The Hows and Whys of Ion Trapping - Analytical Chemistry (ACS

INSTRUMENTATION

The Hows and Whys of Ion Trapping John Allison Richard M. Stepnowski Department of Chemistry Michigan State University East Lansing, Mich. 48824

ion trap (ian trap) n.—a device in which one or more charged, gas phase species (atomic and/or molecular anions and/or cations) can be formed and confined for extended periods of time. Confinement results from the forces exerted on the charged species by electric and/or magnetic fields imposed by the device.

Why trap ions? The localization of a single atomic particle in a vacuum is one of the most fundamental problems of physics and one of enduring interest (1). Working in the gas phase at low pressures, the chemist has the opportunity to study a variety of phenomena, if such species of interest can be contained for a period of time. The facility with which ionic species can be manipulated makes their isolation and containment easier than for neutral species. However, charged species are not of interest just because they can be easily manipulated. The chemistry and spectroscopic study of ionic species are relevant to a number of areas ranging from mass spectrometry (MS) to the chemistry in interstellar media to the development of "atomic clocks" (2). Ions usually are trapped so that their interaction with a second

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entity—an atom, molecule, or photon—may be studied. The technology that has evolved for ion trapping and subsequent manipulation has led to some remarkable feats, such as the trapping and detection of a single ion for weeks and the cooling of ions to temperatures well below 1 K. A number of reviews are available on ion traps and on the specific areas in which they are used (e.g., high-resolution atomic spectroscopy and atomic physics) (3-5). In this article, the bases for the prominent ion-trapping schemes will be presented. Once ions are trapped, a number of manipulations such as heating, cooling, and selection may be performed. Strengths and limitations of some types of ion traps will be discussed, and approaches for the analysis of ions in a trap will be reviewed. Finally, examples in which 0003-2700/87/A359-1072/$01.50/0 © 1987 American Chemical Society

ion trapping has been successful— ranging from fundamental physical measurements to applications in chem istry and MS—will be provided. As a benchmark, consider the ion source of a mass spectrometer. Al though mass spectrometers are de signed to study ionic species, conven tional ion sources typically are not de signed to contain ions for any period of time. Typical residence times (from ion formation to extraction) are about 10 - 5 -10~ 6 s (6). In contrast, ion traps have been used to trap ions for weeks. How to trap ions Ion traps typically are classified as ac tive or passive. The classification re flects whether the fields used to con fine the ions are time-dependent (ac tive) or time-independent (passive). We will first focus on the mechanisms of trapping for the various devices that have been reported. Although a vari ety of ionization methods have been used in conjunction with ion traps, by far the most common is electron im pact ionization (EI). Unless otherwise stated, it may be assumed that EI is

used to form ions in the experiments discussed. Active traps The quadrupole ion storage trap (quistor). The quistor or Paul trap (5), first described in 1953, falls into the same family as, and is a "closed form" of, the quadrupole mass filter (7). The quistor (7, 8) is a small "ion bottle" that has a geometry described as a hyperboloid of one sheet com bined with a hyperboloid of two sheets (9)—it is composed of a ring electrode and two end caps (see Figure la). Elec trons can be injected through a hole in either the end cap (9) or in the ring electrode (10). In the quistor, the end caps are elec trically connected, and dc/rf potentials are applied between them and the ring electrode. The applied voltage takes

the form [U + V cos fit], where U is the dc voltage; the rf voltage has amplitude V and radial frequency Ω. The equations of motion for an ion in the trap with a certain mass-to-charge ratio (m/z) are derived from Newton's Law (F = ma), giving differential equa tions known as the Mathieu equations. The Mathieu equations indicate the time dependence of the forces experi enced by the ion as a function of the applied voltages, physical dimensions, and m/z of the ion. Ionizing electrons are injected into the device, forming ions within the quistor volume. The ions move in re sponse to the forces exerted on them by the time-dependent electric fields within the device. Thus at one time an ion may be moving toward one side of the ring electrode, and at another time it will be moving away, as the oscillat ing component of the field changes sign. If amplitudes and frequencies are properly chosen, the ion will be trapped (11). For ion trapping to occur, the mo tions of the ions should be such that the coordinates at any time do not exceed

the dimensions of the trap. Ions will follow stable trajectories for certain values of the applied dc and rf poten tials. Values for these potentials can be chosen such that only one m/z value has a stable trajectory for all times in all three dimensions, or such that all m/z values are trapped. This has direct analogy to the two modes in which quadrupole mass filters may be used (7, 12), as mass filters or in the "rf-only" mode. An ion with a stable trajectory follows an aperiodic orbit about the center of the field. If there is no dc component to the applied field, the trap ideally will store all ions formed. In fact, the quistor cannot be operated in a mode in which ions of all possible m/z values are trapped (7). Recent ad vances, however, include the use of a relatively high pressure of helium to extend the mass range of this ion trap.

The He damps the ion motion, increas ing the concentration of ions in the cen ter of the trap and trapping higher mass ions than would be possible with out this buffer gas (7). The Mathieu equations lead to two frequency components that character ize motion in the rf trap (13). These frequencies, at which the ions oscillate in the trap, are m/2-dependent and are frequently in the range of hundreds of kilohertz. Typically, kilovolt rf voltages and small dc voltages are used in rf ion traps. Ion densities (9) can be as high as 3 Χ 106 ions/cm 3 , and practical trap ping times can range from 200 jus to 100 ms (13). Such traps are less than ideal for collision studies (14), because the kinetic energies of the ions are con stantly changing, with a mean value generally of about 1-3 eV. Other active traps. A number of other active ion traps have been de scribed in the literature; many are vari ations (10, 15) of the quadrupole ion trap. Dawson et al. (16) reported the construction of a quadrupole ion trap in which the electrodes were not pre cisely machined, but rather "shaped by hand from a coarse steel mesh." Other attempts to simplify the construction of the quistor-type trap include the use of a cylindrical geometry. The cylindri cal ion trap (see Figure lb) has been well characterized (17) and has been used to store ions for up to 100 ms. Also, ring electrodes with spherical rather than hyperbolic cross sections have been proposed (15). Such simpli fications become necessary when the experiment demands the construction of a small ion trap (submillimeter size) for laser spectroscopy (15). These vari ations from the ideal geometry lead to larger ion losses, that is, shorter attain able trapping times (15). A hexapole ion storage trap or histor (12) also has been described (Figure lc). This trap is formed by three two-sheet hyperboloids, each situated normal to the three cartesian axes. Unrelated to these, a novel type of rf ion trap has been constructed that is essentially a quadrupole mass filter with the four rods curved such that a ring is formed: the "storage ring" or "race track" trap (18) (Figure Id). Here, the ions are confined to a much larger volume by moving around the track. Storage times in excess of 14 min have been reported with this device. Passive traps Trapping in an electron beam. This is a simple technique that can enhance the residence time of ions in a conven tional mass spectrometer EI ion source (19). The space charge of the electron beam forms a potential well or "trap ping field," confining positive ions formed within the electron beam.

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Typically, the electron beam energy is briefly at some value sufficient to cause ionization; it is then decreased to a low value to cause trapping. Trapping can be induced with electron currents of approximately 10 μΑ, with electron en ergies of about 5 V. Although not re quired, magnetic fields frequently are used to collimate the electron beam. Well depths higher than thermal ener gies (3/2 kT = 0.04 eV at 300 K) must be created for trapping. Trapping po tentials of 0.3-0.5 eV are attainable (29), and trapping times on the order of milliseconds have been achieved. Ques tions concerning the possible vibra tional excitation of trapped ions by the trapping electron beam have been ad dressed (19). Devices that are characterized by long residence times and that use a dc field only. The development of a unique ionization gauge, the orbitron, is relevant to the construction of simple ion sources that are characterized by millisecond residence times for the ions contained therein (5, 20). In hot fila ment ion gauges, such as the BayardAlpert gauge, positive ions are formed by EI and are detected. The number of positive ions formed per second reflects the pressure of neutrals present. The positive ion current is given by

drawback in some applications (21). Nonetheless, it is an exceedingly sim ple design for increasing the residence time of ions in a fairly small region of space. Devices that use magnetic (B) and electric (Ë) fields. Early work in the development of the residual gas analyzer known as the omegatron (22) led to the technique of ion cyclotron resonance (ICR) MS (23, 24), which now

has matured to the technique of Fourier transform MS (FT-MS) (6). Some passive ion trap configurations, in which trapping occurs because of the application of dc electric fields (Ë) and magnetic fields (B), are shown in Figures 3-5. Figure 3 shows the original omegatron design (22), and Figures 4 and 5 show a variety of ICR traps (commonly called cells) that have been used to create, store, manipulate, and ana-

Ip = adnle where σ is the ionization cross section, d is the average distance an electron travels, η is the neutral number densi ty, and Ie is the electron emission cur rent. To increase the sensitivity of such a device, one must increase d or Ie or both. There are practical limits to the value of Ie. The orbitron, shown in Figure 2, is designed to increase the electron path. It consists of two cylinders, with a po tential difference of 500 V between them. The outer cylinder has a radius of approximately 15 mm, and the inner cylinder is a wire (0.04-mm radius). When electrons are injected into the field between the two concentric cylin ders, their energies and angular mo menta are such that they cannot hit the outer cylinder, and they rarely hit the small inner cylinder (wire) to which they are attracted. The result is that electron paths of approximately 2500 cm have been obtained, yielding an ion gauge with a low pressure limit that has yet to be determined because of the limitations of vacuum technology (20). This design has been extended as a trap for positive ions (21), which can be used to trap on the order of millisec onds. One such design is the Kingdon trap (5). A disadvantage is that, as they do in the active traps, ions achieve a range of kinetic energies along their trajectories; those that occupy "sta tionary orbits" have average kinetic en ergies of about 2 eV, which will be a

Figure 1. Some active ion trap configurations—defining regions of space where ions are confined because of the application of dc and rf electric fields. (a) Cross section of the quistor (9) showing two end caps and a ring electrode. Electrons may be injected through an orifice placed in either an end cap or in the ring electrode (designated by arrows). Although not required, many quistors have a perforated lower end cap, which allows for ion ejection to an analyzer, A, which may be an ion flux detector or a mass spectrometer. (b) Cylindrical ion trap. This design, which maintains the same end cap and ring electrode configuration and applied electric fields as the quistor, has been evaluated as a simplification of the quistor geometry ( 17). (c) The histor, or hexapole ion trap, is ideally constructed of three two-sheet hyperbolic electrodes situated along the three major axes. Shown here is the basic geometry, with five of the six electrodes displayed. Other designs have been proposed in which the curved electrodes are approximated by plane sheets, with the three pairs of electrodes differentially biased ( 12). (d) The storage ring, or race track trap, consists of a quadrupole mass filter configured as a closed loop. A cutaway view of the general design is shown. Both oval and circular designs have been built ( 18).


Figure 2. Orbitron-type ion trap (21). A view from the top is shown. Two concentric cylinders are used, with the outer cylinder held at a positive potential with respect to the inner cylin der. The inner cylinder is typically a thin wire. Electrons are injected through a small hole in the outer cylinder to form ions. Many ions have initial velocities such that they assume stable orbits about the inner electrode (as shown), leading to relatively long residence times. A second aper ture may be added to the outer electrode, and ions may be pulsed out of the trap to a mass spectrometer for analysis (21).

lyze ions. All use strong magnetic fields, created by conventional and so lenoid electromagnets, for ion confine ment. Similar to this design is the Pen ning trap, which uses the same elec trode configuration as the rf trap but no rf field—only a dc field and a static magnetic field along the ζ axis. For these devices, the mechanism of ion trapping is essentially the same. Formed in the presence of a strong magnetic field, the ions precess about the field lines. For an ion with a veloci ty ν perpendicular to.the magnetic field (characterized by a magnetic flux den sity B), the ions experience a force that is perpendicular to both ν and B, which results in a circular motion. The ion, with mass m and charge ze, moves about the field lines with a characteris tic frequency known as the cyclotron frequency, vt = zeB/2-irm. Note that each ion (each m/z) has a unique vc. This interaction with the magnetic field does not^onfine the ions along the ζ axis (along Ë). DC voltages serve this purpose. Thus the ion containment is attributable to the magnetic field-induced cyclic motion in the presence of an electrostatic potential well. As in quistor traps, the actual motion of the ions in electromagnetic traps is complex and can be characterized by more than one frequency of motion (11). For example, in the Penning trap, the motion of the ions in the trap consists of a harmonic oscillation at a specific frequency, vz. All ions also move in small circles at the cyclotron frequency, vc. There is a third periodic motion in which the center of the circular cyclotron trajectories precess about the z-

axis at an angular frequency vm. In ICR cells, ions also exhibit cyclotron, magnetron, and z-mode oscillations (6). Devices of this type have been most popular in ion trapping for the study of ion/molecule reactions because the ions are not heated by the trapping mechanism. Also, ICR methods have some advantages over other ion traps because the trap construction is much simpler. Two types of trapping modes are used in such devices. Penning traps and single-region ICR cells create regions where ions may be confined. In contrast, much work has been done with multisection ICR cells in which ions slowly move through the device in the presence of an Ë Χ Β field. For example, in a cell 8 cm long, times on the order of milliseconds (24) are re quired for an ion to move from the for mation region to the end of the cell. Thus ICR "drift" cells are character ized by drift times sufficiently long to be useful in experiments that require trapping. Ions can easily be trapped for seconds or longer in such devices. Trapping times depend on a variety of mechanisms for ion losses (6), some of which are pressure-dependent. For ex ample, at a pressure of 1 X 10~8 torr, in an FT-MS instrument using a 4.7 Τ electromagnet, after 13.5 h, 30% of the stable ions initially formed remained in

the trap (6). Detecting events occurring in ion traps Ions usually are trapped to study some phenomenon such as their interaction with atoms, molecules, or photons. The detection of the occurrence of such an event obviously depends on the phe nomenon under study. Optical detection of events If the ion mass does not change, and optical emission from ions in excited states is of interest, conventional spec troscopic methods are used. Quadrupole ion traps (10, 25) and ICR (26) traps have been extremely useful in la ser-induced fluorescence (LIF) studies of ionic species. Ion traps can be used to selectively trap an ion with a particular m/z value for spectroscopic analysis, in contrast to earlier discharge-flow methods (25). Mahan et al. (10, 27) have reported extensive work on LIF of species such as CH + , CD + , N2 + , and BrCN + . Spectroscopic constants and radiative lifetimes have been deter mined for ionic species in this way. Uti lizing rf traps with potential well depths of 19 V, up to 3 Χ 107 ions have been trapped for spectroscopic analy sis. In contrast, LIF of ions trapped in an ICR cell (26) has been performed in which 1 Χ 106 ions are available 13 μ$

Figure 3. An exploded view of the omegatron. Electrons enter and exit the side (trapping) plates through small apertures. An electric field is applied per pendicular to the magnetic field across the stacked plates. The trapping plates are both maintained at a small positive potential for confinement of positive ions. A small collector is mounted off the bottom plate. Ionic motions are stimulated at their cyclotron frequencies by applying an rf voltage across the stacked plates. The radius of orbit of the ions in resonance with the applied field increases until they strike the collector and are detected.


after ion formation. The estimated de tection for LIF of C0+ in the ICR ex periment (26) was 2 Χ 105 ions. LIF has been an extremely useful tool for characterizing ground-state species and for determining radiative decay rates for excited states. Prior to the availability of LIF technology, emission spectroscopy of ions was per formed using a variety of excitation methods. In such experiments radia tive transition rates were difficult to determine because of the excitation processes used (10); many highly excit ed states are initially formed that cas cade to the state of interest, complicat ing measurements of decay rates. For example, early work on the A -* X radi ative decay for CH+ yielded lifetimes ranging from 70 to 630 ns for the (0,0) band. However, in a low-pressure rf quadrupole trap using LIF, the A state can be prepared cleanly and character ized with minimal collisions. Such studies yield unambiguous radiative lifetimes—815 ns for this particular system (10). A major spectroscopic accomplish ment achieved using an rf ion trap is the determination of the lifetime of the first excited state of Li + (ls 1 2s 1 , 3 Si). The dominant decay mode of this 2-

electron atom was not discovered until 1969. Previously it was considered to be a 2-photon emission instead of a singlephoton decay. The theoretical value for the lifetime of this metastable state was 49 s. Using an rf trap to contain Li + formed by EI on an Li atomic beam (28), researchers determined a lifetime of 58.6 ± 12.9 s. Lifetimes that varied by almost 14 orders of magnitude were observed for states Li+ (28). Mass spectrometry analysis In most ion trap experiments, the phe nomenon under study results in one or more ionic products, and mass spectrometric analysis (m/z and abundance in formation) is desired. These include photodissociation reactions (in which detection of the photofragment is de sired), bimolecular ion/molecule reac tions (in which reaction products are of interest), photodetachment studies of anions (in which the rate of disappear ance of the anion is determined), and anion-cation recombination studies (in which consumption of the recombina tion partners must be monitored). Such experiments require ion trapping. In photo-induced processes, because of limited photon fluxes and small ion concentrations, a significant irradia-

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Figure 4. Two types of ICR cells. (a) A thrçe-section ICR cell. As a "drift" cell (24), ions are formed in the source region and, because of the Ε Χ Β field, drift to the right into the analyzer region. Ion motion continues to a collector region (C), where the total ion current is measured. Drift times are on the time scale of milliseconds or greater. A small electric field is maintained across the drift (D) plates. The trapping (T) plates are held at a positive potential for trapping positive ions. As a trapped ion cell, a sequence of events is involved in the experi ment. Ions are formed in and confined to the source region by appropriate selection of voltages during some trapping time. After this time, cell voltages are switched to resemble the drift cell voltages, allow ing the trapped ions to move into the analyzer region for analysis. (b) The Mclver trapped ICR cell. Electrons are injected into the cell along magnetic field lines. Typical voltages applied for positive ion trapping are — 1V on both end (E) plates and on both drift plates (D), with + 1 V on both trapping plates (T). Typical ICR cell cross sections are approximately 1 sq. in.

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tion time is required to induce a per ceptible change in the ions in the trap. In the case of ion/molecule reactions, pressures must be kept low to ensure that bimolecular collisions dominate. At low pressures, low collision frequen cies require long reaction times for chemistry to occur. Three modes of obtaining mass spec tra of the trapped ions have been em ployed. One approach is to use the ion trap as the mass analyzer. A second approach is to eject all of the ions out of the trap into a mass spectrometer; here, the trap is being used as an ion source. A third approach, somewhat in termediate between the first two, is to use a selective ejection, in which ions of increasing m/z are sequentially ejected out of the trap to an ion flux detector, yielding a mass spectrum of the trap's ionic population. Ion traps as mass spectrometers. Active and passive traps have been used as mass spectrometers; the ana lyses are based on the concept of "reso nance." It has been shown that ions in traps move with angular frequencies that are unique for a given m/z value. If an oscillating electric field is applied across the appropriate electrodes of a trap that is equal to the one selected frequency of motion of a particular ion, the ion and applied field are said to be in resonance. Under these conditions the selected ion absorbs energy from the oscillator in the form of kinetic en ergy. If the ion in resonance moves fast er while maintaining a fixed frequency of motion, its radius of orbit must in crease. A number of detection schemes have resulted from these principles. In one mode, the orbital radius of the ion can be increased to a value larger than the dimensions of the trap, lead ing to a collision with the trap elec trodes. The resulting current is then detected (5). In the omegatron (22), ions can be selectively excited at their cyclotron frequencies such that their trajectories spiral out until the ions in resonance hit a collector plate that is close to one of the cell walls. For an ion in resonance with an applied field the radius of motion increases with time, depending on the magnitude of the ap plied field (24). A second approach is to bring ions into resonance with an oscillating field, but rather than ejecting the ion and detecting the current, one can monitor the power lost by the field (4). This approach was used with marginal oscil lator detectors in ICR (23,24). Such an approach has been successful, yielding mass spectral information on ions with concentrations as low as 1-10 ions/cm3. The most successful approach in this category is that of FT-MS, in which ion cyclotron motion is excited and image currents, generated as a result of the motion of the excited ions between two

Figure 5. An FT-MS cell. The Nicolet FT-MS uses highly transmissive etched stainless steel mesh instead of solid plates for the cell walls. Shown here is a representation of the dual cell design used in the Nicolet FT-MS 2000. Two square cells share a common trapping plate in which there is a small hole. This plate separates the vacuum system into two differentially pumped regions. Ions are formed on side A and are moved into side Β for analysis. The differential pumping and small ori fice size allows side Β to be maintained at pres sures in the 1 0 - 8 torr range (6), whereas ions are formed at higher pressures on side A. This design allows for the attainment of QC/FT-MS spectra in which resolution is not degraded by collisional line broadening. As in ICR cells, the plates per pendicular to Β are trapping plates. To obtain FT-MS spectra, one set of plates perpendicular to the χ axis (transmitter plates) is used to excite ionic motions with an rf "chirp." The image cur rents generated in the remaining electrodes per pendicular to the y axis (receiver plates), because of the coherent cyclotron motion of the trapped ions, are then amplified, digitized, stored, and processed.

plates of the trap, are detected. The resultant time-dependent waveform has frequency components with magni tudes that are proportional to the ion abundances. A Fourier transform of this time-dependent signal yields a fre quency spectrum from which a mass spectrum can easily be generated (6). Image currents ("emission") from kinetically excited H20 + ions in a 2.5cm FT-MS cell, using a 4.7 Τ magnetic field, were observed for 51 s at 10 -11 torr and yielded a resolution of 100,000,000. The theoretical high mass limit for an FT-MS equipped with a 2.5-cm cubic cell, with a magnetic field of 13 T, was cited as 950,000 daltons (6). Such resolution and mass range are unmatched in other types of mass spec trometers. Ion traps as mass spectrometer sources. Obviously, this marriage is re quired when electron beam trapping (19) and orbitron-type sources are used. A significant number of experi ments have been reported that use the quistor as an ion source for quadrupole (8,13,14,29) and magnetic sector (30) instruments. Beauchamp et al. (31) re


ported the design of an ICR-like source for a quadrupole mass spectrometer that uses an Ë X Ë field that exhibited residence times on the order of milliseconds. Unlike ICR alone, which requires low pressures for operation, this combination could be used with pressures up to 10 -2 torr. Sequential ion ejection to an ion flux detector. This approach has been used with quistor traps and has only recently been shown to yield reasonable resolution with the development of the Finnigan ion trap. This trap is a variation of the quistor, with a number of significant changes. First, a relatively high pressure (10 -3 torr) of helium is used in the trap. This damps ion motions, allowing ions over a wide range of m/z values to be trapped with an increase in resolution and sensitivity. Also, only rf voltages are used (32). In the quistor, dc and rf voltages are used. Ions are created, stored for some time, and then may be ejected to an electron multiplier detector. This ejection is done by varying one or both of the applied voltages, creating conditions where trajectories of ions of consecutive m/z values become unstable (larger than the trap dimensions). The end cap is perforated, thus ions are made to pass out of the end cap to hit the detector. The mass spectrometric resolution was fairly low for this experiment. In the Finnigan ion trap (32,33), the damping effect of the helium results in much better mass resolution. Here, the amplitude of the rf voltage is ramped. This increases the lower bound of the range of masses that may be trapped. Thus ions become unstable in order of increasing m/z and are ejected toward the external ion flux detector. A number of "scan modes" are being developed for this ion trap that facilitate experiments such as chemical ionization (CI) and collision-induced dissociation (CID). In the CID experiment, a second rf signal is used for resonance excitation of a selected ion, which then undergoes energetic collisions with helium atoms and fragments. Typically, rf frequencies as high as 1.1 MHz are used to trap ions. To obtain a mass spectrum, the rf amplitude is scanned to excite ion motion for ejection and detection. Better than unit resolution has been achieved in the 20-650 u mass range, with scan speeds from 0.5 to 4 s. This ion trap is now routinely used, interfaced to a capillary GC, as a compact, low-priced, high-performance mass spectrometric detector with good sensitivity and a dynamic range of 104. Heating, cooling, and selective ejection

Discussed above are methods in which resonant excitation of ionic motion can lead to ejection and detection. Reso-

nant excitation of various motional fre quencies also can be used to increase the kinetic energy (6) of a selected ion within the trap (i.e., heating, which may or may not lead to actual ejection). For an ionic motion in resonance with an applied field, the kinetic energy in creases with the square of the irradia tion time. Thus heating by coupling an oscillator to the angular frequency of a selected ion is a useful tool. Chosen ions can be rapidly ejected from the trap or kinetically heated. In addition to kinetic excitation, ions can be heated in traps by combining rf heating with relatively high pressures (i.e., collisions). If kinetically excited ions collide with neutrals t h a t are heavy with respect to the ions, heating occurs; if the neutrals are lighter, cool ing occurs (5). This concept has been explored by Mahan et al. (10) in their work on LIF of ions such as CH + . They noticed that the line widths detected were indicative of ion temperatures of approximately 4000-5000 K. High-en ergy collisions were used to populate high J levels of selected ions, which were observed by LIF, allowing for the study of energy levels not substantially populated at room temperature (27). Spectra of "cool" and "hot" ions could be obtained by exciting ion motion in the presence of inert collision gases. Impressive results have been ob tained using optical methods of cooling

ions in a trap (34). In laser cooling (also referred to as "lumino-refrigeration" or optical sideband cooling) an ion is irradiated by a monochromatic laser beam that is only slightly lower in fre quency than that of a wavelength cor responding to a strong resonance tran sition. The excited state is formed, which fluoresces over its natural nar row band of wavelengths about the transition frequency, resulting in slightly more energy being given off than was absorbed. Starting from 300 K, up to 104 such events may be required to cool an ion substantially (104 is approximately the ratio of the momentum of an ion to that of a photon [5]). Individual Ba + ions in a small rf trap have been cooled to tem peratures of 10-36 mK (1). On cooling, ion motion decreases and the ions "col lapse" into the center of the trap. It has been suggested that cooled ions could be trapped indefinitely (1). The meth od may be capable of achieving kinetic temperatures of ΙΟ - 8 Κ. Photographs of single cooled, trapped ions have been published (1). "Sympathetic cooling" also has been reported in a Penning trap (35) in which two ions such as Hg + and Be + are held in an rf trap. If Be + is cooled, ion motions are coupled suffi ciently such that the temperature of the Hg + decreases. This is a powerful technique, because ions with compli cated electronic structures (molecular

ions) or those with no optical spectrum (e.g., protons) can be cooled (35). A similar experiment can be performed in which the irradiating frequency is slightly higher than required, which re sults in heating of the ions (5). Ion traps at work Although chemists may be familiar with many of the experiments that can be performed in ion traps, they may not be aware of the other areas in which ion traps have had impact, such as the measurement of fundamental con stants and the characterization of fun damental processes. One application of ion traps and laser cooling is in the de velopment of better atomic clocks (36) and frequency standards (2). One sec ond of time is defined (36) as the dura tion of 9,192,631,770 periods of radia tion corresponding to the transition be tween two hyperfine levels of the ground state of 133 Cs. At the National Bureau of Standards in Boulder, Colo., the frequency of a particular hyperfine transition in the ground state of beryl lium atomic ions was measured to one part in 1013 in a Penning ion trap; fre quency standards and clocks with inac curacies of only one part in 10 15 or bet ter are forseen in the near future (36). Ion traps have been used to measure the proton-electron mass ratio. This fundamental constant has been mea sured (37) in a split ring Penning trap

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at a magnetic field strength of 5.05 T, at cryogenic temperatures. Cyclotron resonances for electrons and protons, alternately trapped, are measured. Because the mass ratio m(proton)/77i(e~) = ^(e^/V^proton) mp/me is found to be 1836.152470(76). Measurements such as these are important in the development of a set of fundamental constants that are self-consistent (37). Ion traps have been used to study the recombination of positive ions with electrons. Recombination plays an important role in the ionization balance of stars and in the evolution of ionic and

molecular species in both interstellar regions and planetary atmospheres (11). A number of important neutral molecules, such as CH, NH, NH 3 , and CH2O, result from the dissociative recombination of electrons and positive ions in dense interstellar clouds. Theoretical rate constants yield recombination rates that are too slow to explain the complex collection of molecules in interstellar space. However, experimentally determined recombination rates of cooled ions in traps lead to predicted densities that are much more consistent with those that have been observed (11). Techniques such as ICR, FT-MS, and the more recent rf-ion trap mass

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spectrometer have been developed for their utility in chemical and mass spectrometric studies. Their use has been extensive and cannot be properly reviewed here. However, a number of areas in which the power of trapping technology is apparent will be highlighted. The spectroscopy of ions is difficult to study without some sort of ion trap. Purely spectroscopic experiments yielding spectroscopic constants were discussed earlier. Spectroscopy, combined with mass spectrometric analysis, is used to obtain other quantities such as bond energies and electron affinities. Traps will confine suitable numbers of ions in a defined region of space for times sufficiently long such that changes in the population attributable to ion/photon interactions can be detected. The area of photodissociation of ions has developed because of the pioneering work of Dunbar (38). Although photodissociation experiments are commonly performed with positive tons, both photodissociation and photodetachment can be studied for gas-phase anions. Lineberger has been one of the most active workers in negative ion spectroscopy (39). The area of gas-phase ion/molecule reactions cannot be adequately discussed here. Using predominantly ICR and FT-MS, organic, inorganic, and organometallic ion/molecule reactions have been characterized (6, 24). In these experiments, reaction pathways have been elucidated; rate and equilibrium constants have been established; and thermodynamic quantities such as ion enthalpies, bond strengths, proton affinities, and gas-phase acidities and basicities have been determined. The quistor-quadrupole combination also has been used to study ion/molecule reactions (8, 13, 14, 40). The quistor can be used as a "selective ion reactor" in which an ion of a single m/z is trapped, after which all of its products are trapped and injected into the quadrupole mass filter for analysis (40). Perhaps the ultimate demonstration of ion trap technology to date is in the study of the reaction of CoFe + with 1pentene using FT-MS, as depicted on the cover of the Oct. 19, 1984, issue of Science. In a sequence of ionic manipulations, the ion CoFe + was generated (using laser vaporization, ion/molecule reaction, and CID), trapped, and reacted with 1-pentene, and an ion/molecule reaction product was analyzed by CID. Finally, a number of developments in ion trap technology have benefited those in the area of MS. New methods have evolved in response to the limitations of ion traps. For example, chemical ionization (CI) experiments typically involve relatively high pressures. The concepts of "self-CI" and lowpressure CI (30) have evolved from ion-

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trapping groups interested in mass spectrometric applications. In self-CI (6), the EI fragments of an ion are used as the chemical ionization reagent ions, reacting with their neutral precursor molecules by proton transfer, for exam ple. Low-pressure CI experiments (8, 30), which are the only type of CI that can be performed in some traps, are useful tools; unlike high-pressure CI, there is not extensive clustering be cause of termolecular reactions. The speed with which mass spectra can be acquired in the FT-MS ion trap has allowed capillary GC/FT-MS to yield promising results in which spec tra can be accumulated at a rate of 0.5 s/scan file, where each file corresponds to 25 averaged scans. Analytical appli cations of the quistor-quadrupole com bination have been reported, such as the use of trapping to enhance "weak peaks" (29). Also, ion traps have been used to obtain electron impact mass spectra of ultra-low pressure com pounds that could not normally be studied by direct introduction tech niques. Most recent developments in the Finnigan ion trap and the demon stration of its utility for MS/MS work are exciting. This trap yields a low-cost approach to MS/MS with (possibly) significant advantages over other tech niques such as triple quadrupole mass spectrometry (TQMS). It has been re ported that the overall MS/MS effi ciency of the ion trap is 25 times better than in the TQMS instrument (41). Conclusions



Future experiments often are predict ed in accounts of work employing ion traps. For example, the utility of ion traps for extreme sample economy has been discussed. An experiment has been suggested (42) in which a small amount of gaseous Co(CO)3NO, con taining the positron emitter 58Co, is ad mitted into a trap. This isotope has a half-life of 70 days. Positrons that are emitted could be captured by the trap and studied. With an ion trap, it has been suggested that elastic atom-posi tron collisions could be studied and the positrons would still be available in the trap to use again! This review shows that many areas of science in addition to MS benefit as ion trap technology continues to develop. Ion traps provide a unique chemical system, approaching that of the unper turbed chemical entity, trapped in time and space and available for a variety of experiments. Ions in the gas phase can be manipulated as no other chemical species can. The ability to store, heat and cool, irradiate, react, and destroy ionic species, coupled with powerful and sensitive MS methods, will contin ue to allow for useful and exotic science to be performed at a level unequaled by other techniques. —*-

References (1) Neuhauser, W.; Hohenstatt, M.; Toschek, P. E.; Dehmelt, H. Phys. Rev. [Sect.] A 1980,22,1137. (2) Wineland, D. J. Proceedings of IEEE 1986; Vol. 74, p. 147. (3) Dehmelt, H. G. In Advances in Atomic and Molecular Physics, Vol. 3; Bates, D. R.; Estermann, I., Eds.; Academic Press: New York, 1967, p. 53. (4) Dehmelt, H. G. In Advances in Atomic and Molecular Physics, Vol. 5; Bates, D. R.; Estermann, I., Eds.; Academic Press: New York, 1969, p. 109. (5) Wineland, D. J.; Itano, W. M.; Van Dyck, R. S. Advances in Atomic and Mo lecular Physics, Vol. 19; Academic Press: New York, 1983, p. 135. (6) Gross, M. L.; Rempel, D. L. Science, 1984,226, 261. (7) Dawson, P. H. Mass Spectrom. Rev. 1986,5,1. (8) Bonner, R. F.; Lawson, G.; Todd, J.F.J.; March, R. E. In Advances in Mass Spectrometry, Vol. 6; West, A. R., Ed.; Applied Science: Essex, England, 1974, p. 377. (9) Armitage, Μ. Α.; Fulford, J. E.; Hughes, R. J.; March, R. E.; Mather, R. E.; Waldren, R. M.; Todd, J.F.J. In Ad vances in Mass Spectrometry, Vol. 8B; Quale, E., Ed.; Hayden and Son, Ltd.: London, 1980, p. 1754. (10) Grieman, F. J.; Mahan, Β. Η.; O'Keefe, Α.; Winn, J. S. Faraday Discuss. Chem. Soc. 1981, 71,191. (11) Walls, F. L.; Dunn, G. H. Physics To day, August 1974,30. (12) Todd, J.F.J.; Lawson, G. In Int. Rev. Sci. Phys. Chem. Ser. Two, Buckingham, A. D.; Maccoll, Α., Eds.; 1983,5, 289. (13) Armitage, Μ. Α.; Fulford, J. E.; Hoa, D. N.; Hughes, R. J.; March, R. E. Can. J. Chem. 1979,57,2108.

John Allison (left) is an associate professor of chemistry at Michigan State versity (MSU) and a codirector of the National Institutes of Health/MSU spectrometry facility. His research interests include gas-phase ion/molecule actions and desorption/ionization methodology for mass spectrometry.

Uni mass re

Richard M. Stepnowski (right) is a graduate student at MSU, where he is study ing gas-phase organometallic chemistry using ion cyclotron resonance MS. He received his B.S. degree from the University of Dallas in 1983. (14) Bonner, R. F.; Lawson, G.; Todd, J.F.J. Int. J. Mass Spectrom. Ion Phys. 197273,10,199. (15) O, C-S.; Schuessler, H. A. Int. J. Mass Spectrom. Ion Phys. 1980,35, 305. (16) Dawson, P. H.; Hedman, J. W.; Whetien, N. R.; Rev. Sci. Instrum. 1969, 40, 1444. (17) Fulford, J. E. et al. Can. J. Spectrosc. 1980,25,85. (18) Todd, J.F.J.; Lawson, G.; Bonner, R. F. In Quadrupole Mass Spectrometry and Its Applications; Dawson, P. H., Ed.; El sevier Scientific Publishing: Amsterdam, 1976, p. 181. (19) Lifshitz, C. Mass Spectrom. Rev. 1982, /, 309.

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(20) Mourad, W. G.; Pauly, T ; Herb, R. G. Rev. Sci. Instrum. 1964, 35, 661. (21) Talrose, V. L.; Harachevtsev, G. V. In Advances in Mass Spectrometry, Vol. 3; Mead, W. L., Ed.; Institute of Petroleum: London, 1966, p. 221. (22) McDowell, C. A. Mass Spectrometry; McGraw-Hill: New York, 1963. (23) Wobschall, D. Rev. Sci. Instrum. 1965, 36,466. (24) Lehman, Τ. Α.; Bursey, M. M. Ion Cy clotron Resonance Spectrometry; WileyInterscience: New York, 1976. (25) Winn, J. S. In Molecular Ions: Geo metric and Electronic Structures; Berkowitz, J.; Groenvald, K. O., Eds.; Ple num Press: New York, 1983. (26) Danon, J.; Mauclaire, G.; Govers, T. R.; Marx, R. J. Chem. Phys. 1982, 76,1255. (27) Mahan, B. H.; O'Keefe, A. J. Chem. Phys. 1981, 74,5606. (28) Knight, R. D.; Prior, M. H. Phys. Rev. [Sect.] A 1980,21,179. (29) Lawson, G.; Todd, J.F.J. Anal. Chem. 1977 49 1619. (30) Mather, R. E.; Lawson, G.; Todd, J.F.J.; Bakker, J.M.B. Int. J. Mass Spec trom. Ion Phys. 1978,28, 347. (31) Miasek, P. G.; Beauchamp, J. L. Int. J. Mass Spectrom. Ion Phys. 1974,15, 49. (32) Stafford, G. C ; Kelly, P. E.; Syka, J.E.P.; Reynolds, W. E.; Todd, J.F.J. Int. J. Mass Spectrom. Ion Proc. 1984,60, 85. (33) Kelley, P. E.; Stafford, G. C.; Syka, J.E.P.; Reynolds, W. E.; Todd, J.F.J.; Louris, J. N. In Advances in Mass Spec trometry 1985, Part B; Todd, J.F.J., Ed.; John Wiley and Sons: New York, 1986, p. 869. (34) Wineland, D. J.; Itano, W. M.; Bergquist, J. C ; Bollinger, J. J.; Prestage, J. D. Ann. Phys. Fr. 1985,10, 737. (35) Larson, D. J.; Bergquist, J. C.; Bol linger, J. J.; Itano, W. M.; Wineland, D. J. Phys. Rev. Lett. 1986,57,70. (36) Wineland, D. Science, 1984,226, 395. (37) Van Dyck, R. S.; Moore, F. L.; Farnham, D. L.; Schwinberg, P. B. Int. J. Mass Spectrom. Ion Proc. 1985,66,327. (38) Dunbar, R. C. In Ionic Processes in the Gas Phase; Almoster Ferreira, Μ. Α., Ed.; D. Reidel: New York, 1984, p. 179. (39) Corderman, R. R.; Lineberger, W. C. Annu. Rev. Phys. Chem. 1979,30, 347. (40) Fulford, J. E.; March, R. E. Int. J. Mass Spectrom. Ion Phys. 1978,26,155. (41) Yost, R. A. Abstracts of Papers, 34th Annual Conference on Mass Spectrome try and Allied Topics, Cincinnati; 1986, p. 1090. (42) Wineland, D.; Ekstrom, P.; Dehmelt, H. Phys. Rev. Lett. 1973,31,1279.

The Hows and Whys of Ion Trapping - Analytical Chemistry (ACS

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