INSTRUMENTATION
Advisory Panel Jonathan W . Amy Jack W . Frazer G. Phillip Hicks
Donald R. Johnson Charles E. Klopfenstein Marvin Margoshes
Harry L. Pardue Ralph E. Thiers William F. Ulrich
Ion Cyclotron Resonance Spectrometry: Recent Advances of Analytical Interest M I C H A E L L. GROSS and CHARLES L. W I L K I N S Department of Chemistry, University of Nebraska Lincoln, Neb. 6 8 5 0 8
Ion cyclotron resonance spectrometry
is becoming established as an important
technique in its own right as well as a useful complement The introduction À S MOST
READERS
to conventional mass
analytical spectrometry.
of digital computer techniques should further expand its usefulness ARE W E L L
AWARE,
~ ri - the instrumentation used in the broad field of mass spectrometry is diverse. There are many types of mass spectrometers, each with its own advantages and limitations. These instruments range from the simple "mass fillers" and residual gas analyzers to very complex, high-resolution instruments only available in a few laboratories. However complex the instrument, the common objective is to determine the mass of ions produced in some source and to use their behavior as an analytical tool to obtain structural and thermodynamic information. Since the design of these spectrometers dictates low ion residence times (ca. l ( h e sec), analytical techniques utilizing ion-molecule reactions have not been extensively employed. Naturally, when ions do not remain in the spectrometer for sufficient time to permit reaction with molecules, it is impossible to employ such reactions analytically. Of course, by suitable modification of the spectrometer and conditions, it may be possible to use higher sample pressures and thus to observe ion-moelcule reactions as a result of the increased reaction rates engendered. Now, even though the residence times are no longer, the probability of an ion colliding with a neutral molecule is much greater (since the concentration of neutrals is orders of magnitude higher), and reaction can occur before the residence time is "used up." Due to a number of
complications which arise through the use of this method, it is often desirable to employ a different type of mass spectrometer which can allow ionmolecule reactions at low sample pressures. I t is that instrument, the ion cyclotron resonance spectrometer, which will be considered in the present article. Ion Cyclotron Resonance: Instrumentation and Principles
The basic phenomenon on which ion cyclotron resonance (ier) spectrometry depends is that observed when a charged particle is subjected to the influence of a magnetic field. Briefly, a magnetic field causes an ion to travel in a circle. As a result, the centrifugal force acting on the ion (mv2/r is balanced by the force from the field (eHv/c), or mv2/r = eHv/c. Rearrangement of this equation leads to an interesting result which is basic to ion cyclotron resonance v/r
eH
(1)
where ν is the velocity of the ion, r is the radius, e is the charge of the ion, Η is the magnetic field strength, m is the mass of the ion, and c is the speed of light. Angular velocity, o>c, is simply the number of radians per unit time, and the frequency of revolu tion in cycles per unit time is cuc/2 -π-. Thus it can be seen that the cyclotron frequency, wc, at constant magnetic
field strength depends only on the mass-to-charge ratio of an ion. If, in addition to the magnetic field, the ion is further subjected to the simultaneous influence of an electric field applied at right angles to the mag netic, the particle departs from the circular motion it had previously pur sued and proceeds in a cycloidal path at right angles to both fields. This motion is represented schematically in Figure 1. I t still follows that the frequency (now the number of com plete cycloids per unit time) is deter mined only by m / e at constant H. This fact can be used to advantage to allow detection of the ions. If a rapidly alternating electric field is ap plied, as the frequency of alternation approaches the cyclotron frequency of an ion which is present, the ion will absorb energy. As this absorp tion takes place, the ion is acceler ated, causing the radius of its cycloi dal path to increase (since the fre quency must remain constant). Fig ure 2 gives a pictorial representa tion of this process, and shows the re sult of an ion's resonance—i.e., the matching of frequencies discussed above. For ions of between 1 and 200 amu, with the magnetic field strength commonly employed, the frequencies range from 50 kHz to about 25 MHz. Therefore, a marginal oscillator detec tion system is practical. Using the cell as a capacitive element in the resonant circuit, a phase-sensitive de tector can be used to observe the en-
ANALYTICAL CHEMISTRY, VOL. 43, NO. 14, DECEMBER 1971
•
65 A
Instrumentation Rcr
REGION 1 ELECTRON IMPACT SOURCE CE LTAGE
REGION 2
TOTAL ION MONITOR
, „ „ v , r o , RADIOFREQUENCY ANALYZER j 0 S C I L L A T O R I DRIFT VOLTAGF^
Figure 1 . Schematic d i a g r a m of an ion cyclotron resonance cell s h o w i n g the nor mal ion t r a j e c t o r y Τ
IONIZATION PRODUCED BY ELECTRON IMPACT
CYC LOI DAL TRAPPING VOLTAGE TRAJECTORY OF NORMAL ION
TOTAL ION COLLECTOR
Figure 2. Trajectory of an ion in reso nance
Ι Γ Ϊ Τ POINT OF IONIZATION
ergy level changes of the oscillator. These changes take place only when ions absorb energy. Therefore, a mass spectrum can be obtained by plotting t h e o u t p u t of the detector circuit vs. a time domain quantity (either the rate of scan of t h e magnetic field at con stant frequency or, alternatively, the rate of change of the electric field fre quency at constant magnetic field). As a result of the circuitous route which ions travel as they pass through a cell of approximately 10 cm in length, the residence times are on the order of milliseconds. Through the use of special techniques, residence times can be m a d e as long as several seconds. This method of detection has a striking advantage over conventional magnetic focusing mass spectrometers. We have mentioned t h a t ω0 is de pendent only on m/e and H, b u t it is not dependent on the velocity of the ion, v. An increase in velocity will increase t h e radius of the cycloidal trajectory, b u t will not, change the frequency. On the other hand, the focusing properties of a conventional mass spectrometer depend strongly on ion velocity. T h u s , studies involving changes in ion kinetic energy can be made by icr without affecting the de tection scheme. Referring back to Figure 1, the op erational procedure is to first form ions in the electron impact source (•Region 1 ) ; they then travel down the cell through the analyzer section (Eegion 2) and eventually reach the total ion monitor (Region 3 ) . To con 66 A
•
' TRAJECTORY OF ION AT RESONANCE
strain the ions to a motion down the center of the cell, t r a p p i n g voltages are applied to side plates. T h e reader is referred to any of t h e excellent review articles of Baldeschwieler (1, 2) or Henis (3) for more complete discus sion of the theory. F o r illustrative purposes, we have included a photo graph of the commercial icr spectrom eter (Figure 3 ) , a close-up of the vac u u m can which encloses t h e cell being lowered between the poles of an electro magnet (Figure 4 ) , and a close-up of the icr cell removed from the vacuum can (Figure 5 ) . Analytical Advantages of Ion Cyclotron Resonance
As Henis points out (3), icr spec t r o m e t r y is well suited for the study of ion-molecule reactions. Physical chem ists have used t h e method for the in vestigation of collisional processes, gasphase kinetic studies, and tests of the oretical predictions regarding funda mental molecular and atomic behavior. Although analytical chemists will find these topics interesting, it is our be lief t h a t the real value of icr spec t r o m e t r y as an analytical tool for quantitative and qualitative analysis will lie in different areas. F o r example, physical chemists have elaborated ele gant methods for quantitative kinetic studies of ions by icr. The analytical chemist can take advantage of these methods in attacking the difficult problems which sometimes face him as he a t t e m p t s to interpret conven
ANALYTICAL CHEMISTRY, VOL. 4 3 , NO. 14, DECEMBER 1 9 7 1
tional mass spectrometry results. A complete understanding of a mass spectrum demands a thorough knowl edge of n o t only ion mass-to-charge ratios, b u t also of ionic structures a n d fragmentation mechanisms. T h e new dimension added by icr makes possible the use of differences in ion-molecule re activity t o distinguish isomeric ions or neutrals. As a complement to conven tional mass spectrometry (which is in capable of easily distinguishing iso meric ions), t h e icr method is uniquely powerful. Ions of different structure will rarely undergo the same bimolecular reactions. Therefore, structural identity of ions m a y be established b y generating t h e m from both known sources and compounds of interest and using ion-molecule reactions to demon strate t h a t identity. For instance, u s ing this approach, we have been able to unequivocally demonstrate the nonidentity of the ions arising from the isomeric compounds, cyclooctatetraene and styrene (4), which were previously thought to be t h e same as a result of conventional mass spectrometric stud ies. Similarily, neutral compounds which might otherwise be extremely difficult to analyze can be characterized b y the use of suitable ion-molecule reactions. Bursey and co-workers (5) have re cently demonstrated t h a t a gas-phase acetylation ion-molecule reaction can be an i m p o r t a n t means of chemical ionization. T h e advantage of chemical ionization over ionization by electron impact is t h a t the former is a "soft" ionization process—i.e., little internal energy is transferred to the ionized species. As a result, fewer fragmenta tions of this new ion will occur, and it is expected t h a t these fragmentations will be more selective t h a n those p r o duced b y electron impact ionization. Certainly, a significant limitation of electron impact ionization is t h a t mole cules of similar structure produce nearly identical fragmentation p a t t e r n s due to the large a m o u n t of internal energy imparted in the ionization process. Ion-molecule reactions, as studied b y icr and other techniques, m a y be a n important complement to conventional mass spectrometry in solving analyti cal problems concerning such similar molecules. T h e acetylation reaction mentioned above occurred with some selectivity depending on t h e nature of heteroatoms with unshared electron pairs present in an organic molecule. I n our laboratory, we have used the 1,3-butadiene molecule ion as a chemi cal ionization agent and have found significant differences in the reactions of this ion with various isomeric pentenes. This series is typical of hydro carbon isomers because nearly identi-
Instrumentation
Figure 3.
A c o m m e r c i a l ion cyclotron resonance spectrometer Magnet and cell are t o the left of console
cal mass spectra are obtained b y electron-impact ionization, and thus complete analysis is very difficult. Implicit in the methods above is the use of kinetic data t o distinguish isomeric species. I t is also worthwhile to examine how kinetic d a t a for reac tions of interest can be obtained through use of a variety of techniques. F o r one thing, precursors of ionic prod ucts can be established by use of double resonance methods. This is extremely difficult to do by high-pres sure mass spectrometric studies. Double resonance techniques depend on the principle t h a t , if one irradiates at the cyclotron resonance frequency of a suspected precursor ion while ob serving t h e p r o d u c t ion, changes in the product abundances indicate their connection. T h e absence of such changes imply t h a t the product does not arise from a reaction involving the suspected precursor. I n t h e event t h a t an ion product arises from two dif ferent precursors, one can be ejected, and thus the reaction of t h e other m a y be studied without interference. This "ion ejection" is either t h e result of irradiating in t h e source region a t the interfering ion's cyclotron frequency a n d t h u s spiraling t h e ion into t h e cell wall before it can react, or of modulating the t r a p p i n g voltage, which results in a similar occurrence. The technical details of the modula tion method arc discussed in detail in a recent p a p e r b y B e a u c h a m p and co workers (β) . Another recent develop ment is t h e use of a t r a p p e d ion ana lyzer cell to extend ion residence times to as long as several seconds ( 7 ) . I n this technique, ions are formed b y a pulsed electron beam a n d then t r a p p e d in a completely enclosed cell, con structed so as to prevent ions from drifting out of t h e cell. This develop
ment will permit t h e study and use of even slower ion-molecule reactions t h a n was hitherto possible. I n addition, studies of the effect of long residence times on mass spectral fragmentations are now possible. Of particular interest to analytical chemists are the potential applications of icr to such fundamental problems as the direct measurement of proton affinities and acidities in the gas phase. I t is extremely i m p o r t a n t t o be able
to measure these if we are to fully understand t h e role of solvent in acidbase chemistry. F u r t h e r m o r e , the data which can be collected will be of in estimable aid in interpreting the large amount of solution d a t a already avail able. Briefly, t h e method is to estab lish relative acidities by studying an ion-molecule reaction involving a pro ton transfer between the two com pounds t o be compared. I n this way, it is relatively easy to measure data for a whole series of compounds and, ultimately, to relate t h a t series to a similar series determined in solution. 1er allows the study of negative ions in exactly t h e same way. So, for ex ample, the gas-phase basicities of alkoxide ions have been measured b y icr and the order found to be dif ferent from t h a t found in solution (
