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Anal. Chem. 1989, 61, 83-85 (3) Tran-Mlnh, C.; Vallln, D. Anal. Chem. 1978, 5 0 , 1874-1878. (4) Rich. S.; Ianniello, R. M.; Jespersen, N. D. Anal. Chem. 1979. 5 1 , 204-206. (5) Fulton, S. P.; Cooney. C. L.; Weaver, J. c. Anal. C h m . 1980, 5 2 , 505-508. (6) Leypoklt. J.; &ugh, D. Anal. Chem. 1984, 5 8 , 2896-2904. (7) Cares, S.; Petelenz, D.; Janata, J. Ana/. Chem, 1985, 5 7 , 1920-1923.
(8) Bischoff, K. AIChEJ. 1965, 1 1 , 351-355.
RECEIVEDfor review November 10,1987. Resubmitted March 29, 1988. Accepted October 11, 1988. This work was performed with SUPPOd by Pants from the Whitaker Foundation and the Arizona Disease Control Research Commission.
CORRESPONDENCE High-Resolution Ion Partitioning Technique by Phase-Specific Ion Excitation for Fourier Transform Ion Cyclotron Resonance Sir: The intrinsic versatility of Fourier transform ion cyclotron resonance (FT-ICR) for analytical studies has been widely demonstrated. The utility of ion traps arises from the ability to manipulate ions that are stored in the trap ( I ) . Ion manipulation is accomplished in FT-ICR by ion ejection (e.g., ion selection for chemical studies (2) and ejection of high abundance ions for enhancement of dynamic range ( 3 , 4 ) ) .Ion ejection is achieved by accelerating unwanted ions (by resonant radio frequency (rf) excitation) until their radii exceed the confines of the ICR cell. During a prolonged ejection sweep, signal loss can occur due to ion-neutral collisions or ion evaporation (5),therefore rapid sweeping or “chirp” excitation is utilized. Because the excitation magnitude decreases as the inverse of the sweep time (6),rapid sweep rates often lead to a loss of selectivity with respect to ion excitation (3). Ions of interest are inadvertently accelerated resulting in (i) ejection of reactant ions and (ii) complication of ion-molecule reaction studies. It has also been demonstrated that the phase relationship of translationally hot ions complicates ion detection (7). Owing to the importance of ion selection in FT-ICR, there has been much work in high-resolution ion isolation (8-10). Marshall and co-workers have demonstrated that the ejection sequence can be performed rapidly while maintaining high selectivity by utilizing an inverse Fourier transform (11). Although this procedure significantly reduces the problems incurred, there can be undefined excitation of ions of interest as well as incomplete ejection (12). We recently demonstrated that ions can be selectively partitioned (in terms of m / z and kinetic energy) in a twosection cell (13). Unwanted ions are excited to radii that exceed the dimensions of the conductance limit orifice and are selectively discriminated against upon partitioning from the source to the analyzer regions of the two section cell. Because the ions are not translationally excited to large radii, ion selection is highly selective. Because ion cyclotron frequency shifts occur as ions reach large orbital radii, any method of ion ejection loses selectivity. Thus, even perfectly selective excitation sources are limited in resolution by the ICR frequency differences between the center and edge of the ion cell. The two-section ion cell partitioning experiment is potentially advantageous because the ions do not have to be totally ejected in order to be removed. Although the excitation magnitude is substantially decreased by accelerating unwanted ions to radii which need only exceed the dimensions of the conductance limit, tailing of an excitation sweep can still result in inadvertent excitation of the ion of interest. We describe here a technique for ion
selection that combines phase-specific excitation and ion discrimination on partitioning in a two-section FT-ICR cell. This method greatly enhances ion isolation by applying the relationship of the excitation phase angle to ion acceleration. The radial velocity of ions is modulated by phase-specific excitation. By control of the radial velocity (i.e., cyclotron radius), selected ions can be manipulated to produce high ion selectivity upon partitioning.
EXPERIMENTAL SECTION All experiments were performed on a prototype Nicolet Analytical Instruments FTMS-1000 spectrometer equipped with a 3-T superconducting magnet. The vacuum system has been modified to accommodate a two-section cell. The two-section cell consists of two cubic cells (3.81 X 3.81 X 3.81 cm) mounted collinearly along the central axis of the magnetic field. The two cells share a common trap plate that also serves as a conductance limit for the differential pumping system. The orifice in the conductance limit has a radius of 2 mm. The vacuum in both sections of the differentially pumped system is maintained by oil diffusion pumps. Background pressures for both sections of Torr or less. Gaseous reagents the vacuum system were 1 X were admitted to the vacuum system by variable leak valves (Varian Series 951). Ionization was performed by electron impact (50-eV electrons). Detection of the ions in either the source or the analyzer region of the cell was performed by electronically switching the rf excite pulses between the cell regions.
RESULTS AND DISCUSSION To illustrate the capabilities of this technique, nominal isobars of Fe2(C0)3+and Fe(CO)S+ ( m / z 195.8546 and 195.9095, respectively) were isolated from one another. Separation of these ions requires a mass resolution of ca.4000. The corresponding cyclotron frequencies (in a 3.028-T field) for these ions are 243.0847 and 243.0165 kHz yielding a frequency difference of ca. 50 Hz. Contained in Figure 1A is a mass spectrum of Fe(C0)5. The peak at m / z 196 corresponds to the isobar of Fe2(C0)3+and Fe(C0)5+. This system was selected because isolation of one of these ions for chemical studies by a simple “chirp” ejection sweep is difficult without inadvertent excitation of the ion of interest. By use of phase-specific ion excitation it is possible to select one of the isobars for either enhancement of dynamic range or chemical studies (Figure 1B). Ion selection is better illustrated when a narrow mass range is used such that the selection of specific peaks can be observed. Contained in Figure 2 is the mass spectrum of Fez(CO),+ and Fe(CO)5+with isotopes at m / z 194,196, and 197. The spectrum of isolated Fez(C0)3+contained in Figure 3 was obtained by using phase-specific ion selection. An initial
0003-2700/89/0361-0083$01.50/00 1988 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 1, JANUARY 1, 1989
19'4
19h
L
196MASS IN 197 A . M . U .198
199
200
Figure 3. A mass spectrum of Fe2(CO),+ isolated in the analyzer region by phase-specific ion partitioning.
MASS
IN A . M . U.
Figure 1. (A) A mass spectrum of Fe(CO), including nominal isobars of Fe2(CO)3+and Fe(CO),+ ( m / z 195.8546 and 195.9095 amu). (B) Ion isolation by phase-specific ion exciation.
Figure 4. A mass spectrum of Fe(CO),+ isolated in the analyzer region by phase-specific ion partitioning. spectra solely to establish the selective isolation of the isobars at m / z 196. The perturbations in the signal intensities for m / t 197 and 198 are the result of acceleration of the ions due to tailing of the initial excitation sweep. Such effects can be minimized, or even eliminated, by varying the sweep range and amplitude of the initial ef excite (6). However, such perturbations in signal intensities are not normally of any real significance when it is necessary only to increase the ion orbital radius to values greater than the dimensions of the conductance limit aperture. The exciteldeexcite step is used to isolate a particular ion on the analyzer side of the two-section ion cell and leave all others in the source region. For example, in an ion-molecule reaction study of a chosen m/z 196 isobar, all ions of mlz ratios other than 196 are not partitioned. The experimental sequence for phase-specific ion partitioning is illustrated in Figure 5. Following the ionization event (Figure 5A), all ions are accelerated to radii that exceed the dimensions of the conductance limit (Figure 5B). Ions that are translationally excited to a larger cyclotron radius by a resonant oscillating electric field can be deexcited (i.e., lose translational energy in the X-Y plane) by inverting the phase of the applied rf field with respect to the phase of the cyclotron orbit of a given ion packet ( m / z ratio) (14,15).In this manner, the final radii of selected ions can be modulated by selective excitation/deexcitation (Figure 5C). Ions that are decelerated to sufficiently small radii are selectively partitioned to the analyzer region of the two-section cell (Figure 5D). As mentioned above, specific ion isolation as a result of "chirp" ejection is difficult because of tailing of the power
LA--?-Jd:h
_i
194
195
1 9 6 t l A S S Iiu l9,7 A I4 U 1 9 8
199
200
Figure 2. A mass spectrum of the nominal isobars of Fe2(C0)3+and Fe(CO)5+( m / z 195.8546 and 195.9095 amu). frequency sweep (120 V/m; 250 ps; 243.2508 to 242.9017 kHz or mlz 195.7208to 196.0021) accelerated both isobars to radii too large to partition from the source to analyzer regions of the two-section cell. Fe2(C0)3+ions were then deexcited by phase-specific ion excitation to an orbit acceptable for partitioning. Because the Fe(CO)5+ions were not deexcited, only the Fe2(C0)3+ions partitioned to the analyzer region. Similarly, the spectrum contained in Figure 4 was obtained by phase selecting the Fe(COI5+ions such that the Fe2(C0I3+ions were not allowed to partition. To demonstrate the ability of the technique to selectively isolate isobars, the spectra contained in Figures 3 and 4 were acquired by accelerating only the isobars at m / z 196 with a narrow excite sweep (237.15 to 236.82 kHz). By use of this approach, the signals for the 67Feand 13C isotopes remain in the spectra. The resolved isobar at m / z 197 was left in the
ANALYTICAL CHEMISTRY, VOL. 61, NO. 1, JANUARY 1, 1989
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S1
DELAY
S2
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PARTITION
Q I
C)
0)
Flgure 5. Extreme selectivity for ion isolation is achieved by phasespecific excitation/deexcitation of the translational energy of selected ions coupled wlth partitioning in a two-sectlon cell: (A) ions are produced in the source region: (e) all ions are “driven” into phase coherence and accelerated to larger radii; (C) selected lons are deexcited to radii suitable for partitioning; (D) upon partitioning deexcited ions are selectively isolated.
Flgure 6. Two ions whlch have distinct cyclotron frequencies will oscillate in- and cut-of-phasewith respect to each other creating a beat frequency. A node of the beat frequency corresponds to a relative phase angle of 180’. curve. Therefore, two ions of close m / z ratios are difficult to separate because rf excitation leads to acceleration of the ion of interest. Because the ions have discrete frequencies, there will be a beat frequency produced as the ion packets oscillate in- and out-of-phase with each other (see Figure 6). A schematic for phase-specific ion partitioning is contained in Figure 7. Ion separation is achieved by the application of an initial excitation pulse (Sl) which increases the radius of the ions to be separated. A second, single frequency excitation pulse (S2) is initiated after a delay (m)corresponding to a node of the beat frequency. Nodes of the beat frequency correspond to a condition a t which the phases of the two ion packets are 180’ out-of-phase with respect to one another. If S2 is 180’ out-of-phase with respect to the cyclotron motion of the ions of interest, these ions can be deexcited to a radius suitable for partitioning. Similarly, the ion packet that is in-phase with 52 (but slightly off-resonance) will be driven to a larger radius. Conditions are chosen such that only deexcited ions partition. It is important that ions of significantly different cyclotron frequencies from S2 will not be perturbed from the excited condition induced by S1 (i.e., these ions cannot partition). It should be noted that the phase-specific scheme of separating ions requires ion packets of small radial and angular distribution with respect to the FT-ICR cell. That is, ions must be formed with low kinetic energies perpendicular to the X-Y plane (e.g. electron impact ionization) such that they can be “driven” into phase coherence by Sl(7). Only ions of well-defined phase coherence give rise to defined nodes in the cyclotron beat frequency. Therefore, sample pressures were maintained at 2 x lo-’ or lower to minimize ion-neutral collisions. If these conditions are met, then this technique affords facile separation of ions of two very similar m / z ratios while virtually eliminating all energy tailing effects. By implementation of phase selection in a two-section cell during the partitioning event, good selectivity can be realized
TIME
Flgure 7. An oscillating electric field which is applied at a node of the beat frequency will spatially separate the two ions of distinct frequency. in terms of ion isolation. This permits routine chemical studies of ions that had previously been difficult to isolate without the complication of inadvertent excitation. Phase selection also provides an increase in the dynamic range of FT-ICR. By use of this method, it is feasible to separate an ion of low abundance from an ion of high abundance even in the case where the frequencies are similar. LITERATURE CITED Allison, J.; Stepnowskl, R. M. Anal. Cbem. 1987, 59, 313. Anders, L. R.; Beauchamp, J. L.; Dunbar, R. C.; Baldeschwleler, J. D. J . Cbem. fbys. 1968, 45, 1062-1083. Castro, M. E.; Russell, D. H. Anal. Chem. 1985, 5 7 , 2290-2293. Wang. T. L.; Ricca, T. L.; Marshall, A. G. Anal. Cbem. 1986, 58, 2935. Remple, D. L.; Huang, S. K.; Gross, M. L. Int. J . Mass Spectrom. Ion Processes 1988. 70, 163. Marshall, A. 0.; Roe, D. C. J . Cbem. f b y s . 1980. 7 3 , 1581. Hanson, C. D.; Kerley, E. L.; M. E. Castro; Russel. D. H., unpublished results. de Konlng. L. J.; Fokkens, R. H.; Pinkse, F. A.; Nibbering, N. M. M. Int. J . Mass SpecWom. Ion Processes 1987, 77. 95-105. Chen, L.; Marshall, A. G. Int. J . Mss Spectrom. Ion Processes 1987, 7 9 , 115-125. Cody, R. B.; Goodman, S. D.; Hlen, R. E.; Weil. D. A. Proceedings of the 36th ASMS Conference on Mass Spectrometry and Allied Toplcs, San Francisco. 1988 pp 824-825. Marshall. A. 0.;Wang, T. L.; Ricca, T. L. J . Am. Cbem. Soc. 1985, 107. 7893-7897. Mullen, S. L. and Marshall, A. G. J . Am. Cbem. SOC. 1988. 110, 1786. Kerley, E. L.; Russell, D. H., Anal. Cbem. 1989, 61, 53-57. Marshall, A. 0.;Wang, T. L.; Ricca, T. L. Cbem. f b y s . Lett. 1984, 105, 233-238. Pfandler, P.; Bodenhausen, G.; Rapin. J.; Houriet, R.; G&urnann, T. Cbem. Pbys. Lett. 1987, 138, 195-200.
Curtiss D. Hanson Eric L. Kerley David H.Russell* Department of Chemistry Texas A&M University College Station, Texas 77843 RECEIVED for review July 5,1988. Accepted October 11,1988. This work was supported by the National Institutes of Health-General Medical Sciences (GM-33780) and the National Science Foundation (CHE-8418457). Some of the equipment was purchased from funds provided by the TAMU Center for Energy and Mineral Resources. We gratefully acknowledge the Texas Agricultural Experiment Station for providing a portion of the funds for purchase of the Nicolet FTMS 1000 mass spectrometer.
