i
and Whys
John Allison Richard M. Stepnowski Department of Chemistry Michigan State University EaS Lansing, Mich. 48824
1072A
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 (I). Working in the gas phase a t 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 he 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
ANALYTICAL CHEMISTRY, VOL. 59, NO. 18, SEPTEMBER 15. 1987
entityatom, molecule, or photon-may be studied. The technology that has evolved for ion trapping and subsequent manipulation has led to some remarkable feata, such as the trapping and detection of a single ion for weeks and the cooling of ions to temperatures well below 1K. A number of reviews are available on ion traps and on the specific arena 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 he 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 .SO10
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ion trapping has been successfulranging from fundamental physical measurements to applications in chemistry and MS-will be provided. As a benchmark, consider the ion source of a mass spectrometer. AIthough mass spectrometers are designed to study ionic species, conventional ion sources typically are not designed 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 active or passive. The classification reflects whether the fields used to confine the ions are time-dependent (active) or time-independent (passive). We will first focus on the mechanisms of trapping for the various devices that have been reported. Although a variety of ionization methods have been used in conjunction with ion traps, by far the most common is electron impact ionization (EI). Unless otherwise stated, it may be assumed that E1 is
used to form ions in the experiments discussed. AcHve traps
T h e quadrupole ion storage trap (quistor). The quistor or Paul trap (9,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 bas a geometry described as a hyperboloid of one sheet combined with a hyperboloid of two sheets (9)-it is composed of a ring electrode ‘andtwo end caps (see Figure la). Electrons can be injected through a hole in either the end cap (9) or in the ring electrode (IO). In the quistor, the end caps are electrically connected, and dclrf potentials are applied between them and the ring electrode. The applied voltage takes
theform[U+ VcosOt],whereUisthe dc voltage; the rf voltage has amplitude V and radial frequency Q. The equations of motion for an ion in the trap with a certain mass-to-charge ratio (mlz) are derived from Newton’s Law (F= ma), giving differential eq-uations known as the Mathieu equations. The Mathieu equations indicate the time dependence of the forces experienced by the ion as a function of the applied voltages, physical dimensions, and mlz of the ion. Ionizing electrons are injected into the device, forming ions within the quistor volume. The ions move in response 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 oscillating 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 motions 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 potentials. Values for these potentials can be chosen such that only one mlz 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 he used (7, 12),as mass filters or in the “rf-only” mode. An ion with a stable trajectory follows an aperiodic orbit ahout 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 mlz values are trapped (7). Recent advances, however, include the use of a relatively high preeaure of helium to extend the mass range of this ion trap.
The He damps the ion motion, increasing the concentration of ions in the center of the trap and trapping higher mass ions than would be possible without this buffer gas (7). The Mathieu equations lead to two frequency components that characterize motion in the rf trap (13).These frequencies, at which the ions oscillate in the trap, are mlz-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 X lo6 ionslcms, and practical trapping times can range from 200 pa to 100 ms (13).Such traps are less than ideal for collision studies (14),because the kinetic energies of the ions are constantly changing, with a mean value generally of about 1-3 eV. Other active traps. A number of other active ion traps have been described in the literature; many are variations (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 precisely machined, but rather “shaped by band from a coarse steel mesh.” Other attempts to simplify the construction of the quistor-type trap include the use of a cylindrical geometry. The cylindrical ion trap (see Figure lh) has heen 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 heen proposed (15).Such simplifications become necessary when the experiment demands the construction of a small ion trap (submillimeter size) for laser spectrwopy (15).These variations from the ideal geometry lead to larger ion losses, that is, shorter attainable trapping times (15).A hexapole ion storage trap or histor (12)also has been described (Figure IC). 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 heen 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 inexcess of 14 min have been reported with this device. Passive traps Trapping in a n electron heam. This is a simple technique that can enhance the residence time of ions in a conventional masa spectrometer E1 ion source (19).The space charge of the electron beam forms a potential well or “trapping field,” confining positive ions formed within the electron heam.
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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 A,with electron energies of about 5 V. Although not required, magnetic fields frequently are used to collimate the electron beam. Well depths higher than thermal energies (3/2 kT = 0.04 eV at 300 K) must be created for trapping. Trapping potentials of 0.3-0.5 eV are attainable (19),and trapping times on the order of milliseconds have heen achieved. Questions concerning the possible vibrational excitation of trapped ions by the trapping electron beam have been addressed (19). Devices that are characterized by long residence times and that use a de 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 filament ion gauges, such as the BayardAlpert gauge, positive ions are formed by E1 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 simple design for increasing the residence time of ions in a fairly small region of space. Devices t h a t use magnetic (B) and electric (e) 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, Za), 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 (E) and magnetic fields are shown in Figures 36. 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-
(e),
A
I, = a d d , where a is the ionization cross section, d is the average distance an electron travels, n is the neutral number density, and I. is the electron emission current. To increase the sensitivity of such a device, one must increase d or I. or both. There are practical limits to the value of I.. The orbitron, shown in Figure 2, is designed to increase the electron path. It consists of two cylinders, with a potential 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 cylinders, their energies and angular momentaare 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 em 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 milliseconds. 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 “stationary orbits” have average kinetic energies of about 2 eV, which will be a 1074A
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Flgun 1. Some actbe ion trap wnfigvations-defining regions of space where ions are confined because of the application of dc and rf electric fields. (a) Crws s~ctionof qui& (9dhowing two end caps and a ring eiecbode. EiecbOns may be inlscted IhroW an uific8 p i e d in eimer an end cap or in me ring eieotmde (designated by arrowah AiUwugh not required, many quislm have a picrated lower end cap. which allows for ion ejeotbn to an meiyzer, A. Whidl may be an ion flux detector w a m858 specemeler. (b) cyiinhicai ion trap mls design, whim maintains me same end cap and ring elecb& configuratbnend applied BIecuic field, as me quisw, haa bsan evaluated as a oimpiiRcetion of me quMw aeanebv ( 17). (0)me hislor, W he,xapole ion trap. is ideally constructedof three t w o - h e t hyperbolic elenmdes dluated abng me mree melor axes. shown here is me basic geanetrl,wim ~ v ofe ne six eiecbodes d i a played. Omer designs have been p o p o s d in which me c u m elecbodes are approximated by plane sheets. wim me mree pairs of e m & dinerentially b W ( 13. (d) me storage ring, or race track trap. consists d a quaavpole mass filter configured as a closed loop. A cutaway view of me general design is shown. Bom WBI and circular destgns have been buin (161.
ANALYTICAL CHEMISTRY, VOL. 59. NO. 18, SEFTEMBER 15, 1987
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Aviewlmm the mp l a s h n . Two oo(1o(MMc cylinders are used. wim wtor Winder held at a positin, potenWalwHh Io lhs Inner cylinder. Ra inner cylindsr is lypic4)y a lhin wire. EIBcbons are injectedm h a small hole in the ahKoylWermfwm ions. Many ions have inltiai veilociws(lsucil mat mey BI)wmostable orbits a!xul thrr inner e W & (as shown), W i n g to re!ativeiy lag residencetimes. A s e d h.s may be added mtheouterelecbode. and Iwu, may be pulsed out ofthe oap m a mass aps*mmeter I W miysis (20.
m
--
lyze ions. All use strong magnetic fields, created by conventional and solenoid electromagnets, for ion confinement. Similar to this design is the Penning trap, which uses the same electrode configuration as the rf trap but no rf field-only a dc field and a static magnetic field along the z axis. For these devices, the mechanism of ion trapping is essentially the m e . Formed in the presence of a strong magnetic field, the ions precess about the field l i e s . For an ion with a velocity 0 perpendicular to.the magnetic field (characterized by a magnetic flux density B),the ions experience a force that is perpendicular to both 0 and 8,which results in a circular motion. The ion, with masa rn and charge re, moves about the field lines with a characteristic frequency known as the cyclotron frequency, uC = zeBl2rrn. Note that each ion (each mlz) has a unique uO. This interaction with the magnetic field does not confine the ions along the z axis (along 8).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. Aa 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, v,. 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 z1076A
axis at an angular frequency urn.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 ionlmolecule 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 re$on8 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 fi X 8 field. For example, in a cell 8 em long, times on the order of milliseconds (24) are required for an ion to move from the formation region to the end of the cell. Thus ICR “drift” cells are characterized by drift times sufficiently long to he 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 (I?), some of which are pressure-dependent. For example, at a pressure of 1 X 10-8 torr, in an FT-MS instrument using a 4.7 T electromagnet, after 13.5 h, 30%of the stable ions initially formed remained in
the trap (6). Detecmg events OCeurrLg 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 phenomenon under study. OpNcd d e t e c t h of events
If the ion mass does not change, and optical emission from ions in excited states is of interest, conventional spectroscopic methods are used. Quadrupole ion traps (10, 25) and ICR (26) traps have been extremely useful in laser-induced fluorescence (LIE) studies of ionic species. Ion traps can be used to selectivelytrap an ion with a particular mlz value for spectroscopic analysis, in contrast to earlier discharge-flow methods (2.5). Mahan et aL (10, 27) have reported extensive work on LIE of species such as CH+, CD+, Nz+,and BrCN+. Spectroscopic constants and radiative lifetimes have been determined for ionic species in this way. Utilizing rf traps with potential well depths of 19 V, up to 3 X lo7ions have been trapped for spectroscopic analysis. In contrast, LIF of ions trapped in an ICR cell (26)has been performed in which 1 X 108 ions are available 13 ps
Flgure 8. An exploded view ofthe ornegatron.
ms
E W m enta and exn side (Wapping) plates m h smell apenures. An e w i c Held is applied perpendicular m the madlnetic fleld a c r w the stacked plates. trapping plates are born maintained at a small posnive potential fa mnlimment 01 positive ions. A small coibctw is mounted on bOtmrn plate. Ionic mo1lons are stimulated at their oyoimm frsquendes by applying an rf wiiage athe stacked plates. Ra radius 01 orbn d me Ions in resonance wim the applied field in-ases un(il hey strike milecta a d are detected.
me
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after ion formation. The estimated detection for LIF of Cot in the ICR experiment (26)was 2 X 1lV 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 performed using a variety of excitation methods. In such experiments radiative transition rates were difficult to determine because of the excitation processes used (IO);many highly excited states are initially formed that cascade to the state of interest, complicating measurements of decay rates. For example, early work on the A X radiative decay for CHt 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 cbaracterized with minimal collisions. Such studies yield unambiguous radiative lifetime-815 ns for this particular system (IO). A major spectroscopic accomplishment achieved using an rf ion trap is the determination of the lifetime of the first excited state of Li+ (1~1281,3SI). The dominant decay mode of this 2-
d
-
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 thii metastable state was49s. UsinganrftraptocontainLit formed by E1 on an Li atomic beam (28), researchers determined a lifetime of 58.6 f 12.9 8. Lifetimes that varied by almost 14 orders of magnitude were observed for states Li* (28).
spacbomemc -MIS In most ion trap experiments, the phenomenon under study results in one or more ionic products, and mass spectrometric analysis (m/z and abundance information) is desired. These include photodigsociation reactions (in which detection of the photofragment is desired), bimolecular ion/molecule reactions (in which reaction prcducta are of interest), photodetachment studies of anions (in which the rate of diappearance of the anion is determined), and anion
