ANALYTICAL OF lob RESONANCE By
J. M. S.
Henis, Monsanto
Ion cyclotron resonance spectrometry represents a rather unique combination o f mass spectroscopy and reaction kinetics. Some analytical applications are obvious although others await a different orients tion for development
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AS PHASE ION-MOLECULE REAC-
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 t o operate under conditions where the ions produced by electron impact could make 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. The instruments most commonly used for such studies were sector, time-of-flight, and, more recently, tandem 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 t o the properties of the instruments used. Mass spectrometers of the types mentioned above cannot generally operate a t pressures much above mm. Unfortunately, ions will not undergo a large number of collisions (and hence extensive reaction will not occur) unless the pres22 A
TIONS
ANALYTICAL CHEMISTRY
sure in the reaction zone is greater than mm. Naturally, the path length of an ion in the reaction region, the collision cross section, and the pressure, are all important in determining the extent of reaction. I n sector and time-of-flight mass spectrometers, reaction takes place in the ion source and the ion path length is thus limited to less than 1 cm. Similarly, in tandem 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 path length dictates that pressures in excess of mm will be required to obtain significant reaction for reactions with typical cross sections (10-100 A * ) . 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. It 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 a t relatively high pres-
sures in the source. Other difficulties which relate to operation a t high pressures are the short lifetime of many 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 (10-6-10-4 mm) and therefore the problems associated with high pressure operation are not encountered. The phenomenon of ion cyclotron resonance was first applied to mass spectrometry by Sommer, Hipple, 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 many years. Indeed icr mass spectrometers are more limited with respect to mass range and resolution than are other types of mass ape c t r omet ers . Principles
Consider an ion moving in space with a velocity component which is
Cc
REPORT FOR ANALYTICAL CHEMISTS
30 N.
Lind6ergh
Blvd.,
St. Louis,
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 trajectory in the plane which is perpendicular 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 Equation l ,
eB
wc = -
Cill
(1)
Figure 1. Motion of a charged particle in a magnetic field Magnetic field in Y direction
M o . 63766
where wC is the natural cyclotron frequency of the ion, e is the electronic charge, B is the magnetic field strength, M is the mass of the ion, and c is the speed of light. The radius of the circular trajectory is given in Equation 2, y =
V
-
WC
(2)
where r is the radius, v is the ion velocity perpendicular t o the magnetic field, and oC is the natural cyclotron frequency. From Equation 1 it can be seen t h a t oc is independent of the ion velocity. Hence two ions of the same mass, but with different kinetic energies will have the same natural cyclotron frequency. From Equation 2 it can be seen t h a t the faster moving ion will have a larger radius than will the slower one. Typical values for an argon ion (m/e 40), with thermal velocity, in an 8000 gauss magnetic field, are oc = 307 kc and r = .02 cm. If an rf electric field (o,.f) is introduced in the XZ plane (Figure 2) and o l j is equal to oc,the ion will absorb energy from the field and be accelerated. I n the low pre,
