magnetic field, improvements in magnetic field homogeneity and increases in magnetic field have had an immediate impact on the capabilities of FT-ICR MS. The majority of high-performance FT-ICR mass spectrometers utilize superconducting magnets to achieve high magnetic fields (some over 15 T) and uniform magnetic fields. As an exception to this trend are the resistive and hybrid resistive/superconducting magnets at the National High Magnetic Field Laboratory in Florida. The advances in magnet technology are well demonstrated by the FT-ICR mass spectrometer in Fig. 3, which has a 12 T magnet with active shielding and cryogen refrigeration systems that significantly reduce the weight and footprint of the magnet. Additionally, the active shielding of the magnetic field reduces the stray magnetic field outside of the magnet housing. This is accomplished by superconducting shield coils surrounding the magnet, which produce magnetic fields to actively counteract the magnetic field of the main superconducting coil. Recently, ultra high field FT-ICR MS systems have been developed that incorporate 21 T magnets and the latest in ion accumulation and focusing technology in their spectrometer designs. These systems, at the National High Magnetic Field Laboratory in Tallahassee FL and at EMSL/Pacific Northwest National Laboratory in Richland WA, are both available to the scientific community as NSF and DOE user facilities respectively. The higher fields provide a number of analytical advantages, most notably higher mass resolution for very complex environmental, biological, or energy-related samples.
The majority of the ion path in FT-ICR MS occurs under vacuum conditions (Fig. 4), and extensive vacuum chambers and differential pumping schemes are required to achieve suitable vacuum pressures. The pressure within a vacuum chamber has a significant impact on the performance of the FT-ICR instrument. For example, the length of time that an excited ion packet will remain coherent is related to the pressure inside the ion-trapping cell. The lower the pressure, the fewer the collisions per unit of time and the longer the useful signal information that can be obtained from the FID. Vacuum improvements have enabled increases in resolution as transient lengths have increased due to the reduced vacuum chamber pressures. Some care must be given to the design and maintenance of vacuum systems as different regions of the FT-ICR require different vacuum pressures for optimal performance. Commercial FT-ICR MS platforms employ turbomolecular pumps to achieve the required ultrahigh vacuum. Custom FT-ICR systems can be found with diffusion and cryo-pumps. As FT-ICR MS, like mass spectrometry in general, analyzes charged atoms or molecules, the target analyte must be ionized and introduced into the mass spectrometer. Many ionization methods have been developed, which tend to perform better for certain analyte classes than for others. Thus, the nature of the analysis dictates the type of ionization method utilized. Ionization methods include electron impact (EI), electrospray ionization (ESI), atmospheric pressure ionization (API), atmospheric pressure chemical ionization (APCI), matrix-assisted laser desorption ionization (MALDI), and others. The development of ionization methods enables the analysis of new classes of analytes or analysis with new MS experimental procedures. For example, the development of ESI and MALDI ionization methods for biomolecules played a significant role in a number of FT-ICR MS publications and citations (Fig. 5). Note the rapid and sustained publication rate from the early 1990s, which corresponds to the development of ESI and MALDI techniques. The source region of the FT-ICR mass spectrometer is critical as ions are prepared for injection into the ICR trapping cell (Fig. 4). This region has the greatest number of differences
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Fig. 5 The number of publications (left axis, blue curve) and citations (right axis, green curve) per year for publications with Fourier-transform ion cyclotron resonance or FT-ICR MS as the search criteria. Note the rapid rise in publications starting in the early 1990s.
between FT-ICR MS instruments. Significant improvements and innovations have been made regarding ion selection, reaction, and transmission. For example, ion funnel technologies have been demonstrated to significantly increase the ion current transmitted through the source region (Fig. 4). Further, a significant number of FT-ICR mass spectrometers utilize an external trapping multipole to enhance analytical sensitivity. Ions of particular analytical interest can then be selectively accumulated by isolating the ion of interest using a selection quadrupole before the accumulation multipole (Fig. 6). Note that some implementations combine the selection and accumulation regions into a single device, such as a linear ion trap. An alternative method, DREAMS, selectively ejects the most abundant ions and then accumulates the less abundant ions in the external accumulation multipole, which extends the dynamic range of FT-ICR MS measurements. The DREAMS method is particularly useful in the analysis of samples with a significant range of analyte concentrations, as DREAMS aids in removing highly abundant analytes that can obscure less abundant analytes of interest (Fig. 6). An important feature of ion accumulation is the possibility of dynamically modulating the time or flux of ions into the external accumulation multipole as a means to regulate the total number of ions transferred to the analytic ICR trapping cell. This type of regulation can significantly improve the mass measurement accuracy when utilizing a dynamic ion source. For example, such dynamic control of ion population has proven beneficial in liquid chromatography (LC) ESI FT-ICR MS analysis. Accumulated ions, with or without isolation, can then be fragmented by either collisional activation methods (CAD, CID, etc.) or chemical reaction, like ETD. Also external ion accumulation can be performed during the analysis of ions in the ICR trapping cell, which improves the overall duty cycle. External accumulation multipoles vary from relatively simple quadrupole and hexapole ion traps to sophisticated linear ion traps in so-called hybrid instruments. Multipole ion guides or high-voltage ion optics are typically used to control the transmission of ions from the external accumulation multipole to the ICR trapping cell due to
Fig. 6 The top diagram illustrates ion transmission with no active selection of ions. The most abundant ions, represented by blue and green circles, dominate the ion population of the external accumulation multipole. The middle diagram illustrates the selective accumulation strategy. Here ions of a known m/z (yellow circles) are transmitted through the selection quadrupole, whereas all other ions are ejected. This results in the accumulation of only the selected ions in the external accumulation multipole. Frequently, the selected ions are then subjected to tandem mass spectrometry methods. The bottom diagram illustrates the DREAMS method to extend the dynamic range of an analysis by selectively ejecting only the most abundant ions, which enables the accumulation of lower abundant ions (yellow and brown circles) and even enables the accumulation of ions not observed in nonselective accumulation (red circles).
magnetic fringe fields, which can deflect or even reflect ions (magnetic mirror effect) en route to the ICR trapping cell. This process of ion transmission into the ICR trapping cell is also referred to as ion injection. Improving this ion injection process has also been an active area research. These efforts have focused on improving the overall ion transmission, minimizing the ion kinetic energy, and minimizing spread of ions with differing m/z due to time-of-flight effects between the external accumulation source and the ICR trapping cell, that is, maximizing the m/z range of ions, which can be efficiently trapped in the ICR cell and sampled in a mass spectra. The heart of FT-ICR MS is the ion-trapping cell (Figs. 2 and 4). Situated inside the magnet at ultrahigh vacuum, the ICR trapping cell is where the analytical measurements are performed. Although the details of ICR trapping cell design continue to be refined for additional analytical enhancements, the fundamental aspects of ICR trapping cell design have remained unchanged. The consistency of these fundamental aspects of ICR trapping cell design have led some to state that the construction of ICR trapping cells is straightforward and
Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry
further design is unnecessary. This is an oversimplification that is similar to stating that the fundamental design aspects of making a car are well known and that further design is unneeded. Just like cars need only four wheels and a motor, the required components of an ICR trapping cell are rather simple. As the magnetic field confines ion motion in two dimensions, all that is needed to confine the ions to a specific region is an additional electric field applied to confine the ions in the remaining dimension, typically referred to as the z-axis with the magnetic field confining ions in the xy-plane. As electric fields for ion trapping are not needed in the xy-plane, a pair of excitation plates for applying the RF excitation waveform to coherently excite the ions and a pair of detection plates to measure the ion cyclotron frequency are placed in the xyplane. The mass measurement accuracy, sensitivity, and resolution of an ICR trapping cell depend on the quality of design and construction, like the performance of cars depends on these factors. FT-ICR MS also utilizes a collection of electrical components to acquire mass spectra. Specialized operations generate the appropriate excitation waveforms, which are then amplified to the required amplitude. The image current of the excited ions on the detection plates is amplified, filtered, and then passed to digital-to-analog converters. The acquired data is then typically transferred and stored on a computer where a fast Fourier transform (FFT) is performed followed by application of a calibration function to convert the measured ion cyclotron frequencies to m/z. Technical advancements in the electronic and computer systems have enabled significant reductions in noise and increases in the rate, quality, and total quantity of data collected during FT-ICR mass spectra acquisition.
Applications FT-ICR MS can be utilized in many applications, particularly those that have stringent requirements for mass measurement accuracy and resolution, the strengths of FT-ICR MS. For example, owing to the resolution and mass measurement accuracy of FT-ICR MS, the elemental composition of small molecules can be determined by a mass measurement alone. For larger molecules, such as proteins, tandem mass spectrometry methods can be utilized to provide additional information to identify and characterize the larger molecules. Indeed, the analysis of complex mixtures, such as top-down proteomics, is greatly aided by capabilities of FT-ICR MS. Bottom-up proteomics strategies, such as the accurate mass and time (AMT) tag approach, also benefit from the use of FT-ICR MS. The
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analysis of complex polymer distributions is another application in which FT-ICR MS has found application. The analysis of crude oil, perhaps the most complex polymer solution, has been performed utilizing FT-ICR MS in procedures known as petroleomics. Some applications take advantage of the ion trapping combined with the capabilities of FT-ICR MS to perform ion kinetic measurements, ion–ion and ion-neutral reactions, determination of molecular properties, and others.
See also: Electrospray Ionization in Mass Spectrometry; Fragmentation in Mass Spectrometry; Ion Trap Mass Spectrometers; MS–MS and MSn; Peptides and Proteins Studied Using Mass Spectrometry; Proteomics, Top-Down; Proteomics; Time of Flight Mass Spectrometers.
Further Reading Beu SC and Laude DA (1992) Open Trapped Ion Cell Geometries for Fourier-Transform Ion-Cyclotron Mass Spectrometry. Int. J. Mass Spectrom. Ion Process. 112: 215–230. Campbell S, Rodgers MT, Marzluff EM, et al. (1995) Deuterium Exchange Reactions as a Probe of Biomolecule Structure. Fundamental Studies of Gas Phase H/D Exchange Reactions of Protonated Glycine Oligomers With D2O, CD3OD, CD3CO2D, and ND3. J. Am. Chem. Soc. 117: 12840–12854. Comisarow MB and Marshall AG (1974) Fourier Transform Ion Cyclotron Resonance Spectroscopy. Chem. Phys. Lett. 25: 282–283. Donald WA, Leib RD, O’Brien JT, et al. (2009) Directly Relating Gas-Phase Cluster Measurements to Solution-Phase Hydrolysis, the Absolute Standard Hydrogen Electrode Potential, and the Absolute Proton Solvation. Chemistry 15: 5926–5934. Hendrickson CL, Quinn JP, Kaiser NK, Smith DF, Blakney GT, Chen T, Marshall AG, Weisbrod CR, and Beu SC (2015) 21 Tesla Fourier Transform Ion Cyclotron Resonance Mass Spectrometer: A National Resource for Ultrahigh Resolution Mass Analysis. J. Am. Soc. Mass Spectrom. 26: 1626–1632. Marshall AG, Hendrickson CL, and Jackson GS (1998) Fourier Transform Ion Cyclotron Resonance Mass Spectrometry: A Primer. Mass Spectrom. Rev. 17: 1–35. Marshall AG and Hendrickson CL (2008) High-Resolution Mass Spectrometers. Annu. Rev. Anal. Chem. 1: 579–599. McLafferty FW, Horn DM, Breuker K, et al. (2001) Electron Capture Dissociation of Gaseous Multiply Charged Ions by Fourier-Transform Ion Cyclotron Resonance. J. Am. Soc. Mass Spectrom. 12: 245–249. Pas a-Tolic´ L, Harkewicz R, Anderson GA, et al. (2002) Increased Proteome Coverage for Quantitative Peptide Abundance Measurements Based Upon High Performance Separations and DREAMS FTICR Mass Spectrometry. J. Am. Soc. Mass Spectrom. 13: 954–963. Senko MW, Hendrickson CL, Emmett MR, et al. (1997) External Accumulation of Ions for Enhanced Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry. J. Am. Soc. Mass Spectrom. 8: 970–976. Smith RD, Anderson GA, Lipton MS, et al. (2002) An Accurate Mass Tag Strategy for Quantitative and High-Throughput Proteome Measurements. Proteomics 2: 513–523. Tolmachev AV, Robinson EW, Wu S, et al. (2008) Trapped-Ion Cell With Improved DC Potential Harmonicity for FT-ICR MS. J. Am. Soc. Mass Spectrom. 19: 586–597. Zubarev RA (2004) Electron-Capture Dissociation Tandem Mass Spectrometry. Curr. Opin. Biotechnol. 15: 12–16.
FT-ICR MALDI MS
Abbreviations ESI ETD FID
Electrospray ionization Electron transfer dissociation Free induction decay
Introduction Melvin B. Comisarow and Alan G. Marshall published a series of papers in 1974 first describing and reporting results of Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS). FT-ICR MS to date has the highest mass measurement accuracy and resolution of any mass spectrometry platform. In recent years, Orbitrap mass spectrometers have narrowed the performance gap considerably relative to other mass spectrometers. This is in part due to the many similarities of Orbitrap and FT-ICR mass spectrometers and Fouriertransform mass spectrometry (FTMS) as terminology has come to refer to both of these high-resolution methods. Occasionally, it has been stated that FT-ICR MS was always going to be the highest performing mass spectrometer. Such statements have been made for a variety of justifications, including that fundamentally FT-ICR MS is about measuring frequencies, a measurement that can be performed with unparalleled precision and accuracy, and FT-ICR MS is inherently a multichannel detection method, meaning all possible ion frequencies are detected simultaneously. There is no scanning across an ion mass-to-charge ratio (m/z) range; all ions are detected at the same time. Thus, the superior FT-ICR MS analytical performance results from the physics and analytical method foundation of the technique.
Fourier-transform ion cyclotron resonance Matrix-assisted laser desorption ionization Mass spectrometry
Under suitable conditions of magnetic field and ion mass, charge, and velocity, the trajectory of the ion is bent into a circle, or cyclotron orbital. The rate at which the ion circles around, or the ion cyclotron frequency, is determined by the magnetic field, ion charge, and ion mass. Once the ion begins the circular cyclotron motion, barring external/additional forces, the ion will remain on the page indefinitely. In practice, this type of spatial confinement is known as ion trapping. In most FT-ICR MS applications, the ions actually enter the magnetic field in the same direction of the magnetic field, or in terms of our previous example, into the page. The ion will still undergo cyclotron motion, due to the components of ion motion in the plane perpendicular to the magnetic field, for example, the plane of the page. The combination of ion motion into the page and cyclotron motion results in the ion precessing around its axis of motion, into the page, at the ion cyclotron frequency which is proportional to the magnetic field and ion mass to charge ratio, m/z. An additional electric field is required to confine the ions in the z-axis, as the magnetic field confines the ions only in the xy-plane. Fig. 2 illustrates one type of ICR trapping cell geometry and
Theory Fundamentally, FT-ICR MS measures the frequency of ion motion in a magnetic field, known as cyclotron motion. Movement of ions in a magnetic field generates a radial component of force, which causes a deflection in the direction of ion motion. Assuming a magnetic field going into this page, the trajectory of a charged ion moving across the page will bend due to forces arising from the magnetic field interacting with the charged particle (Fig. 1). The force bending the ion motion is known as the Lorentz force. As the ion is still moving, the Lorentz force continues to bend the direction of ion travel. ☆ Change History: April 2016. DW Koppenaal updated text throughout. Errol W. Robinson, Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS), In Encyclopedia of Spectroscopy and Spectrometry (Second Edition), edited by John C. Lindon, Academic Press, Oxford, 2010, Pages 714-719
Encyclopedia of Spectroscopy and Spectrometry, Third Edition
Fig. 1 By convention the magnetic field is defined to align with the z-axis. An ion moving through a magnetic field experiences a force, called the Lorentz force, which bends the trajectory of the ion. Under suitable conditions the trajectory of the ion will bend into a circle. Note, as illustrated, that the velocity, or kinetic energy, of the ion does not change the cyclotron frequency.
http://dx.doi.org/10.1016/B978-0-12-409547-2.12676-9
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the use of trapping plates to provide the required electric field to trap ions. Typically, when the ions enter the magnetic field, the resulting ion cyclotron motion is a relatively tight circle. If multiple ions enter the magnetic field, even if the ions have the same mass and charge, the cyclotron motion of the ions will not be correlated; that is, the ions will not be in-phase. Owing to these two factors, the cyclotron frequency of the ions cannot be measured. What is needed is coherent ion motion, that is, inphase, in a cyclotron orbital with a relatively large radius. Both of these needs are satisfied by uniformly increasing the ion kinetic energy, a process known as excitation. As the frequency of ion motion is independent of ion energy, increasing the ion kinetic energy increases the ion cyclotron orbital radius without changing the ion cyclotron frequency (Fig. 1). In practice, ions are coherently excited by applying an RF electric field, or excitation waveform, between two metal plates (Fig. 2). If the applied excitation waveform has a constant magnitude, independent of frequency, then the
ions will all be excited to the same cyclotron orbital radius. The result of this excitation is that ions with the same m/z circle around as a coherent packet with an increased cyclotron orbital radius. The ion cyclotron frequency of the excited coherent ion packet is determined by measuring the image current that the packet of ions induces ina metal plate as the ions circle past the plate (Fig. 2). This image current is recorded as a function of time and is known as the transient or free induction decay (FID). A Fourier-transform of this recorded time series of image current corresponds to the number of ions rotating past the plate as a function of frequency. Simple calibration functions then convert the frequencies to values of m/z. The resolution of the peaks in FT-ICR mass spectra is directly related to the length of time for which the image current is recorded after ion excitation. The frequency at which the image current must be sampled is determined by the minimum m/z (fastest cyclotron frequency). This is based on known sampling constraints to correctly determine frequency.
Instrumentation Significant improvements in mass spectrometry instrumentation have occurred since the first commercial FT-ICR MS was introduced by Nicolet Instruments in 1980. An FT-ICR mass spectrometer consists of an intricate and precise collection of components, which will be discussed as in functional subsystems (Figs. 3 and 4). Improvements in one area can have a cumulative improvement effect or present additional challenges. For example, improvements in magnetic field shielding have allowed for the use of turbomolecular vacuum pumps, whereas overall high magnetic fields have introduced
Fig. 2 The top illustration is a schematic of a cylindrical ICR trapping cell. Note the cylindrical trapping plates are used to confine ions in the z-axis (the magnetic field confines ions in the xy-plane). The middle drawing is a rendering of the PNNL compensated cylindrical ICR trapping cell with segmented capacitive coupled trapping (rendering courtesy of James Ewing, EMSL). The bottom photo is the same PNNL compensated ICR trapping cell rendered above. The photo reveals the cell wiring, insulation, and titanium support structure that holds the cell plates in position. Typically, only nonmagnetic, ultrahigh vacuum compatible components are used in ICR trapping cell construction.
Fig. 3 A typical commercial 12 T FT-ICR mass spectrometer utilizing active shielding and a refrigerated cryogenic system to reduce the physical and magnetic footprints and the overall system cost. (photo courtesy of Bruker Daltonics and used with permission)
Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry
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Fig. 4 Diagrammed here are some of the major components typically used to generate and transmit ions through the FT-ICR mass spectrometer. Many alternative designs have been implemented with some variations noted in the text. Electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) are illustrated though many other ionization methods have been successfully implemented with FT-ICR MS. The illustration also demonstrates the use of an ion funnel, though historically many systems have utilized a nozzle skimmer. After the ion funnel, ions travel through a series of differentially pumped regions until they pass through a selection quadrupole and are trapped in an external accumulation multipole. The selection and accumulation regions can be operated in several modes, some of which are further illustrated in Fig. 6. Ion trajectories during injection to the ICR trapping cell from the external accumulation multipole are typically guided using either RF multipole ion guides or high-voltage ion optics to maximize ion transfer into the ICR trapping cell. After excitation of the trapped ions, the induced image current on the detection plates is first amplified and then passes through a series of signal processing steps, resulting in mass spectra.
additional challenges in ion transfer. Overall, significant progress continues to be made in every generation of FT-ICR MS platforms. Some improvements have led to new analytical methods that have found broad application in many areas of mass spectrometry. Electron capture dissociation (ECD) developed exclusively in FT-ICR MS led to electron transfer dissociation (ETD), which has been more broadly applied in MS, as just one example. All FT-ICR mass spectrometers require a homogenous magnetic field in which to trap ions and generate the ion cyclotron motion of the ions (Fig. 3). As many of the fundamental properties of FT-ICR MS directly relate to the magnetic field, improvements in magnetic field homogeneity and increases in magnetic field have had an immediate impact on the capabilities of FT-ICR MS. The majority of high-performance FT-ICR mass spectrometers utilize superconducting magnets to achieve high magnetic fields (some over 15 T) and uniform magnetic fields. As an exception to this trend are the resistive and hybrid resistive/superconducting magnets at the National High Magnetic Field Laboratory in Florida. The advances in magnet technology are well demonstrated by the FT-ICR mass spectrometer in Fig. 3, which has a 12 T magnet with active shielding and cryogen refrigeration systems that significantly reduce the weight and footprint of the magnet. Additionally, the active shielding of the magnetic field reduces the stray magnetic field outside of the magnet housing. This is accomplished by superconducting shield coils surrounding the magnet, which produce magnetic fields to actively counteract the magnetic field of the main superconducting coil. Recently, ultra high field FT-ICR MS systems have been developed that incorporate 21 T magnets and the latest in ion accumulation and focusing technology in their spectrometer designs. These systems, at the National High Magnetic Field Laboratory in Tallahassee FL and at EMSL/Pacific Northwest National Laboratory in Richland WA, are both available to the scientific community as NSF and DOE user facilities respectively. The higher fields provide a number of analytical advantages, most notably higher mass resolution for very complex environmental, biological, or energy-related samples.
The majority of the ion path in FT-ICR MS occurs under vacuum conditions (Fig. 4), and extensive vacuum chambers and differential pumping schemes are required to achieve suitable vacuum pressures. The pressure within a vacuum chamber has a significant impact on the performance of the FT-ICR instrument. For example, the length of time that an excited ion packet will remain coherent is related to the pressure inside the ion-trapping cell. The lower the pressure, the fewer the collisions per unit of time and the longer the useful signal information that can be obtained from the FID. Vacuum improvements have enabled increases in resolution as transient lengths have increased due to the reduced vacuum chamber pressures. Some care must be given to the design and maintenance of vacuum systems as different regions of the FT-ICR require different vacuum pressures for optimal performance. Commercial FT-ICR MS platforms employ turbomolecular pumps to achieve the required ultrahigh vacuum. Custom FT-ICR systems can be found with diffusion and cryo-pumps. As FT-ICR MS, like mass spectrometry in general, analyzes charged atoms or molecules, the target analyte must be ionized and introduced into the mass spectrometer. Many ionization methods have been developed, which tend to perform better for certain analyte classes than for others. Thus, the nature of the analysis dictates the type of ionization method utilized. Ionization methods include electron impact (EI), electrospray ionization (ESI), atmospheric pressure ionization (API), atmospheric pressure chemical ionization (APCI), matrix-assisted laser desorption ionization (MALDI), and others. The development of ionization methods enables the analysis of new classes of analytes or analysis with new MS experimental procedures. For example, the development of ESI and MALDI ionization methods for biomolecules played a significant role in a number of FT-ICR MS publications and citations (Fig. 5). Note the rapid and sustained publication rate from the early 1990s, which corresponds to the development of ESI and MALDI techniques. The source region of the FT-ICR mass spectrometer is critical as ions are prepared for injection into the ICR trapping cell (Fig. 4). This region has the greatest number of differences
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Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry
Fig. 5 The number of publications (left axis, blue curve) and citations (right axis, green curve) per year for publications with Fourier-transform ion cyclotron resonance or FT-ICR MS as the search criteria. Note the rapid rise in publications starting in the early 1990s.
between FT-ICR MS instruments. Significant improvements and innovations have been made regarding ion selection, reaction, and transmission. For example, ion funnel technologies have been demonstrated to significantly increase the ion current transmitted through the source region (Fig. 4). Further, a significant number of FT-ICR mass spectrometers utilize an external trapping multipole to enhance analytical sensitivity. Ions of particular analytical interest can then be selectively accumulated by isolating the ion of interest using a selection quadrupole before the accumulation multipole (Fig. 6). Note that some implementations combine the selection and accumulation regions into a single device, such as a linear ion trap. An alternative method, DREAMS, selectively ejects the most abundant ions and then accumulates the less abundant ions in the external accumulation multipole, which extends the dynamic range of FT-ICR MS measurements. The DREAMS method is particularly useful in the analysis of samples with a significant range of analyte concentrations, as DREAMS aids in removing highly abundant analytes that can obscure less abundant analytes of interest (Fig. 6). An important feature of ion accumulation is the possibility of dynamically modulating the time or flux of ions into the external accumulation multipole as a means to regulate the total number of ions transferred to the analytic ICR trapping cell. This type of regulation can significantly improve the mass measurement accuracy when utilizing a dynamic ion source. For example, such dynamic control of ion population has proven beneficial in liquid chromatography (LC) ESI FT-ICR MS analysis. Accumulated ions, with or without isolation, can then be fragmented by either collisional activation methods (CAD, CID, etc.) or chemical reaction, like ETD. Also external ion accumulation can be performed during the analysis of ions in the ICR trapping cell, which improves the overall duty cycle. External accumulation multipoles vary from relatively simple quadrupole and hexapole ion traps to sophisticated linear ion traps in so-called hybrid instruments. Multipole ion guides or high-voltage ion optics are typically used to control the transmission of ions from the external accumulation multipole to the ICR trapping cell due to
Fig. 6 The top diagram illustrates ion transmission with no active selection of ions. The most abundant ions, represented by blue and green circles, dominate the ion population of the external accumulation multipole. The middle diagram illustrates the selective accumulation strategy. Here ions of a known m/z (yellow circles) are transmitted through the selection quadrupole, whereas all other ions are ejected. This results in the accumulation of only the selected ions in the external accumulation multipole. Frequently, the selected ions are then subjected to tandem mass spectrometry methods. The bottom diagram illustrates the DREAMS method to extend the dynamic range of an analysis by selectively ejecting only the most abundant ions, which enables the accumulation of lower abundant ions (yellow and brown circles) and even enables the accumulation of ions not observed in nonselective accumulation (red circles).
magnetic fringe fields, which can deflect or even reflect ions (magnetic mirror effect) en route to the ICR trapping cell. This process of ion transmission into the ICR trapping cell is also referred to as ion injection. Improving this ion injection process has also been an active area research. These efforts have focused on improving the overall ion transmission, minimizing the ion kinetic energy, and minimizing spread of ions with differing m/z due to time-of-flight effects between the external accumulation source and the ICR trapping cell, that is, maximizing the m/z range of ions, which can be efficiently trapped in the ICR cell and sampled in a mass spectra. The heart of FT-ICR MS is the ion-trapping cell (Figs. 2 and 4). Situated inside the magnet at ultrahigh vacuum, the ICR trapping cell is where the analytical measurements are performed. Although the details of ICR trapping cell design continue to be refined for additional analytical enhancements, the fundamental aspects of ICR trapping cell design have remained unchanged. The consistency of these fundamental aspects of ICR trapping cell design have led some to state that the construction of ICR trapping cells is straightforward and
Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry
further design is unnecessary. This is an oversimplification that is similar to stating that the fundamental design aspects of making a car are well known and that further design is unneeded. Just like cars need only four wheels and a motor, the required components of an ICR trapping cell are rather simple. As the magnetic field confines ion motion in two dimensions, all that is needed to confine the ions to a specific region is an additional electric field applied to confine the ions in the remaining dimension, typically referred to as the z-axis with the magnetic field confining ions in the xy-plane. As electric fields for ion trapping are not needed in the xy-plane, a pair of excitation plates for applying the RF excitation waveform to coherently excite the ions and a pair of detection plates to measure the ion cyclotron frequency are placed in the xyplane. The mass measurement accuracy, sensitivity, and resolution of an ICR trapping cell depend on the quality of design and construction, like the performance of cars depends on these factors. FT-ICR MS also utilizes a collection of electrical components to acquire mass spectra. Specialized operations generate the appropriate excitation waveforms, which are then amplified to the required amplitude. The image current of the excited ions on the detection plates is amplified, filtered, and then passed to digital-to-analog converters. The acquired data is then typically transferred and stored on a computer where a fast Fourier transform (FFT) is performed followed by application of a calibration function to convert the measured ion cyclotron frequencies to m/z. Technical advancements in the electronic and computer systems have enabled significant reductions in noise and increases in the rate, quality, and total quantity of data collected during FT-ICR mass spectra acquisition.
Applications FT-ICR MS can be utilized in many applications, particularly those that have stringent requirements for mass measurement accuracy and resolution, the strengths of FT-ICR MS. For example, owing to the resolution and mass measurement accuracy of FT-ICR MS, the elemental composition of small molecules can be determined by a mass measurement alone. For larger molecules, such as proteins, tandem mass spectrometry methods can be utilized to provide additional information to identify and characterize the larger molecules. Indeed, the analysis of complex mixtures, such as top-down proteomics, is greatly aided by capabilities of FT-ICR MS. Bottom-up proteomics strategies, such as the accurate mass and time (AMT) tag approach, also benefit from the use of FT-ICR MS. The
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analysis of complex polymer distributions is another application in which FT-ICR MS has found application. The analysis of crude oil, perhaps the most complex polymer solution, has been performed utilizing FT-ICR MS in procedures known as petroleomics. Some applications take advantage of the ion trapping combined with the capabilities of FT-ICR MS to perform ion kinetic measurements, ion–ion and ion-neutral reactions, determination of molecular properties, and others.
See also: Electrospray Ionization in Mass Spectrometry; Fragmentation in Mass Spectrometry; Ion Trap Mass Spectrometers; MS–MS and MSn; Peptides and Proteins Studied Using Mass Spectrometry; Proteomics, Top-Down; Proteomics; Time of Flight Mass Spectrometers.
Further Reading Beu SC and Laude DA (1992) Open Trapped Ion Cell Geometries for Fourier-Transform Ion-Cyclotron Mass Spectrometry. Int. J. Mass Spectrom. Ion Process. 112: 215–230. Campbell S, Rodgers MT, Marzluff EM, et al. (1995) Deuterium Exchange Reactions as a Probe of Biomolecule Structure. Fundamental Studies of Gas Phase H/D Exchange Reactions of Protonated Glycine Oligomers With D2O, CD3OD, CD3CO2D, and ND3. J. Am. Chem. Soc. 117: 12840–12854. Comisarow MB and Marshall AG (1974) Fourier Transform Ion Cyclotron Resonance Spectroscopy. Chem. Phys. Lett. 25: 282–283. Donald WA, Leib RD, O’Brien JT, et al. (2009) Directly Relating Gas-Phase Cluster Measurements to Solution-Phase Hydrolysis, the Absolute Standard Hydrogen Electrode Potential, and the Absolute Proton Solvation. Chemistry 15: 5926–5934. Hendrickson CL, Quinn JP, Kaiser NK, Smith DF, Blakney GT, Chen T, Marshall AG, Weisbrod CR, and Beu SC (2015) 21 Tesla Fourier Transform Ion Cyclotron Resonance Mass Spectrometer: A National Resource for Ultrahigh Resolution Mass Analysis. J. Am. Soc. Mass Spectrom. 26: 1626–1632. Marshall AG, Hendrickson CL, and Jackson GS (1998) Fourier Transform Ion Cyclotron Resonance Mass Spectrometry: A Primer. Mass Spectrom. Rev. 17: 1–35. Marshall AG and Hendrickson CL (2008) High-Resolution Mass Spectrometers. Annu. Rev. Anal. Chem. 1: 579–599. McLafferty FW, Horn DM, Breuker K, et al. (2001) Electron Capture Dissociation of Gaseous Multiply Charged Ions by Fourier-Transform Ion Cyclotron Resonance. J. Am. Soc. Mass Spectrom. 12: 245–249. Pas a-Tolic´ L, Harkewicz R, Anderson GA, et al. (2002) Increased Proteome Coverage for Quantitative Peptide Abundance Measurements Based Upon High Performance Separations and DREAMS FTICR Mass Spectrometry. J. Am. Soc. Mass Spectrom. 13: 954–963. Senko MW, Hendrickson CL, Emmett MR, et al. (1997) External Accumulation of Ions for Enhanced Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry. J. Am. Soc. Mass Spectrom. 8: 970–976. Smith RD, Anderson GA, Lipton MS, et al. (2002) An Accurate Mass Tag Strategy for Quantitative and High-Throughput Proteome Measurements. Proteomics 2: 513–523. Tolmachev AV, Robinson EW, Wu S, et al. (2008) Trapped-Ion Cell With Improved DC Potential Harmonicity for FT-ICR MS. J. Am. Soc. Mass Spectrom. 19: 586–597. Zubarev RA (2004) Electron-Capture Dissociation Tandem Mass Spectrometry. Curr. Opin. Biotechnol. 15: 12–16.