1450
Anal. Chem. 1989, 61, 1458-1460
(14) McNeal, C. J.; Macfarlane. R. D.;Jardine, I. Biochem. Biophys. Res. Commun. 1988, 139, 18-24. (15) Barber, M.; Bordoli, R. S.; Sedgwick, R. D.; Tyler, A. N. J. Chem. Soc., Chem. Commun. 1981, 325-326. (16) Fenselau, C. I o n Formation from Organic Solids; Benninghoven, A,, Ed.; Springer Series in Chemical Physics 25; Springer: New York, 1983; p 90. (17) Gaskell, S. J.; Brownsey, B. G.; Brooks, P. W.; Green, 8. N. Biomed. Mass Specfrom. 1983, 10,215. (18) Williams, D. H.; Bradley, C.; Bojeson, G.; Santikarn, S.; Taylor, L. C. E. J. Am. Chem. SOC. 1981, 103,5700. (19) Willlams, D. H.; Bradley, C. V.; Santikarn, S . ; Bojeson, G. Biochem. J . 1982, 201, 105. (20) Desiderio, D. M.; Katakuse. I. Biomed. Mass Specfrom. 1984, 1 7 , 55. (21) Barber, M.; Bordoli, R. S.; Sedgwick, R. D.;Tyler, A. N. Biomed. Mass Specfrom. 1982, 9 ,206. (22) Dell, A.; Morris, H. R.; Egge. H.; Von Nlcolai, H.; Strecker, G. Carbohydr. Res. 1883, 115,41-52. (23) Dell, A.; Ballou, C. E. Org. Mass Spectrom. 1983, 18,50-56. (24) Dell, A.; Ballou, C. E. Carbohydr. Res. 1983, 120,95-1 11. (25) Balbu, C. E.; Dell, A. Carbohydr. Res. 1985, 140, 139-143. (26) Strecker. G.: Wieruszeski, J. M.; Martel, C.; Montreuil. J. Glvcoconjugate J. 1987, 4 ,329-337. (27) Reinhold, V. N.; Carr, S. A.; Green, B. N.; Petitou, M.; Choay, J., Sinay, P. Carbohydr. Res. 1987, 161, 305-313. (28) Dell, A ; Rogers, M E.; Thomas-Oates, J E.; Huckerby, T N.; Sander-
son, P. N.; Nieduszynski, I. A. Carbohydr. Res. 1988, 779,7-19. (29) Yang, V. C.; Linhardt, R. J.; Bernstein, H.; Cooney, C. L.; Langer. R. J. Biol. Chem. 1985, 260, 1849-1857. (30) Rice, K. G.; Kim, Y. S . ; Grant, A. C.; Merchant, 2. M.; Linhardt, R. J. Anal. Biochem. 1985, 150,325-331. (31) Linhardt, R. J.; Rice, K. G.; Kim, Y. S . ; Lohse, D. L.; Wang, H. M. Loganathan, D. Biochem. J. 1988, 254, 761-787. (32) Lehmann, W. D.; Kessler, M.; Konig, W. A. Biomed. Mass Specfrom. 1984. 1 7 , 217-222. (33) Mallis, L. M.; Jardine, 1.; Linhardt, R. J.. unpublished results of experiments performed at the Mayo Clinic, Rochester, MN, 1984, and discussions with I . Jardine of Finnigan MAT, San Jose, CA, 1986. (34) Gower, J. L. Biomed. Mass Spectrom. Ig85, 12, 191-196. (35) Rice, K. G.; Linhardt, R . J. Carbohydr. Res., in press.
RECEIVED for review November 9,1988. Accepted March 24, 1989. This work was supported by a grant from the National Institute of Health-General Medical Sciences (R.J.L., GM38060). Support for L.M.M., of the University of Iowa High Resolution Mass Spectrometry Facility, and funds for the purchase of mass spectrometry equipment for these studies were obtained from the University of Iowa Office of Educational Development and Research.
CORRESPONOENCE Photodissociation in a Reflectron Time-of-Flight Mass Spectrometer: A Novel Mass Spectrometry/Mass Spectrometry Configuration for High-Mass Systems Sir: The relative merits of time-of-flight (TOF) mass spectrometers have been discussed frequently. TOF instruments have the advantage of multichannel mass detection, high-frequency data acquisition, and high mass range. However, the applications of these instruments have been limited because of their low mass resolution. In recent developments, resolution limits on TOF instruments have been improved dramatically by the use of the “reflectronn configuration (1-6). Resolution in reflectron TOF systems has been demonstrated in the range of several thousand atomic mass units (1-6). This performance is now acceptable for many demanding applications. Nevertheless, TOF instruments have the remaining difficulty that they are not easily adaptable to tandem mass spectrometry experiments (i.e. MS/MS). In this correspondence, we describe a novel scheme involving a reflectron tandem instrument that overcomes many of the previous limitations on TOF systems for MS/MS experiments. This instrument configuration, and variations on this general theme, may have significant applications in a variety of mass spectrometry environments, particularly those involving large molecules. Before we describe the new instrument, it is important to consider the problems inherent in MS/MS experiments using conventional TOF instruments. Collisionally activated dissociation (CAD) is fundamentally incompatible with TOF measurements because collisions destroy the required welldefined ion kinetic energy. Modified TOF instruments have been used for CAD studies, but with unsatisfactory results compared to those obtained with other instruments (7, 8). TOF instruments have been used successfully for one stage of analysis in MS/MS CAD experiments (9,lO). Additionally, new experiments have been described employing tandem TOF instruments with a solid surface for collisional activation (11).
However, gas-phase CAD has not proven to be very useful for TOF/TOF experiments. In principle, laser radiation can be used instead of CAD for photodissociation with less distortion of the time-of-flight information. There have in fact been some successful examples of end-to-end tandem TOF experiments using laser dissociation (12-14). However, these experiments are difficult in practice because the fast ion beam must be overlapped in time and space with a pulsed (usually nanosecond) laser. Deceleration and reacceleration of the ion beam provides a partial solution to the problem, but the timing considerations are still critical. The reflectron configuration described below provides optimal laser overlap with the ion beam, while at the same time minimizing the uncertainties in the photodissociation laser firing time. The schematic diagram of the reflectron system in our laboratory is shown in Figure 1. Sample species for photodissociation experiments in this system are introduced in a pulsed molecular beam. However, this sample introduction scheme is not critical for the operation of the reflectron system. Ions are formed by intersecting the collimated molecular beam with the unfocused output of an excimer laser operating at either 193 or 157 nm (ArF or F, mixtures, respectively). Ions are accelerated into the first arm of the flight with a two-stage acceleration plate configuration, imparting a constant kinetic energy to all species. Mass selection occurs at the end of the first flight tube (1.2 m from the ionization point). Parallel plates (3-cm spacing) at this position are biased with a positive voltage (typically 100 V), which deflects positive ions off the flight tube axis. The rise time of this pulse, which is about 1 ws in the present system, is the limiting factor in mass selection capability. At the precise arrival time of the ion packet to be selected, the deflection voltage pulses to ground, transmitting that ion packet to the reflection region. The
0003-2700/89/0361-1456$01.50/0 0 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 13, JULY 1, 1989
1459
Reflection Grid
Asselbly
-
/
4
4
.
/
'
/
.
/
Fraglen tat ion Laser ,
ti H
c
*IO0 v 0
v
Pulsed
1Mas5 I I Gate
L - - c i - H - I - - c + !
1
5
CLLSTER
Einzel Lens
Cluster Source
Deflection Plates
1-
/)
l
7
f
,, .
I
1
1 I
Electron bltipl ier Tube
-
I::=
Ionization Laser
Acceleration Plates
Skim
Flgure 1. Apparatus configuration used for MS/MS experiments in the reflectron time-of-flight mass spectrometer.
reflection region of our instrument is constructed like those described previously (1-6). The angle between the two flight tubes is 12O, determined by other geometrical constraints in the molecular beam machine. However, windows are added to the vacuum system, allowing the introduction of a photodissociation laser in the reflection region. Specifically, the laser position and reflection voltages are adjusted so that the laser intersects the ion beam at the peak of its trajectory. At this point, the average vertical velocity component of the ion beam is zero, making it possible to fire the dissociation laser with the minimum timing uncertainty. Dissociation is accomplished with a second excimer laser. Residual parent ions and daughter fragment ions resulting from photodissociation are reaccelerated and mass analyzed by their flight time through the second arm of the flight tube (1.0-m length). The lasers and pulsed nozzle for this experiment are synchronized with a digital delay generator (Stanford Research Systems). Arrival time spectra are recorded with a transient digitizer system (Transiac Model 2101), triggered so that time zero is the fragment laser firing. Computer differences are accumulated for a sequence of 20 cycles with the fragment laser on and off for 20 laser shots each per cycle (800 total shots). The final spectrum presented is the resulting difference (laser on-laser off), indicating depletion in the parent ion channel and positive-going daughter fragments. An example of the performance of this instrument is shown in Figure 2 for the metal cluster ion PbI7+at m / z 3522. This species was produced by pulsed vaporization of a solid lead sample with a Nd:YAG laser a t 532 nm, followed by photoionization a t 193 nm. The 17-atom ion was size-selected out of a distribution containing clusters from two to 20 atoms in size and dissociated at 308 nm (XeCl excimer laser). The depletion in the parent ion channel represents approximately 5% of the available parent ion density per shot for a dissociation laser power of 1mJ/cm2. Higher laser powers produce greater depletion, but at the expense of multiphoton rather than single-photon absorption. Mass spectral studies of these cluster systems have been discussed in detail in previous reports from our laboratory (15). As shown, in this experiment
s
10
I
!
;5
SIZE
Flgure 2. Representative photodissociation spectrum obtained with the reflectron system. In this spectrum, the parent ion is a 17-atom cluster of lead atoms. Dissociation at 308 nm produces fragment ions containing from six to 13 atoms.
the primary charged fragments are in the size range of six to 10 atoms. In principle, the integrated intensity of the fragment ions should add up exactly to the parent ion depletion, but this is not exactly the case in Figure 2. In part, this nonadditive effect is caused by the poor shot-to-shot reproducibility of our experiment (primarily due to the laser vaporization cluster source), amplified by the difference technique used in data acquisition. Another more interesting effect is caused by the ion optics of the reflectron itself. Trajectory calculations show that the corresponding parent and fragment ions do not follow exactly the same trajectory and are laterally displaced from each other on the detector. Our present detector has a small acceptance aperture (1 cm2), making it difficult to achieve optimum focusing for both parent and fragment ion beams. An increased area detector (e.g. 1-in. diameter) would eliminate this problem. Under the typical operating conditions used here, the first-stage acceleration voltages total 1700 V, and the delay between the ionization and dissociation laser pulses is 179 ps. The time interval within which the dissociation laser beam can be made to intersect the parent ion beam is 3.0 ps. This same experimental configuration without the fragmentation laser can be used to detect unimolecularly produced, or metastable, fragment ions resulting from the laser photoionization process. While we have detected metastables for other cluster systems (e.g. benzene clusters), we have not detected metastable lead cluster fragments. It should be noted that laser dissociation experiments in reflectron spectrometers have been described previously (16, 17). However, this is the first such experiment in which dissociation occurs in the reflection field at the turning point in the ion trajectory. We have tried experiments with dissociation a t other positions just prior to the reflection field or within the field prior to the turning point and find that the timing considerations are quite severe in these configurations. Although the timing is convenient for dissociation a t the turning point, it is important to consider the possible effects of mass discrimination in this geometry. To do this we have used trajectory calculations on a variety of parent ion and fragment ion masses, with dissociation at different points along the trajectory. If there is no significant kinetic energy release in the lighter fragment ions, fragmentation at the turning point results in the least possible deviation between the parent ion and fragment ion trajectories. However, if there is significant kinetic energy release, fragment ion trajectories may be sufficiently displaced from the parent ion axis so that they will miss the dectector. Significant kinetic energy release is not expected for unimolecular decay of large molecules. However, the exact amount of energy release necessary to cause a fragment ion to miss t h e detector depends on the mass of the
1460
Anal. Chern. 1989, 67, 1460-1465
fragment, the distance to the detector, the size of the detector, and the angular distribution of the fragment. In our instrument, even a few tenths of an electronvolt of energy is sufficient to cause fragment loss if the energy is directed exactly perpendicular to the ion beam axis. Fragment ejection parallel to the beam axis may cause broadening in the arrival time spectrum. While we have made preliminary observations of fragment ion loss from diatomic and triatomic metal parent ions, we have not measured any noticeable broadening in arrival time spectra. One of the initial applications of reflectron instruments was in the study of metastable ion decay ( 2 , 5 , 6 ) .These experiments used the slow overall time scale for ion drift in the first flight tube, deceleration, and reacceleration to probe microsecond metastable lifetimes. Unimolecular dissociation lifetimes are another common problem in photodissociation studies of large molecules. In the configuration described above, however, irradiated ions remain in the turning region for up to 3-4 p s (mass dependent) before reacceleration. This instrument is therefore sensitive to “slow” unimolecular fragmentation processes. It should be emphasized that the present system is not yet fully optimized for the study of large molecules. The system described in Figure 1is in fact equivalent to two low-resolution spectrometers connected by the reflection/dissociation region. Mass selection of parent ions and mass analysis of fragment ions are both limited to unit resolution a t about m / z 200 (typical for a simple linear TOF). However, the same PbI7+ ion can be detected without photodissociation by its time of flight through the full reflectron instrument with a resolution of about 1OOO. To incorporate this general idea into the design of a genuine high-mass/high-resolution instrument, then, it should be possible to include a reflectron for high-resolution analysis prior to mass selection and another reflectron after the dissociation region for high-resolution analysis of fragments. This “triple reflectron” instrument would not be simple geometrically, but designs based on the linear reflectron (3, 18) concept could be used to achieve a more compact system. The general design philosophy described here may have significant applications for the study of large molecules, particularly for biological systems. Kinematic effects associated with large mass differences between parent ions and collision partners make collisionally activated dissociation less attractive than high-energy laser dissociation for these systems (19,20). In the reflectron configuration the timing problems associated with laser experiments are effectively eliminated, and the benefits of essentially unlimited mass range can be realized without sacrificing resolution. Additionally, the pulsed nature of these laser experiments lends them naturally to coupling with recently developed laser desorption sources (21-25) for the study of involatile species. These and other
applications of reflectron systems are under current investigation in our laboratory.
ACKNOWLEDGMENT We thank Fred McLafferty and Jon Amster for helpful discussions about these experiments. LITERATURE CITED (1) Mamyrin, B. A.; Karataev. V. I.; Shmikk. D. V.; Zagulin. V. A. Zh. Eksp. Teor. Fiz. 1973, 64,82. (2) Boesl, U.;Neusser, H. J.; Weinkauf, R . ; Schlag, E. W. J . Phys. Chem. 1982, 86,4857. (3) Lubman, D. M.; Bell, W. E.; Kronick, M. N. Anal. Chem. 1983, 55, 1437. (4) Kuehlwind. H.;Neusser, H. J.; Schlag, E. W. Int. J . Mass Spectrom. Ion Phys. 1983, 51,25. (5) Kuehlwind, H.;Neusser, H. J.; Schlaa. E. W. J. Phys. Chem. 1984. 88,6104. (6) Kuejlwind, H.;Neusser, H. J.; Schlag, E. W. J. Phys. Chem. 1985, 89, 5600
(7) Hadden, W. F.; McLafferty, F. W. J. Am. Chem. SOC. 1988, 90, 4745. (8) Hadden, W. F.; McLafferty. F. W. Anal. Chem. 1969, 41,31. (9) Stults, J. T.; Enke, C. G.; Holland, J. F. Anal. Chem. 1983, 55,1323. (10) Glish, G. L.; Goeringer, D. E. Anal. Chem. 1984, 56,2291. (11) Cooks, G. L. 11th International Mass SDectrometw Conference, Bordeaux, 1988. (12) Geusic, M. E.; Jarrold, M. F.; McIlrath, T. J.; Freeman, R. R.; Brown, W. L. J . Chem. Phys. 1987, 8 6 , 3862. (13) Brucat. P. J.; Zheng, L. S.; Tittel. F. K.; Curl, R . F.; Smalley. R . E. J . Chem. Phys. 1988, 84,3078. (14) Liu, Y.; Zhang, Q. L.; Tittel, F. K.; Curl, R . F.; Smalley, R . E. J. Chem. Phys. 1988, 85,7434. (15) LaiHing, K.; Wheeler, R . G.; Wilson, W. L.; Duncan, M. A. J . Chem. Phys. 1987, 87,3401. (16) Alexander, M. L.; Levinger, N. E.;Johnson, M. A,; Ray, D.; Lineberger, W. C. J . Chem. Phys. 1988, 88.6200. (17) Posey, L. A.; Johnson, M. A. J. Chern. Phys. 1988, 89,4807. (18) El-Sayed, M. A.; Tai, Tsong-Lin J. Phys. Chern. 1988, 92,5333. (19) Neumann, G. M.; Sheil, M. M.; Derrick, P. J. 2 .Naturforsch. 1984, 39a,584. (20) Kiplinger, J. P.: Bursey, M. M. Org. Mass Spectrum. 1988, 23, 342. (21) Grotemeyer, J.; Boesl, U.; Walter, K.; Schlag, E. W. J. Am. Chem. Sac. 1988, 108. 4233. (22) Ternbreull, R . ; Lubman, D. M. Anal. Chem. 1986, 58, 1299. (23) Engelke, F.; Hahn, J. H.; Henke, W.; Zare, R. N. Anal. Chem. 1987, 59,909. (24) Tembreull, R.; Lubman, D. M. Anal. Chem. 1987, 59, 1082. (25) Spengler, B.; Karas, M.; Bahr, U.; Hillenkamp. F. J. Phys. Chem. 1987, 9 1 , 6502.
K. LaiHing P. Y. Cheng T. G. Taylor K. F. Willey M. Peschke M. A. Duncan* Department of Chemistry School of Chemical Sciences University of Georgia Athens, Georgia 30602 RECEIVED for review December 2, 1988. Accepted March 21, 1989.
Polishable Modified Carbon Fiber Composite Electrodes Containing Copolymers of Vinylferrocene or Vinylpyridine in a Cross-Linked Polystyrene Matrix Sir: The carbon fiber composite electrodes described herein are polishable, random arrays of polymer-modified ultramicroelectrodes. These electrodes illustrate two new general approaches for obtaining modified electrodes that have renewable surfaces, but with electrochemical properties resem0003-2700/89/0361-1460$01.50/0
bling those of their nonrenewable polymer-film counterparts. These new types of electrodes may be designed and fabricated to have the selectivity, sensitivity, and detection limit of surface-modified electrodes, the signal-to-charging-current ratio and steady-state behavior of ultramicroelectrodes ( I , 21, 0 1989 American Chemical Society
