ANALYTICAL OF ION RESONANCE By J. M.
S. Henis, Monsanto
Ion cyclotron resonance spectrometry represents a rather unique combination of mass spectroscopy and reaction kinetics. Some analytical applications are obvious although others await a different orientation for development
G
AS PHASE ION-MOLECULE REACTIONS have been investigated by a variety of methods for almost 20 years. Most of the instruments used in such studies were mass spectrometers which were modified to operate under conditions where the ions produced by electron impact could m a k e collisions before they were extracted from the spectrometer ion source. Once extracted, both product and reactant ions were focused through a magnetic field and collected. T h e instruments most commonly used for such studies were sector, time-of-flight, and, more recently, t a n d e m mass spectrometers. The techniques and instrumentation have quite naturally been improved and become more refined since the earliest studies were carried out, but the experiments themselves still have certain inherent limitations which are related to the properties of the instruments used. Mass spectrometers of the types mentioned above cannot generally operate a t pressures much above 10~ 3 mm. Unfortunately, ions will not undergo a large number of collisions (and hence extensive reaction will not occur) unless the pres22 A
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sure in the reaction zone is greater t h a n 1 0 - 3 mm. N a t u r a l l y , the p a t h length of an ion in the reaction region, the collision cross section, and the pressure, are all important in determining the extent of reaction. In sector and time-of-flight mass spectrometers, reaction takes place in the ion source and the ion p a t h length is thus limited to less t h a n 1 cm. Similarly, in t a n d e m mass spectrometers a specific ion is deflected, and passes through a reaction chamber with linear dimensions of about 1 cm. I n both cases the relatively short ion p a t h length dictates t h a t pressures in excess of 10~3 m m will be required to obtain significant reaction for reactions with typical cross sections (10—100 Â 2 ). This high pressure in the source can be reached by differentially pumping the reaction zone (or ion source) but this requires extensive modification or design changes in the instrument. I t is not possible to use a commercial mass spectrometer for ion molecule studies without such modifications. Furthermore, the changes made in such instruments often compromise sensitivity and resolution for the ability to operate at relatively high pres-
sures in the source. Other difficulties which relate to operation a t high pressures are the short lifetime of m a n y filaments, and pyrolysis and decomposition of gas on the hot filament surface which often give spurious results. On the other hand, reaction in ion cyclotron resonance spectrometers occurs a t relatively low pressures (10 _ e —10~ 4 mm) and therefore the problems associated with high pressure operation are not encountered. T h e phenomenon of ion cyclotron resonance was first applied to mass spectrometry by Sommer, Hippie, and Thomas in 1949 (1). However, as a straightforward mass spectrometric technique, icr has few advantages over the magnetic sector, electrostatic focusing, and timeof-flight mass spectrometers which have been used by analytical chemists for m a n y years. Indeed icr mass spectrometers are more limited with respect to mass range and resolution than are other types of mass spectrometers. Principles
Consider an ion moving in space with a velocity component which is
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perpendicular to a magnetic field as shown in Figure 1. The ion will experience a constant acceleration in a direction which is perpendicular to its direction of motion as long as it is in the magnetic field. Hence, the ion will follow a circular tra jectory in the plane which is per pendicular to the magnetic field (i.e., the XZ plane in Figure 1). The frequency of revolution (i.e., the natural cyclotron frequency) in the magnetic field is given in Equa tion 1, ω< = ^
(1)
Figure 1. Motion of a charged particle in a magnetic field Magnetic field in Y direction
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where a>c is the natural cyclotron frequency of the ion, e is the elec tronic charge, Β is the magnetic field strength, M is the mass of the ion, and c is the speed of light. The radius of the circular trajec tory is given in Equation 2, ν r = ~ ω0
(2)
where r is the radius, υ is the ion velocity perpendicular to the mag netic field, and rf is equal to o>e, the ion will absorb energy from the field and be accelerated. In the low pressure limit where no collisions or reac
Figure 2. Trajectory of an ion in sonance with rf electric field magnetic field in Y direction rf electric field in Ζ direction
tions occur, the equations of motion for an ion in such a combination of electric and magnetic fields have been treated and solved by several authors (2). These treatments have shown that the rf electric field may be treated as two contrarotating electric fields, and further that only the component which has the same sense of rotation as the ion will be effective in accelerating it. In the earliest icr spectrometers (called omegatrons) ions were pro duced by electrons as in most mass spectrometer ion sources, and in the same region an rf electricfieldwas
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Figure 3.
END VIEW SIDE VIEW Schematic of early icr mass spectrometer discussed in Reference 1
introduced (Figure 3). If rt was equal to o>0 for a given ion, it would be accelerated, and its trajectory would be that of an Archimedes' spiral. A suitably placed electrode could therefore collect ions in reso nance while ions not in resonance would presumably not be detected. By sweeping either the magnetic field (changing the 2 2eV
(4)
From Equation 4 it is clear that the highest resolution will be achieved when ω is as large as pos sible. But from Equation 1 it is also clear that a large ω0 will also require a correspondingly large Β in order for an ion of given m/e to pass through resonance. The largest magnetic fields which can be reached with the relatively simple 24 A
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9" electromagnets used in most icr spectrometers are --15,000 gauss. The most common method of re cording spectra is to fix ωΓ/ and sweep B. At fixed ωΓ; the resolu tion of all ions will be the same, but a compromise must always be made between mass range and resolution. For instance, at rf = 765 kc a theoretical resolution of ~5000 can be achieved in the absence of col lisions. However, the highest mass which can be observed at 15,000 gauss is m/e 30. At 153 kc the mass range is increased to m/e 150, but the resolution is decreased to — 1000. It would appear then that the useful mass range could be in creased quite a bit more by lower ing iart if the sole requirement were unit resolution. However, it must be remembered that icr experiments are most useful when applied to the study of ion molecule reactions which occur only when the ions in question undergo collisions. Un fortunately, collisions will broaden the observed peaks and at higher pressures the resolution is pressure dependent as well as frequency de pendent. As a matter of practical utility, unit resolution cannot be main tained at pressures which are high enough for significant reaction to occur if (orf is much below 120 kc. This limits the useful mass range to ~ 180-200 insofar as the study of reaction products is concerned. Furthermore, an effective base to base resolution of about 150 is the most that can be achieved under such conditions. As stated earlier, higher resolution can be achieved by increasing o>rf only at the expense of
mass range. This situation could be improved by using larger mag nets, but the interdependence of mass range and resolution will al ways have to be dealt with. The cell design currently used for the study of ion-molecule reactions is somewhat different from that de scribed in Figure 3. Figure 4 shows schematically the behavior of a charged particle in a crossed dc electric field and constant magnetic field. Solving the equations of mo tion for an ion in such a situation shows that the ion will drift in a direction which is perpendicular to both the electric and magnetic fields. The ion will follow a cy cloid trajectory along the X axis, and its drift velocity in the X direc tion is given by Equation 5
V, = f
(5)
where Es is the static electric field strength and Β as before is the magnetic field strength. Hence an ion produced at the origin in Figure 4 can be moved from its point of origin (and its drift velocity con trolled) by applying a variable dc electric field in the Ζ direction. For an argon ion (m/e 40) in an 8-kilogauss magnetic field, a dc po tential of 0.25 V/cM will result in a drift velocity of ~500 cm/sec. This is several orders of magnitude less than the thermal velocity of ar gon and so the kinetic energy of the
F i g u r e 4. T r a j e c t o r y of an ion in c r o s s e d dc e l e c t r i c a n d m a g n e t i c f i e l d s magnetic field in Y direction electric field in Ζ direction
Report
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Figure 5. 1er cell used for study of ion molecule reactions
ion is not appreciably perturbed by t h e application of the dc electric field. T h e instrument (commercially available from V a r i a n Associates: Syrotron) and cell used in our work have been discussed previously (3, 4) • T h e cell is shown in Figure 5. Region A is denoted as the ion source although there are no p h y s i cal boundaries (focusing electrodes, etc.) separating this region from the remainder of t h e cell. Electrons are produced by a heated rhenium fila ment and are accelerated through the ion source. T h e magnetic field serves t o collimate t h e electrons and as in a n y mass spectrometer ions will be produced along the t r a c k of electron beam. B y applying a suitable dc elec tric field between plates 1 and 2, ions are caused to drift down the long axis of t h e cell and a w a y from their point of origin along the elec tron beam. However, the ions are not restricted b y t h e magnetic field with respect t o motion in the Y di rection—i.e., parallel to the m a g netic field—and therefore a small trapping potential m u s t be applied to plates 2 and 4 to keep the ions from drifting to the cell walls. A positive potential serves to t r a p positive ions while pulling negative ions and scattered electrons out of the coll. AVhen negative ions are studied, the t r a p p i n g potential is simply reversed. T h e ions will cycloid into region Β (the detector region) where a n rf electric field is applied between plates 5 and 6. An ion in resonance with this field will absorb energy and be accelerated. I n s t e a d of col
lecting the ions, a m a r g i n a l oscilla tor is used to detect the energy which is absorbed b y t h e ion. T h i s method of detection permits usable signals t o be obtained from ion densities as low as 5—10 ions/cc. I t also eliminates the problems associated with s t r a y ions finding their w a y to the collector electrode. Such problems were common in earlier instruments using ion collec tion techniques. I n s t r u m e n t a l l y , the technique is quite similar to n m r and epr ex periments in which energy absorp tion occurs from a n oscillating elec tric field. I n such experiments, it is necessary to m o d u l a t e one of the variables affecting energy absorp tion and in the case of ion cyclotron resonance, experiments have been carried out using magnetic field modulation (3, 5), source drift modulation (3, δ), and most re cently electron energy modulation (4) • T h e latter technique has proved t o be the most flexible and useful of t h e various icr modulation schemes used to obtain simple mass spectra and has been used almost exclusively in our laboratory as well as several others since it was de veloped. N a t u r a l l y , only one ion can be in resonance in the detector region a t any given time, and a dc electric field is therefore applied between plates 5 and 6 to keep those ions not in resonance drifting down the long axis of t h e cell. E v e n t u a l l y , these ions reach t h e t o t a l ion collector (region C ) . H e r e t h e four elec trodes are a t ground potential, and are connected to an electrometer. Since the ions have no directed m o -
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Figure 6. 1er mass spectrum of n-decane pressure 1 X 10 « mm election energy 40 ev parent ion = 4% of total ionization
sources of conventional sector mass spectrometers make it quite difficult to obtain low electron energy spectra. In Figure 7, the icr mass spect r u m of n-decane a t 12.0 electron volts is shown. As this is just 2.8 eV above the ionization potential of n-decane, much less fragmentation takes place. T h e parent ion represents 2 8 % of the total ion current under these conditions compared to 4 % in Figure 6. T h e cell dimensions of the icr spectrometer are 2.5 X 2.5 X 10 cm. Since drift velocities on the order of 500 cm/sec are common, typical ion residence times are 0 . 5 25 milliseconds, and typical path lengths for thermal velocity ions are 1-50 meters. This compares to residence times of 1 0 ~ e sec and path lengths in the source of 0.5 cm in sector type mass spectrometers. Since the p a t h length of an ion m a y be 1000 times longer in an icr spectrometer t h a n it is in a sector mass spectrometer, it will suffer 1000 times as many collisions at any given pressure in an icr spectrometer. I t is this feature of the icr spectrometer which accounts for its usefulness as a tool for the study of ion molecule reactions a t low pressures. Experimental Techniques
Figure 7. 1er mass spectrum of n-decane p r e s s u r e I X 10 6 m m electron energy 12 eV parent ion = 28% of total ionization
tion in this region, and no trapping potential is applied, they will drift to the side plates and be recorded as an ion current. I t is possible therefore to collect all of the ions in region C while simultaneously measuring the power absorption of any given ion in region B. A t y p i cal simple mass spectrum for ndecane is shown in Figure 6. T h e spectrum in Figure 6 was obtained at 40 electron volts, and is similar to the 70-volt mass spectrum obtained with a conventional mass 26 A
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spectrometer reported in the A P I tables for n-decane. However, from the previous discussion, it should be clear t h a t there are no strong electric fields present in the icr spectrometer to perturb the kinetic energy of either t h e ions or the electrons in the source region. As a result it is easy to obtain spectra at low electron energies (and with well defined electron energies) in the icr spectrometer. On the other hand, the accelerating fields and repeller fields which are present in the
Even with the mass range limitation discussed above, many reactive systems can be studied. At low electron energy in 1-butene, only the parent olefin ion is present and neither fragmentation nor reaction takes place. However, in Figure 8 1-butene is examined at a pressure of 10~ 5 mm, and product ions are present at masses which are higher t h a n the molecular weight of the neutral molecule. Since only one primary ion is produced by electron impact at 10 eV, it is reasonable to presume t h a t the products observed in Figure 8 result from the reaction of 1-C 4 H 8 + with neutral 1-C 4 H 8 . However, the situation can be much more complicated when more t h a n one primary ion is present in the system. Consider the spectrum shown in Figure 9. Here 1-C 4 H 8 is mixed with 2-C 4 D 8 . At low pressure and low electron energy only the molecular ion from each com-
Report
Figure 8.
Figure 9. mixture
1er mass spectrum of 1-butene pressure5 X 1 0 s mm electron energy 10.5 eV
Figure 10. mixture
1er mass spectrum of 55% 1-butene, 45% Z-butene pressure 5 X 10 s mm electron energy 10.5 eV
1er mass spectrum of 55% 1-butene, 45% 2-butene pressure 1 X 10 ° mm electron energy 10.5 eV
pound is produced. At high pressure in Figure 10 extensive reaction has occurred and a quite complicated distribution of reaction products and H-D exchange products is evident between m/e 70 and m/e 96. Indeed it is not possible to determine which of the two possible reactants is involved in the production of each product ion from a 28 A
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simple inspection of the spectrum. However, it is possible to identify specific reactions in the ion cyclotron resonance spectrometer by means of an "icr double resonance experiment." Consider the general reactions
1) A+ +M->C+ + N 2) B+ +M ^ D+ +N'
where A+ and B+ are primary ions and C+ and D+ are secondary product ions. In a sector mass spectrometer (under conditions where most of the primary ions were not used up by reaction) A+ and B+ would be linearly dependent on the pressure while C+ and D+ would be dependent on the square of the pressure. If C+ and D+ reacted further, the products of those reactions would be tertiary products and would show a cubic pressure dependence. Pressure dependence experiments therefore give information about the number of collisions involved in the production of a given product and were the most common technique used to interpret ion molecule reactions in sector mass spectrometers. However, such an experiment will not distinguish which of the two reactants is involved in the production of the two products in Reactions 1 and 2, since both A+ and B+ are primary ions while C+ and D+ are secondary In the icr spectrometer this determination can be made in the following way. The marginal oscillator is set to continuously observe the C+ product ion at
