2417
Anal. Chem. 1983, 55,2417-2418
CORRESPONDENCE High-Energy Collision- Induced Dissociation with Fourier Transform Mass Spectrometry Sir: Collision-induced dissociation (CID) of mass selected ions plays an important role in analytical mass spectrometry (1,2). In a series of recent papers, Freiser and co-workers described the methodology for performing CID by using a commercially available Fourier transform mass spectrometer (FTMS) ( 3 , 4 ) . This development greatly expands the analytical capabilities of FTMS; however, the results reported by Freiser are limited to low translational energy CID (in the 10-100 eV range). In the original theoretical treatment of CID-FTMS, McIver discussed high energy (keV) CID and pointed out that instrument modifications are required to achieve these energies on typical instruments (5). For example, the maximum translational energy an ion can receive without being ejected from the cell can be calculated by using eq 1 (6, 7). As can be seen, E,,(max) is proportional to the &(mu) = e2r2B2/2m (1) charge of the ion (e2 = 1.6 X C), the square of the ion cell radius (r in meters), and the magnetic field strength ( B in tesla) (3). We have enlarged the ion cell on our Nicolet FTMS-1000 in order to perform CID in the kiloelectronvolt energy range. For example, with the larger cell (r = 2.4 cm) ions of m / z 100 can be accelerated to translational energies of approximately 2.1 keV, whereas the original system (r = 1.27 cm) was limited to translational energies of approximately 280 eV a t mlz 100. In our laboratory we are investigating the utility of FTMS for the study of large biomolecules. We feel the applicability of FTMS to these types of studies can be greatly enhanced by the development of CID-FTMS. However, it is obvious from other studies and our own preliminary studies that CID-FTMS will be of limited utility when the collisional activation (CA) is performed at low (10-100 eV) translational energies. For these reasons we are interested in developing instruments for high-energy (keV) CID-FTMS. This report presents the first results of such work.
EXPERIMENTAL SECTION The experiments described here were performed with a Nicolet FTMS-1000 system. The technique and instrumentation used for obtaining the CID spectra are similar to those described by Freiser and co-workers ( 3 , 4 ) . The magnetic field employed for these studies was nominally 3 T with a trapping voltage of 1.35-2.5 v (17.7 V/m-32.8 V/m). The c&6+'ions subjected to CID were generated by 12-eVionization of benzene. At this ionizing energy, fragment ions, viz., C&5+, C4H4+a,and C3H3+,are not observed, thus it was not necessary to eject background ions from the cell prior to the acceleration step. The CID was performed using argon or benzene as the target gas. The benzene was admitted to the analyzer, using a varible leak valve, to a pressure of ca. 5 x lo-" torr. The argon was introduced through a separate leak valve to a pressure of ca. (1-2) x IO-' torr. Ion acceleration was performed using the "off-frequency"acceleration procedure described by Freiser (3). This procedure was found to increase the overall efficiency of CID and minimize ion losses due to ejection. However, great care was taken to ensure that the acceleration time was short compared to the average time between bimolecular collisions. At long acceleration times spurious results were observed due to multiple collision effects. All samples and gases were obtained from standard commercial sources. 0003-2700/83/0355-2417$01.50/0
Table I. Effect of Target Gas and C,H,'. Ion Translational Energy on the Branching Ratio for the C, Fragment Ionsa argon benzene excitation time, ms excitation time, ms fragment ion 9 6 3 9 6 3 0.42 0.43 0.38 0.32 0.27 0.26 C,H,'. 0.31 0.28 0.31 0.45 0.49 0.50 C,H:. 0.27 0.29 0.31 0.23 0.24 0.24 C,H:. a C,H,'. was accelerated at a constant frequency of 589 Hz using different excitation times. Resonant frequency of m/z 78 is (594.8 kHz). Trap voltage is 1.5 V (see Experimental Section). For a given excitation frequency the translational energy of the ion increases as t2 (see ref 4).
RESULTS AND DISCUSSION A very interesting result of Freiser's work and work in our own laboratory using a low field system (1.9 T) with a standard ion cell (2.54 cm3) is that C6H6+' (ionized benzene) and other highly stable carbocations, viz., C7H7+and C7H70+,cannot be collisionallydissociated even though the ion's translational energy is relatively high, e.g., 100-200 eV. Using our modified system (r = 2.22 cm and B = 3 T), we find that the ionic systems listed above readily undergo CID in a manner similar to that observed for beam type CID experimenta,e.g., 4-8 keV translational energy. The CID-FTMS spectrum of C6H6+* (ionized benzene) obtained at high energy is contained in Figure 1. To a first approximation the spectrum is independent of E, (for E greater than ca.500 eV) for a given target gas. Small differences (6% relative abundance) are detectable in the fragment ion branching ratios as a function of E,, e.g., C4H2+.and C4H3+relative to C4H4+e.On the other hand, significant changes in the branching ratios are detected for target gases resulting in higher center-of-massenergies (see Table I). This observation is consistent with other studies of high energy CID (8). Additional supporting information for the proposed high energy CID-FTMS is the observed charge stripping peak at m / z 38.5 (C6H?+). The C6HS2+ion arises from fragmentation of CgHS2+formed by charge stripping of C6H6+*(eq 2). It
C6H6"
4-
N
-
CgH':
e-
-
C&52+
He
(2)
should be noted that C6HS2+is not observed due to overlap with C3H3+. In other systems, e.g., ionized toluene (C,H8+-) and C7H7+ (from ionized toluene), similar charge stripping reactions are observed both for CID-FTMS and CID using a beam type instrument (Et, = 8 keV) (9). The results from earlier CID-FTMS studies raise a very fundamental question concerning the translational energy at which CID is performed, i.e., why is it not possible to collisionally dissociate ions (say C6H6+.)at translational energies corresponding to center-of-mass energies sufficiently high for CID to be facile? These results from low energy CID studies have analogies for CID experiments performed at kiloelectronvolt energies. For example, as the size of molecules 0 1983 American Chemlcal Society
2418
ANALYTICAL CHEMISTRY, VOL. 55, NO. 14, DECEMBER 1983
states leading to photodissociation of C7H7+(toluene precursor) and C7H70+(methyl phenol precursor) also have high energies, e.g., ,A, 300 nm or 3 eV (13,14). In these ionic systems it is prdbable that CA gives rise to low energy states (states below the dissociation threshold) rather than the higher energy states yhich results in dissociation, especially at low translational energies. In larger molecules, say m/z 1000 and grgater, the density of low lying states is extremely high relative to small systems (C6-ClO),thus leading to very low cross sections for formation of high energy dissociating states. We are presently using CID-FTMS and (4-8 keV) beam type CID to investigate fundamental parameters of collision spectroscopy of polyatomic systems over a range of energies (100 eV-8 keV). We feel these comparative studies are potentially fruitful because of the enormous versatility afforded by FTMS. For example, the translational energy of the mass selected ion as well as the reaction time and the time of ion acceleration can be controlled. Also, experiments can be performed under single or multiple collision conditions. In the present study, great care was taken to ensure single collision conditions; however, future studies will address specifically the effects of multiple collisions.
-
30
40
50
60
70
80
!
m/2
Figure 1. High energy CID-FTMS spectrum of C8H6+. (ionized benzene). The translational energy of C6H,+. is ca. 2 keV. C6H6+. was produced by 12 eV ionization of benzene (partial pressure ca. 5 X 10" torr) and CID was performed wlth argon (partlal pressure ca. 1 X IO-' torr) target gas.
subjected to collisional activation (CA) increases, CID is less able to induce structurally significant dissociation reactions. We have found cases where the (unimolecular)metastable ion spectrum is 10 times more intense than the CID spectrum. These observations were made on a beam instrument operated at 8 keV beam energy using ions in the range of m/z 7 W m / z 1500. The inefficient CID can in part be attributed to rapid energy randomization (high density of states) in these large molecules. Also, it is possible that kiloelectronvolt collisional activation (CA) of large molecules, as well as low energy CA of small molecules, involves vibrational excitation rather than electronic excitation. The fundamental assumption of CA is that the bimolecular collision results in a vertical electronic transition, and by internal conversion a vibrationally "hot" molecule is produced which ultimately dissociates (the twostep model) (IO). However, in order for dissociation to occur the energy of the excited state must be greater than the dissociation energy. For molecules with a high density of low lying states, e.g., polyatomic molecules, it is highly probable that CA will favor production of these low lying states, especially when Et, is low (10-100 eV). Furthermore, based on the energy gap considerations, Le., the energy difference between the excited state and the dissociation products, these low lying states will preferentially decay via nonradiative modes, e.g., isomerization/rearrangement reactions (1I ) . Also, dissociation from these lower energy states will favor the lowest energy reaction channels (1,2). With the above considerations in mind, it is interesting to note that benzene C6H6+-and other ions which are difficult to dissociate by CID-FTMS, viz., C,H7+ and C,H,O+, have relatively high dissociation energies. In particular, benzene CeH6+-is known to have two low energy states (E: and A:, 11.49 eV and 12.4 eV, respectively) below the minimum energy dissociation channel (13.8 eV for loss of H (12)). The excited
LITERATURE CITED (4) [2) (3) (4)
(5)
(6) (7)
(8) (9) (IO) (11) (12) (13) (14)
McLaffenty, F. W. Acc. Chem. Res. 1980, 13, 33. McLaffenty, F. W. Science 1981, 214, 280. Cody, R. B.; Burnler, R. C.; Freiser, B. S. Anal. Chem. 1982, 54, 96. Cody, R. B.; Burnler, R. C.; Cassady, C. S.; Frelser, B. S.Anal. Chem. 1988, 54, 2225. McJver, R. T. Workshop on newer Aspects of Ion Cyclotron Resonance (Fourier Transform Mass Spectrometry), 29th Annual Conference of Mass Spectrometry and Allied Topics, Minneapolis, MN, 1981; p 798. Beauchamp, J. L. Annu. Rev. Phys. Chem. 1971, 22, 527. Comisarow. M. B. Int. J. Mass. Spectrom. Ion. Phys. 1978, 26, 369. Cooks, R. G. "Collision Spectroscopy"; Cooks, R. G., Ed.; Plenum Press: New York, 1978; Chapter 7. Brlcker, D. L.; Russell, D. H., unpublished results, Department of Chemistry, Texas A&M University, May 1983. Los, J.; Govers, T. R. "Colllsion Spectroscopy"; Cooks, R. G., Ed.; Plenum Press: New York, 1978; Chapter 6. Krailler, R. E.; Russell, D. H., unpublished results, Department of Chemistry, Texas A I M Unlversity, May 1983. Rosenstock, H. M.; Dannacher, J.; Liebman, J. F. Radiat. Phys. Chem. 1982, 20, 7. McCrery, D. A,; Frelser, B. S. J. Am. Chem. SOC.1978, 100, 2902. Cassldy, C. J.; Freiser, B. S.; Russell, D. H. Org. Mass Spectrom., in press.
D. L. Bricker T. A. Adams, Jr. D. H. Russell* Department of Chemistry Texas A&M University College Station, Texas: 77843 RECEIVED for review August 4,1983. Accepted September 12, 1983. This work was supported in part by grants from the Robert A. Welch Foundation and the U S . Department of Energy, Office of Basic Energy Sciences. The authors also wish to acknowledge the Texas A&M University Center for Energy and Mineral Resources and the Office of University Research for providing funds to purchase the FTMS. T.A.A. gratefully acknowledges financial support as a Robert A. Welch graduate fellowship.
