Anal. Chem. 1091, 63,2200-2203
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Modular Data System for Selectlve Waveform Excitation and Trapping Experiments in Fourier Transform Mass Spectrometry S. C. Beu and D. A. Laude, Jr.* Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712 The rapid development of Fourier transform mass spectrometry (FTMS) has been facilitated by the concurrent development of sophisticated electronic hardware that constitutes the data systems required for instrument control, signal handling, and data processing. Advances in computers, analog/digital signal processing, and data storage have greatly increased the power of FTMS and made practical the implementation of such data acquisition and computation-intensive techniques as stored waveform inverse Fourier transform (SWIFT) excitation (1)and two-dimensional FTMS (2). A variety of approaches to the construction of data systems for FTMS have been described (3-9), and several commercial systems are now available. These commercial systems offer high performance derived from software-intensive integration of sophisticated electronic components but are prohibitively expensive for many potential users. Another disadvantage is a dependence on the vendor to incorporate experimental or technological advances into a data system architecture that is either inflexible or inscrutable to the operator. As an alternative to commercial packages, FTMS data systems have been constructed in several laboratories (3-8). Approaches taken are varied but are typically based upon minicomputer or multiple microcomputer configurations that require construction of additional complex electronic circuitry. Such systems further require that the operator possess advanced computer programming skills. A more recently described microcomputer based system (9) has greatly reduced the cost of construction but still requires custom circuit board assembly and sophisticated programming skills. Here we describe an alternative FTMS data system that employs inexpensive and readily available "off-the-shelf" components in a modular system design. No hardward construction and minimal programming skills are required. System complexity is dramatically reduced with an arbitrary waveform generator that drives, with appropriate amplification, all forms of radial-ion excitation and z-axis trapping and excitation. As examples, broad-band SWIFT excitation with suppression windows is demonstrated, and a linear ramp of trap potentials is used to compress the z-axis amplitude of an ion population and thereby minimize z-axis ejection during excitation. EXPERIMENTAL SECTION FTMS Instrumentation. The spectrometer portion of the FTMS has been described elsewhere (10). The system includes a 2.0-T superconducting magnet and is differentially pumped to base pressures in the low 1O4-Torr range. For this work, an axial probe-mountedelectron gun was installed to permit conventional electron ionization in a 5- X 5- X 10-cm rectangular trapped ion cell. Volatile samples were introduced directly into the spectrometer through a high-precision leak valve. Data System. A block diagram of the data system is presented in Figure 1. The primary components of the system are a Standard 286 AT-compatiblepersonal computer (PC) (Compuadd Corp., Austin, TX), a LeCroy Model 9100 dual-channel arbitrary function generator (AFG),and a Lecroy Model 9410 digital storage oscilloscope (DSO)(LeCroy Corp., Chestnut Ridge, NY). A National Instruments GPIB interface card (National Instruments Corp., Austin, TX) installed in the PC provides communication between all components. The PC is further equipped with a 40-Mbyte hard disk and an 80287 math coprocessor and is used for system control and data processing. These functions are
implemented with W.AV.E. Version 1.1(Vespine Software,Urbana, IL), a commercial software package that provides a powerful waveform processing and instrument control environment. The AFG provides two channels of concurrent arbitrary waveform output, one fiied external trigger, and one programmable external trigger. One output channel is used to synthesize the excitation waveform which is amplified with a high-voltage differentialoutput amplifier and applied to the excite plates of the ion cell. The second output channel is used to provide the potential applied to the trap plates of the ion cell. The trigger outputs are used to control external events such as electron beam gating and triggering of the DSO. The DSO digitizes and displays the output from a differential-input preamplifier, which is connected to the receive plates of the ion cell. Procedure. The FTMS experiment is generally described with a pulse sequence diagram in which each pulse represents an experiment event. A typical pulse sequence is presented in Figure 2a and includes ion formation, ion excitation, signal acquisition, and quench events. The modular data system analog to this sequence is executed in the form of waveforms and triggers, as shown in Figure 2b. These waveforms are first created on the PC by using W.AV.E. and then downloaded and stored in the AFG memory for oatput when the experiment is initiated. For example, the top line of Figure 2b shows the AFG fixed trigger that occurs at the start of waveform output and is used to trigger the DSO for image-current signal acquisition. The second line of the sequence depicts channel 1 output from the AFG, that controls the constant potentials and z axis, or trapping, excitation waveforms applied to the trap plates. In this simplest of examples, the trapping electrode output is held at a constant value throughout the experiment until the terminating quench event. The third line of the figure shows the programmable trigger output, which is set to OCCUI at a desired delay after pulse sequence initiation and is used to activate the electron gun. Timing for the resulting electron beam event, indicated by the fourth line of the figure, is manually set with the electron gun control hardware and is typically a few milliseconds. The fifth line in the f i e depicts channel 2 output from the AFG that determines input to the excitation amplifier. In this example, a single swept excitation event is executed by maintaining the output at 0 V until a desired delay after ionization when the excitation wave form is output, amplified, and applied to the excite plates of the ion cell. The bottom line in the figure indicates the DSO trigger and scan event. After excitation, expiration of the DSO triggdk delay initiates the signal acquisition scan. During acquisition, the ion image current (11) is amplified by the preamplifier and then digitized, stored, and displayed by the DSO. The stored signal is transferred to the PC for processing.
RESULTS AND DISCUSSION A powerful feature of the data system is the versatility that results from the use of an AFG to control the excitation and trapping potentials. Many different approaches to excitation of trapped ions have been described in the literature with advantages and disadvantages particular to each (1,12-17). Similarly, a variety of experiments that require various programmed manipulations of trapping potentials have been described (7,13, 18-27). Described below are some of the experiments that are accessible when the AFG controls all cell potentials. Excitation Waveforms. The simplest excitation waveform is the single-frequency sine wave (12),which is useful when it is desirable to observe or eject selected ions of a single mass-to-charge ratio. For experiments that require the ob-
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 19, OCTOBER 1, 1991 Personal computer
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servation or ejection of a small number of selected ion masses, the AFG extends the advantagesof this type of waveform form to include multiple frequencies. In this case, the resulting waveform is a linear combination of sine waves corresponding to the desired resonance frequencies. The amplitude of each
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frequency component may be individually selected to effect either observation or ejection of all ions of interest with a single waveform. If the excitation of a large number of ion masses or a range of masses is desired, a swept frequency excitation waveform may be employed (13). In this case, the wave form is still sinusoidal but the frequency is varied linearly with time. Such waveforms are useful for observing or ejecting a broad mass range of ions with a single waveform, but it is not generally possible to both observe and eject different ions with a single frequency sweep. As an alternative to swept excitation for broad-band experiments, the AFG can be used to create SWIFT waveforms (1) to achieve simultaneous observation and ejection of ions. The desired waveform is initially described in the frequency domain, and then an inverse Fourier transform is applied to create a time-domain waveform. Here quadratic phase encoding is employed to keep the dynamic range of the resulting waveform within the limits of the excitation amplifier (14). Alternatively, more general phase encoding and frequency domain smoothing techniques may be implemented (28, 29). Other excitation wave forms that have been reported may be similarly implemented with the data system. These include waveforms required for excitation/deexcitation experiments (15, 16) and for impulse excitation (19,although impulse excitation requires a more powerful amplifier than employed here. Trap Potential Programming. A notable advantage of this AFG based system compared to other system designs is that the second output channel is used to generate versatile trapping and excitation waveforms for application to the trap plates. This capability will increasingly be exploited as the importance of z-axis ion cloud manipulation is recognized. For example, changes in trap potentials are required in even the simplest FTMS experitnents to remove ions from the cell at the end of the pulse sequence. Similar potential changes are also required to implement experimentssuch as gated trapping of ions introduced from outside the cell (7,10,22)and suspended trapping for space charge alleviation (19,20). In many cases, it is desirable to effect more gradual changes in trap potential. One reason is that abrupt increases in trap potentials while ions are in the cell may result in axial ejection of a significant fraction of the ions (21,22). Gross describes an adiabatic potential ramp as a means to accomplish axial ion compression and thus minimize temporal variation in image current signal strength (23) and minimize axial ion ejection during excitation (24). Programmed potential changes have been further employed in dynamic trapping experiments where they are used to control the mass range of ions collected from an external ion source (10). A more complex manipulation of trap potentials may be employed to deliberately excite the trapping motion of ions. In this case, an excitation waveform of the appropriate frequency composition is superimposed on the trap potential. The resulting waveform can be used to selectively eject unwanted ions or electrons (25, 26) or excite them for direct detection of trapping motion (27). AFG Examples. The modular data system was used to generate two examples of the experiments described above. A selective broad-band SWIFT excitation waveform was applied to PFI’BA ions formed by electron ionization. Presented in Figure 3a is the AFG-generated frequency-domain representation of the waveform, which includes suppression windows corresponding to ions at m / t 131 and 264. The timedomain representation of the waveform is shown in Figure 3b. The resulting broad-band spectrum with desired suppression of the selected ions is presented in Figure 3c. Figure 4 contrasts a conventional static trapping potential experiment
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Figure 3. (a)Time domain excitation waveform output by the AFG for selective SWIFT excitation over the mass range 50-600 Da with suppression of Ions at m l z 13 1 and 264; (b) frequency domain representation of the wave form In part a; (c)electron ionization FTMS spectrum of PFTBA that results from application of the selective SWIFT excitation waveform.
with a linear ramp of the trapping potential that adiabatically compresses the ion cloud to minimize axial ejection during excitation of the cyclotron motion. Shown in Figure 4a is an E1 spectrum of PFTBA that exhibits severe mass discrimination because it is acquired with a 0.5-V potential applied to the trap plates. Shown in Figure 4b is the resulting spectrum if the trap potential is abruptly raised to 3.0 V; ion ejection is pronounced and severe low mass discrimination remains. Finally, shown in Figure 4c is a spectrum acquired following a linear ramp from 0.5 to 3.0 V to compress the ion cloud and reduce subsequent z-axis ejection. Data System Limitations. Although the data system described here offers a number of advantages, the system does exhibit limitations involving dynamic range and memory size. The dynamic ranges of both the Model 9100 AFG and Model 9410 DSO are limited to 8 bits or 256 discrete signal levels. This limitation is overcome in the DSO to some extent with the signal-averagingfunction, which has a 16-bit range. The 8-bit range for the AFG is not theoretically optimal for experiments such as SWIFT, although in practice this range is often adequate. In any event, higher resolution models of both the DSO and AFG are available at greater cost if required. The waveform memory sizes of the AFG and DSO are 64 and 10 kbytes, respectively. The AFG memory limitation is overcome to a great extent by the ability to create large waveforms by linking smaller discrete waveforms with a preset repetition number for each component. This allows constant portions of a desired waveform to be defined with a small number of data points with high repetition number and thus conserve memory for description of changing output such as excitation waveforms. For experiments requiring a large number of resonance events, it may be necessary to add ad-
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Figure 4. PFTBA spectra acqulred following broad-band swept excitation with (a)trap potentials maintained at 0.5 V, (b) trap potentials maintained at 0.5 V during Ionization and raised abruptly to 3.0 V prior to excitation and detection, and (c) trap potentials maintained at 0.5 V during ionization and raised in a linear adiabatlc ramp to 3.0 V prior to excitation and detection.
ditional memory that is available as an option for the AFG. The 10-kbyte DSO memory limits the signal acquisition time for a given sampling rate and thus limits the frequency resolution of the transformed signal, although this problem may be minimized by using heterodyne detection (30).
LITERATURE CITED
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(1) . . Marshall. A. 0.: Lln-Wens. T.4.: Rlcca, T. L. J . Am. Chem. Soc. 1985, 107, 7893. (2) Pfandler, P.; Bodenhausen, 0.;Rapln, J.; Walser, M.; Gaumann, T. J . Am. Chem. SOC. 1988, 110, 5825.
Anal. Chem. 1991, 63,2203-2206 (3) Dawson, J. H. J. Lecture Notes /n Chem/sfry;Springer: Berlin, 1982; Vol. 31, p 331. (4) Doyle, R. J., Jr.; Buckley, T. J.; Eyler, J. R. Lecture Notes ln Chemkfry; Sprlnger: Berlin, 1982; Vol. 31, p 365. (5) &to, Y.; Chlkara, C.; Inoue, M. Shlhuvyo Bunsekl 1983, 37, 115. (6)Hays, J. D.; Dunbar, R. C. Rev. Scl. Insfrum. 1984, 55, 1116. (7) Alford, J. M.; Wllllams, P. E.; Trevor, D. J.; Smalley, R. E. Int. J . Mess Spectrom. Ion Processes 1988, 72, 33. (8) Meek, J. T.; Stockton, G. W. Fourier Transform Ion Cyclotron Resonance Mass Spectrometer with Spatially Separated Sources and Detector. U S . Patent 4,686,365, 1987. (9) Guan, S.; Jones, P. R. Rev. Scl. Insfrum. 1988, 59, 2573. (10) Beu, S. C.; Laude, D. A,, Jr. Znt. J . Mess Specfrom. Ion Processes 1991, 704, 109-127. (11) Comlsarow, M. 6. J . Chem. Phys. 1978, 69, 4097-4140. (12) Comisarow, M. E.; Marshall, A. G. Chem. Phys. Lett. 1974, 25, 282. (13) Comisarow, M. E.; Marshall, A. G. Chem. Phys. Lett. 1974, 26, 489. (14) Chin, L.; Lin-Wang, T.-C.; Ricca, T. L.; Marshall, A. G. Anal. Chem. 1987, 59, 449. (15) Marshall, A. 0.; Lln-Wang, T.C.; Rlcca, T. L. Chem. phys. Lett. 1984, 105, 233. (16) Kerley, E. L.; Russell, D. H. Anal. Chem. 1989, 67, 53. (17) McIver, R. T., Jr.; Hunter, R. L.; Baykut, G. Anal. Chem. 1989, 67, 489. (18) Kofel, P.; Allemann, M.; Kellerhals, Hp.; Wanczek, K.-P. Int. J. Mess Spectrom. Ion Processes 1988, 72, 53. (19) Laude, D. A., Jr.; Beu, S. C. Anal. Chem. 1989, 67, 2422. (20) Hogan, J. D.; Laude. D. A., Jr. Anal. Chem. 1990. 62, 530.
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(21) Honovich, J. P.; Markey, S. P. Int. J . Mess Specfrom. Ion Processes 1990. 98. 51. (22) Hofstadler, S. A.; Lade, D. A., Jr. Int. J . Mess Specfrom. Ion Processes 1990, 707, 65. (23) Rempel, D. L.; Huang, S. K.; Gross, M. L. Int. J . Mass Specfrom. Ion Processes 1988, 70, 163. (24) Huang, S. K.; Rempel, D. L.; Gross, M. L. Int. J . Mess Specfrom. Ion Processes 1988, 72, 15. (25) Beauchamp, J. L.; Armstrong, J. T. Rev. Scl. Insfrum. 1989, 40, 123. (26) Van De Guchte, W. J.; Van Der Hart, W. J. Int. J . Mess Specfrom. Ion Processes 1990. 95. 317. (27) Scheikhard, L.; Blundeschling, M.; Jertz, R.; Kluge, H.J. Int. J . Mess Specfrom. Ion Processes 1989, 89, R7. (28) Guan, S. J. Chem. phvs. 1989, 97, 775-777. (29) Goodman. S. Proceedings of the 37th ASMA Conference on Mass Spectrometry and Allied Topics, Miami Beach, FL, May 21-26, 1989; p 1218. (30) Comisarow, M. 6. A&. Mess Specfrom. 1980, 8 , 1698-1708.
RECEIVED for review February 15, 1991. Accepted June 14, 1991. This work is supported by the Welch Foundation (F-11381, the Texas Advanced Technology and Research Program (No. 4515), and the National Science Foundation (CHE9013384 and CHE9057097).
Preparatlon of Solid Membrane Chloride Ion Selective Electrodes by Ion Implantation Robert S. Glass,* Ronald G. Musket, and Keith C. Hong Chemistry and Materials Science Department, Lawrence Livermore National Laboratory, Livermore, California 94550 The development of ion-selective electrodes (ISEs)has had a major impact in analytical measurement science, and they will continue to enjoy widespread application. Currently, there are a wide range of electrodes that are commercially available to the experimenter. ISEs come in many shapes and sizes, and use various chemical and material designs. Several all solid-state ISEs are available. Within recent years, a sizeable effort has also been devoted to the development of ion-sensitive field effect transistors (ISFETS), which, like ISE’s, are also potentiometric sensors. There are several excellent recent monographs dealing with the current state of ion-selective devices available to the interested reader (1-6). As an ISE for chloride ion, the solid-state membrane electrode has been available for some time. Common membrane materials include single crystals, disks cast from melts, sintered materials, and pressed polycrystalline pellets. Often, AgCl is dispersed in a matrix of Ag,S as this has been found to decrease photosensitivity and increase sensitivity toward the chloride ion (3). The theory of operation for ISEs based upon solid-state membranes has been extensively discussed ( 3 , 4 , 7). For the chloride ISE, a Nernstian response is expected. We have found that if chloride ions are implanated into a Ag substrate, a solid-state membrane ISE can be produced, which shows (approximately)the expected Nemstian response in solutions containing chloride ion over several decades of concentration. In addition, very good reproducibility of the potential-concentration response between different implanted specimens was obtained. To our knowledge, the only previous work using ion implantation to prepare ion-selective devices was creation of membranes operating on the principle of cation-exchange equilibria similar to those relevant to the glass electrode. In one study (8),using low fluences (1013-1016ions/cm2),Si+ and Li+ were implanted into alumina wafers in order to create a Na+ ISE. ISFETS sensitive to Na+ and K+ have been created by implantation into the gate region of a FET using moderate ion fluences (1016-1017ions/cm2). For example, Li+ and Al+ 0003-2700/9 110383-2203$02.50/0
have been implanted into the gate SiN layer covering a layer of Si02to create a Na+ ISFET (8);Na+ has been implanted into an oxidized gate Si3N4layer (initially covered by an A1 buffer layer, which was removed following implantation) to create a Na+ ISFET (9);and Na+ and Al+ or K+ and Al+ have been implanted into a gate SiOz layer to create ISFETS sensitive to Na+ and K+, respectively, (10). In these previous studies, the sensitivity depends upon the number of surface-active ion-exchange sites (e.g., (AlOSi)-), created by the implantation process. We believe our study is the first where ion implantation has been used to create solid-state membrane ISEs of the AgX type, with solid metal contact, which respond to components that the solution and the membrane have in common. As is well-known, the mechanism of anion response of these ISEs can be explained in terms of a buffering action on the free Ag ions near the membrane surface (3, 4 , 7). Our ultimate motivation in undertaking this work was to develop a convenient and reproducible method for fabricating arrays of ISE’s and “micro-ISE’s”,with individual elements selective to different chemical species. The study reported here is a first step in that direction. Using ion implantation as a fabrication method allows precise control over the structure and composition of the ion-sensing membrane layer. Therefore, special properties can be realized (i.e., rapid response). In addition, the mass production and low cost features of this method of fabricating ISEs implies that they may be disposed of following use.
EXPERIMENTAL SECTION Specimens for implantation consisted of Ag cylinders approximately 0.64 cm in diameter and 1.27 cm in height. Prior to implantation, the end to be implanted was polished to a mirror finish by using a slurry of silica powder with average particle size of 0.5 pm. Along with the implantation samples,two unimplanted control specimens were also prepared. The samples for implantation were then mounted in a fixture that was emplaced into a 200-kV ion implanter with the sample surfaces perpendicular to the ion beam. Hydrogen chloride gas was fed into the hot cathode ion source. The %C1+ions were mass spectrometrically selected 0 1991 American Chemical Society
