Broad-Band Fourier Transform Quadrupole Ion Trap Mass

A survey of recent research activity in quadrupole ion trap mass spectrometry 1 1A celebratory contribution to honor Professor R. Graham Cooks on the ...
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Anal. Chem. 1996, 68, 3314-3320

Broad-Band Fourier Transform Quadrupole Ion Trap Mass Spectrometry Manish Soni, Vladimir Frankevich, Mario Nappi,† Robert E. Santini, Jonathan W. Amy, and R. Graham Cooks*

Department of Chemistry, Purdue University, West Lafayette, Indiana 47907

Broad-band nondestructive ion detection is achieved in a quadrupole ion trap mass spectrometer by impulsive excitation of a collection of trapped ions of different masses and recording of ion image currents induced on a small detector electrode embedded in but isolated from the adjacent end cap electrode. The image currents are directly measured using a simple differential preamplifier, filter, and amplifier combination and then Fourier analyzed to obtain broad-band frequency domain spectra characteristic of the sample ions. The use of the detector electrode provides a significant reduction in capacitive coupling with the ring electrode. This minimizes coupling of the rf drive signal, which can saturate the front-end stage of the detection circuit and prevent measurement of the relatively weaker ion image currents. Although impulsive excitation is preferred due to its broad-band characteristics and simplicity of use, results are also given for narrow-band ac and broad-band SWIFT (stored waveform inverse Fourier transform) excitation. Data using argon, acetophenone, and n-butylbenzene show that a resolution of better than 1000 is obtained with a detection bandwidth of 400 kHz. An advantage of nondestructive ion detection is the ability to measure a single-ion population multiple times. This is demonstrated using argon as the sample gas with an average remeasurement efficiency of >90%. Tandem mass spectrometry experiments using a population of acetophenone ions are also shown.

The quadrupole ion trap mass spectrometer (ion trap), also known as the Paul trap, employs a hyperbolic three-electrode system consisting of a central ring electrode and two adjacent end cap electrodes to trap, manipulate, and mass-analyze sample ions. The well-known Mathieu equations describe the complex ion motion in an ion trap.1 Of particular interest here is the fact that, for a fixed set of operating conditions, the fundamental axial frequency of an ion (fz) is dependent only on its mass-to-charge (m/z) ratio (units: Thomson, Th ) dalton/charge). In a common mode of operation,2 mass analysis is achieved by scanning the radio frequency (rf) voltage applied to the ring electrode while maintaining the two end cap electrodes at ground potential. This causes the ejection of ions, in order of m/z ratio, into an externally placed conversion dynode/electron multiplier detector. Upon † On leave from the II. Physikalisches Institut, Justus-Liebig Universita ¨t Giessen. (1) March, R. E.; Todd, J. F. J. Practical Aspects of Ion Trap Mass Spectrometry; CRC Press: New York, 1995; Vol. I. (2) Stafford, G. C.; Kelley, E. P.; Syka, J. E. P.; Reynolds, W. E.; Todd, J. F. J. Int. J. Mass Spectrom. Ion Processes 1984, 60, 85.

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striking the detector surface, the ions produce a response proportional to their abundance; mass/charge calibration with respect to the rf amplitude yields an m/z vs abundance plot, i.e., a mass spectrum. This form of external ion detection is inherently destructive; i.e., the sample ions are lost upon collision with the detector surface. The original ion population is thus unavailable for further studies, and a new population of ions has to be generated at the end of each detection cycle. External ion detection is, however, straightforward to implement and highly sensitive and hence routinely used in ion trap mass spectrometers. Ion detection can also be nondestructive, sometimes known as in situ detection, where the original ion population is retained during and after the detection cycle, thus permitting further studies and measurements (remeasurement). The principles of nondestructive detection, which are fundamentally different from destructive detection, have been known since the early days of ion trap development. Paul et al.3 first employed nondestructive detection as did Fischer4 and Rettinghaus.5 Their research laid the groundwork for later attempts at nondestructive detection. In general, nondestructive mass analysis in ion traps has been performed using variations of the following two techniques: (i) resonant power absorption, where a marginal oscillator circuit is utilized to measure the power absorbed when the oscillator frequency matches the frequency of ion oscillation, and (ii) image current measurement, where the alternating current induced on adjacent electrodes by the coherent motion of a population of ions is measured. The first technique was utilized by Fischer while the second method, as adapted by Rettinghaus, involved the use of a tuned transformer circuit for measuring image currents. It is important to note that neither of these methods is broad band; i.e., they cannot achieve simultaneous detection of ions over a wide m/z range (broad detection bandwidth). A simpler and extremely powerful approach involves direct broad-band measurement of ion image currents using a differential preamplifier detection circuit and Fourier transform (FT) analysis of the image signal to obtain a frequency (hence mass) spectrum. This method depends on the fact that ions oscillating in an ion trap do so at unique frequencies dependent only on their m/z ratios. An ion population, excited coherently in the axial direction, will therefore generate an image current in the end cap electrodes and the frequency of the image current will be the same as that of the ion axial frequency (fz). When ions of different masses are present, the image current contains a complex sum of the different ion frequencies. Hence an FT analysis of the image signal will yield its individual frequency components, i.e., a spectrum that is characteristic of the m/z values of the sample ions, i.e., a mass (3) Paul, W.; Reinhard, H. P.; Von Zahn, U. Z. Phys. 1958, 152, 143. (4) Fischer, E. Z. Phys. 1959, 156, 1. (5) Rettinghaus, G. Z. Angew. Phys. 1967, 22, 321. S0003-2700(96)00577-X CCC: $12.00

© 1996 American Chemical Society

spectrum. Only the fundamental axial (z-direction) frequency, fz, is usually considered because it strongly couples with a dipolar electric field applied to the end caps to excite the ions. The feasibility of nondestructive FT detection in quadrupole ion trap mass spectrometers was first demonstrated by Syka and Fies6 although only over a narrow frequency range. They utilized resonant ion excitation followed by narrow-band image current measurement and Fourier analysis. Resonant excitation in the axial direction was achieved by applying a dipolar sinusoidal signal of frequency equal to fz, across the end cap electrodes. Recently, Parks et al.7 and Goeringer et al.8 demonstrated improved performance in narrow-band nondestructive detection in an ion trap using variations of the above two approaches. The method adopted by Parks and co-workers yielded high detection sensitivities (∼100 ions) while the Goeringer method had the advantage of high (>99%) ion remeasurement efficiency. Broad-band excitation of ions over a range of frequencies (hence m/z values) is essential for broad-band FT detection. It can be achieved by employing frequency chirps or broad-band SWIFT (stored wave-form inverse Fourier transform) pulses.9,10 Alternatively, impulsive excitation can be used; in this simple method, a high-voltage dc pulse of short duration is applied to either one or both end cap electrodes. This causes all ions, irrespective of mass/charge ratio, to be abruptly accelerated in the same direction, making the motion of ions of the same m/z values phase coherent. It is interesting to note that modern ion cyclotron resonance (ICR) mass spectrometers routinely employ broad-band nondestructive ion detection and Fourier transform techniques for signal processing (FT-ICR),9,11,12 even though ion traps employ destructive detection. The ICR mass spectrometer is of course conceptually similar to the ion trap but involves ion trapping and manipulation using both magnetic and electric fields.9,11,12 In both types of instruments, trapped ions oscillate at frequencies that depend on their m/z ratios and ions of the same m/z value have the same frequency of oscillation irrespective of initial velocity and position. It is this property that makes massselective destructive or nondestructive detection possible in both types of instruments. Unlike ICRs, ion traps operate with high rf trapping voltages (kV range) that capacitively couple to the adjacent end cap electrodes making nondestructive image current measurements extremely difficult. This rf noise signal on the end caps is orders of magnitude larger than the image current due to the oscillating ions and saturates the front-end stage of the detection circuit. This has been the main limitation to the adoption of nondestructive detection to ion traps despite its evident advantages. Most research in this area3-8 has involved minimizing the rf noise pickup at the detection circuit using various electronic schemes, and this approach has been successful but at the expense of detection bandwidth. In this paper, a novel approach is demonstrated that allows, for the first time, routine broad-band nondestructive detection of ions in a quadrupole ion trap mass spectrometer. An important (6) Syka, J. E. P.; Fies, W. J., U.S. Patent 4,755,670, 1988. (7) Parks, J. H.; Pollack, S.; Hill, W. J. Chem. Phys. 1994, 101, 6666. (8) Goeringer, D. E.; Crutcher, R. I.; McLuckey, S. A. Anal. Chem. 1995, 67, 4164. (9) Marshall, A. G. Acc. Chem. Res. 1985, 18, 316. (10) Marshall, A. G.; Wang, T. C.; Ricca, T. L. J. Am. Chem. Soc. 1985, 107, 7893. (11) Comisarow, M. B.; Marshall, A. G. Chem. Phys. Lett. 1974, 25, 282. (12) Amster, I. J.; McLafferty, F. W.; Castro, M. E.; Russell, D. H.; Cody, R. B.; Ghaderi, S. Anal. Chem. 1986, 58, 483.

innovation is the use of a detector electrode embedded in, but insulated from, the end cap electrodes and acting as an antenna to receive image currents from the coherently oscillating ion population. This modification to the classical trap design provides a significant drop in rf noise pickup by the detection circuit due to a reduction in capacitance between the detector and the ring electrode. The image currents are directly measured using a simple, low-noise, broad-band differential preamplifier, filter, and amplifier. The resulting image signal is analyzed using standard Fourier transform techniques to obtain broad-band frequency (and hence mass) spectra. Coherent ion oscillation, a prerequisite for Fourier analysis, is achieved using simple impulsive excitation, although resonant excitation methods like narrow-band dipolar ac and broad-band SWIFT are used in some comparative studies. A practical advantage of the design is the physical separation of the detector electrode from the excitation (end cap) electrode, which provides temporal and spatial isolation of the excitation source from the detector circuit. An important feature of nondestructive detection is ion remeasurement. This is demonstrated here using argon and is also utilized in conjunction with tandem mass spectrometry (MS/MS). In this latter experiment, a single population of acetophenone ions is manipulated to achieve parent ion isolation and subsequent dissociation in the presence of a collision gas. The ability to measure ions nondestructively is used to acquire mass spectra during various stages of the MS/ MS experiment. EXPERIMENTAL SECTION Hardware Modifications. A research grade Finnigan ITMS (Finnigan Corp., San Jose, CA) ion trap mass spectrometer was used for all experiments although the methods described can be applied to any ion trap mass spectrometer. The rf trapping field in this instrument has a frequency of 1.1 MHz, which is therefore the frequency of the rf noise at the detector. The standard Finnigan ion trap configuration including the two end cap electrodes, a ring electrode, an internal (in situ) electron ionization (EI) source, and externally placed conversion dynode/electron multiplier (destructive) detector is shown in Figure 1A. The modified ion trap, including the detector electrode in the exit end cap, but with no external detector, is shown in Figure 1B. The detector electrode is made from stainless steel and has a diameter of 3.2 mm and a length of 32.5 mm. It is fitted in the exit end cap electrode and electrically isolated using a Teflon support. The detector tip projects through the end cap electrode hole (5 mm diameter) and is machined to conform with the hyperbolic surface of the end cap. The position of the detector electrode from the ion trap center is adjustable and influences the quality of the spectra recorded. It was set to 7.3 mm to obtain the data shown here, i.e., to protrude 0.5 mm above the end cap electrode surface. Note that the end cap spacing from the center of the trap is 7.8 mm (i.e., the conventional extended geometry was used) while the internal diameter of the ring electrode is 20 mm. Control Electronics. The standard ITMS data system and electronics were used to control the operation of the ion trap and provide the necessary trigger sequence for various auxiliary devices. These include a modified high-speed dc pulser13 (DEI, Fort Collins, CO), an arbitrary wave-form generator (ARB; Model 395, Wavetek Corp., San Diego, CA), a delay generator (Model DG535, S.R.S Corp., Sunnyvale, CA), and a digital oscilloscope (13) Lammert, S. A.; Cooks, R. G. J. Am. Soc. Mass Spectrom. 1991, 2, 487.

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Figure 2. (A) Basic scan function (timing diagram) describing the experimental sequence for nondestructive ion detection. It consists of an ionization stage followed by an ion-cooling period, dc excitation, and data acquisition stages. Note that the rf level is held constant throughout the experiment. (B) Scan function used for ion remeasurement experiments where multiple excitation/data acquisition stages are used in addition to the stages in (A).

Figure 1. (A) Standard Finnigan ion trap configuration with two end cap electrodes, ring electrode, EI source, and external conversion dynode/electron multiplier; (B) modified trap. The latter includes a small detector electrode fitted into but electrically isolated from the exit end cap for receiving image currents from oscillating ions.

(Model TDS 540, Tektronix, Beaverton, OR). The dc pulser is used for impulsive excitation, a process in which a 2 µs dc pulse (10-80 V) having rise and fall times in the nanosecond range is applied to the entry (unmodified) end cap electrode. The ARB is used to produce either a burst of single-frequency ac signal having a selected frequency and magnitude or a SWIFT pulse for resonant excitation; both excitation signals are used either to excite ions for detection or to excite them for collision-induced dissociation (CID). The SWIFT pulses were created and applied as reported previously.14 Although the excitation signal is applied only to the entry end cap in these experiments, it can also be applied across both the entry and exit end caps in a dipolar fashion. The delay generator provides the necessary delays for sequential triggering of the various devices. Image Current Detection. The ion image currents received at the detector electrode are converted into an image voltage at the input of the preamplifier using a 10 MΩ resistor. The preamplifier circuit was home-built and consists of three stages. The first stage is a Burr-Brown INA-111 differential instrumentation amplifier with a 10 MΩ resistance at each input. For experiments described here, one of the inputs of the differential amplifier was connected to the detector electrode while the second input was grounded. The first stage has a gain of 10 and is followed by a single-pole low-pass RC network with a highfrequency breakpoint of 500 kHz. The final (third) stage of the preamplifier is another Burr-Brown differential amplifier set to provide a gain of 50. The entire device is ac coupled with a 10 kHz low-frequency breakpoint. The preamplifier thus provides a (14) Soni, M. H.; Cooks, R. G. Anal. Chem. 1994, 66, 2488.

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gain of 500, a bandwidth of 500 kHz, and a current transfer ratio of 5 mV0-p/1 pA current at the detector electrode. The output of the preamplifier is fed into an analog filter/amplifier (Model 4302, Ithaco, Ithaca, NY) set at 100 gain and 400 kHz bandwidth. The final detector bandwidth is thus 400 kHz. The output of the filter/ amplifier is connected to the digital oscilloscope. A computer with a GPIB interface allows downloading of image signals from the oscilloscope for further processing. Data Acquisition and Processing. The digital oscilloscope was set at 1 MHz sampling frequency, 1 MΩ input impedance, and 15K data points horizontal resolution to yield a data acquisition window of 15 ms. An acquisition window of 50 ms (50K data points at 1 MHz) was used for some experiments, as indicated. The oscilloscope was operated in the signal-averaging mode (running average over 100 scans) for all experiments. Background signals were obtained under identical conditions but with the excitation source turned off. The frequency domain background spectrum was subtracted from that of the sample to remove peaks due to systematic noise and improve S/N ratios. The built-in real-time FFT capability of the oscilloscope was used during tuning while optimized time domain transients were downloaded to the computer for FFT processing and background subtraction using the STATMOST statistical spreadsheet program (version 2.1, Datamost Corp., Salt Lake City, UT). All frequency domain data were converted to mass spectra by manually calculating the m/z value for each peak in the frequency domain using the Mathieu equations.1 Scan Function. The scan function (timing diagram) that describes the sequence of events in the experiment is shown in Figure 2A for the basic experiment of recording a mass spectrum. It consists of the following stages: (i) ionization (0.9-20 ms) followed by (ii) a 10-30 ms delay period for ion cooling and baseline stabilization, (iii) an excitation period during which the dc pulser (or the ARB) is triggered to apply the excitation signal, and (iv) a data acquisition period during which the ion image signal is recorded using the oscilloscope. The rf level is kept constant throughout these events at a value corresponding to a

Table 1. Values of Experimental Parameters

a

Figure

sample

low-m/z cutoff

pressurea (Torr)

ionization time (ms)

excitation parameters

4 5 6 7 8 9

argon acetophenone n-butylbenzene argon argon acetophenone

20 30 25 20 17 25

1 × 10-6 9 × 10-7 1 × 10-6 2 × 10-6 2 × 10-6 5 × 10-6 b

20 1 10 10 2 1

(23 V/2 µs) (40 V/2 µs) (113.8 kHz/2.4 V/100 µs) (8 V/2 µs) SWIFT (500 kHz/ 0.4 V/ 8.2 µs (15 V/2 µs)

Uncorrected. b He was added in this experiment.

Figure 3. Experimental data showing a reduction in coupled rf noise at the detector electrode by a factor of 200 compared to the end cap electrode. The rf noise level at the detector and end cap electrodes was measured as a function of the rf drive voltage applied to the ring electrode, using the oscilloscope in the absence of sample ions.

low-mass cutoff in the range of m/z 20-30. For ion remeasurement, the excitation/data acquisition sequence is performed multiple times as shown in Figure 2B while ion isolation and CID stages are added for tandem mass spectrometry experiments. The values for various experimental parameters are tabulated in Table 1. Chemicals. Argon (Airco, Murray Hill, NJ), acetophenone (Fischer Scientific, Fair Lawn, NJ) and n-butylbenzene (Aldrich Chemicals, Milwaukee, WI) samples were directly leaked into the ITMS through a leak valve (Granville-Phillips, Boulder, CO) to obtain a pressure reading in the range of (0.5-2) × 10-6 Torr (uncorrected). Helium gas was similarly added together with the sample as the collision gas for tandem mass spectrometry experiments to obtain an uncorrected pressure reading of 5 × 10-6 Torr. RESULTS AND DISCUSSION Background Signals: rf Pickup. The magnitude of the rf image noise was recorded in the absence of sample on (i) the end cap electrode and (ii) the detector electrode. The two sets of results are shown in Figure 3 as a function of the rf voltage applied to the ring electrode. A reduction in the 1.1 MHz rf noise pickup by a factor of 200 at the detector electrode is observed. This significant reduction, essential for the success of the experiments described below, is due to the reduction in capacitive coupling between the ring electrode and the detector (viz. C3) i.e., C3 < C2 (see Figure 1B). Note that this result arises not only because of the smaller surface area of the detector electrode

but also because of its larger distance from the ring electrode in comparison to that of the end cap itself. The capacitance between the end cap and detector (C1) is also small due to the relatively large spacing and small size of the detector. Thus rf coupling is minimized to prevent saturation of the preamplifier by the drive signal. Experimentally, the physical isolation of the detector electrode from the end cap electrodes is also an advantage. It allows temporal as well as spatial separation of the excitation source and preamplifier, eliminating the need for switching circuits. Note that attempts to improve the S/N ratio by modifying electrode designs have also been made in FT-ICR,15,16 and the importance of small electrodes in FT-ICRs has been emphasized by Russell.15 Coherent Ion Excitation. Impulsive excitation17 was achieved by the application of a short high-voltage dc pulse (δ or impulse function) to one end cap electrode. This causes all ions irrespective of their mass (broad band) to be abruptly accelerated in the same direction. We previously investigated this procedure through simulations of ion motion using the program ITSIM18 and found that impulsive excitation produces coherent ion motion with frequencies characteristic of the m/z values of the ions.19 These simulations also showed that mass spectra could be recorded using the image currents. Impulsive excitation does not involve resonance and hence is not mass selective. This is an important advantage. It is also simpler to implement and use since complicated mathematical algorithms, electronic circuitry, and tuning procedures required for tailored broad-band resonant excitation techniques are not needed. In the experiments described here, it was found that a pulse width of 2 µs and a few tens of volts are sufficient to excite ions over more than 500 kHz bandwidth without tuning from experiment to experiment. Image Signal. A coherently moving population of ions generates an image current, the magnitude of which increases with the size of the ion orbit.20,21 Figure 4 shows the argon time domain signal transient (A), background (B), and their fast Fourier transforms (C, D), respectively, obtained using impulsive excitations. Conditions used to record these data are summarized in Table 1, together with those used for other experiments. In the presence of a bath gas, in this case the argon sample, the excited (15) Hanson, C.; Castro, M.; Kerley, E.; Russel, D. H. Anal. Chem. 1990, 62, 521. (16) Vartianian, V.; Anderson, J.; Laude, D. A. Mass Spectrom. Rev. 1995, 14, 1. (17) McIver, R. T., Jr.; Hunter, R. L.; Baykut, G. Anal. Chem. 1989, 61, 491. (18) Reiser, H. P.; Julian, R. K.; Cooks, R. G. Int. J. Mass Spectrom. Ion Processes 1992, 121, 49. (19) Cooks, R. G.; Cleven, C.; Horn, L.; Nappi, M.; Weil, C.; Soni, M.; Julian, R. Int. J. Mass Spectrom. Ion Processes 1995, 146/147, 147. (20) Dunbar, R. Int. J. Mass Spectrom. Ion Processes 1984, 56, 1. (21) Amano, C. Int. J. Mass Spectrom. Ion Processes 1980, 35, 47.

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Figure 4. Nondestructive detection using argon sample and impulsive excitation. The time domain signal and background transients (A, B) and their FFTs (C, D) are shown. The FFT of the time domain signal and the zoom inset show Ar+ (m/z 40) at 179.5 kHz with a resolution of ∼1000. The spectrum also shows a harmonic at 2fz due to the nonsinusoidal nature of the ion oscillation.

ions relax back to the ion trap center as a result of energy loss due to collisions. These stochastic processes yield an image signal that resembles a free induction decay, viz. a transient. In practice, transient decay also occurs due to the dephasing effect of higher order fields that are present in ion traps. The duration of the argon ion transient was ∼20 ms at the nominal pressure of 1 × 10-6 Torr used. The FFT of the time domain transient (C) shows the characteristic Ar+ peak at fz 179.5 kHz, corresponding to m/z 40, and its second harmonic (2fz) at 359 kHz. The presence of harmonics indicates that the ion motion is not truly sinusoidal. The inset shows an enlarged view of the Ar+ peak. Peak width (∆fz ) fwhm) is 190 Hz, which corresponds to a resolution (fz/∆fz) of ∼1000. The frequency domain spectra also show systematic noise peaks, which are easily removed by subtracting the background from the signal. Note that appropriate positioning of the detector electrode was needed for optimum collection of field lines from the coherent ion population and that this procedure increased image current magnitude. Note that the frequency at which the Ar+ ion is observed, 179.5 kHz, is substantially different from the calculated frequency of 184.6 kHz. Similar systematic frequency shifts were observed in all the experiments, and they are believed to arise, at least in part, from the hexapole field component associated with the detector electrode. Experiments done as a function of qz show a constant shift between the observed and theoretical values under standard operating conditions. The shifts were increased when larger ion populations were studied, presumably due to space charge effects. The mass assignment problems encountered under these condi3318 Analytical Chemistry, Vol. 68, No. 19, October 1, 1996

tions are similar to those that occur in FT-ICR.22 These issues and the origins of the shifts will be discussed further in a followup publication. A theoretical analysis of the image signal employing the rotating monopole model of Comisarow23,24 will also be reported on elsewhere. Broad-Band Mass Spectrum. The mass spectrum of acetophenone, recorded by the FT-ITMS procedure, using the values of the experimental parameters given in Table 1, is shown in Figure 5. Total acquisition time for obtaining this spectrum was 45 ms × 100 scans ) 4.5 s. The spectrum is background subtracted and includes a 10-times enlarged y-axis view (same x-axis), which is shown as the inset. The expected characteristic ions at m/z 120 (M•+), 105, 77, 51, and 43 are all present. The 2fz harmonics, already noted for argon, can be seen for m/z 120 and 105. The low-mass cutoff value of m/z 20 does not allow observation of the expected fragment ion at m/z 15. Note that the abundances of the lower mass ions are smaller than those observed in standard electron impact mass spectra. Narrow-Band Excitation. A single cycle or a short (100 µs) single-frequency ac burst also provides coherent ion excitation but over a narrow bandwidth. This experiment was performed for comparison with impulsive excitation and found to yield very similar results over a narrow range of m/z values. Figure 6 shows the mass spectrum recorded for n-butylbenzene using narrowband dipolar ac excitation at a resonance frequency appropriate for the ions m/z 91 and 92 and using the other conditions (22) Ledford, E. B.; Rempel, D. L.; Gross, M. L. Anal. Chem. 1994, 66, 2744. (23) Comisarow, M. B. J. Chem. Phys. 1978, 69 (9), 1. (24) Frankevich, V.; Soni, M. H.; Nappi, M.; Cooks, R. G.; Santini, R. E.; Amy, J. Non-destructive Ion Trap Mass Spectrometer and Method; U.S. Patent applied, 1996.

Figure 7. Ion remeasurement demonstrated using argon as the sample. In this experiment the same population of Ar+ ions is measured four times with an average remeasurement efficiency better than 90%.

Figure 5. Background-subtracted frequency spectrum of acetophenone obtained using impulsive excitation. The characteristic peaks at m/z 120 (M•+), 105, 77, 51, and 43 and harmonics (2fz) for m/z 120 and 105 can be seen.

Figure 8. Data obtained using broad-band SWIFT excitation using an argon sample. The SWIFT pulse was synthesized as previously described14 using 8K data points and 500 kHz bandwidth. The Ar+ peak (m/z 40) can be seen at 147 kHz; the fwhm is 250 Hz (inset).

Figure 6. Background-subtracted frequency spectrum for n-butylbenzene obtained using narrow-band ac excitation. Note that the detection bandwidth was still 400 kHz. The characteristic peaks at m/z 91 and 92 are resolved with a resolution better than 1000.

mentioned in Table 1. Because the excitation is narrow band it must yield a narrow-band spectrum even though the detection bandwidth is still 400 kHz. The characteristic ions at m/z 91 and 92 are observed at a resolution better than 1000 although their relative abundances depend on the exact excitation frequency chosen. As expected, frequencies closer to the fz values of either of the individual ions lead to discrimination against the other. Ion Remeasurement. A cooled population of trapped ions can be re-excited and remeasured multiple times. This capability has been demonstrated previously using FT-ICR25 and also in narrow-band ion trap measurements.8 Figure 7 shows an example of ion remeasurement using argon as the sample. The same argon ion (Ar+) population is re-excited and remeasured four times using impulsive excitation (see Table 1). Only the time domain signals are shown, but the quality of the data actually improves in the second and third measurements. The average remeasurement efficiency is estimated to be better than 90% and can probably be improved by optimizing impulse parameters for each re-excitation event. SWIFT Excitation. The tailored broad-band resonant excitation method of SWIFT is widely used in modern FT-ICRs 9,10 since (25) Williams, E.; Henry, K.; McLafferty, F. W. J. Am. Chem. Soc. 1990, 112, 6157.

it provides excellent control over the shape and magnitude of the excitation spectrum. In these experiments, this previously used capability for ion isolation and excitation in ion traps14,26 was applied as an alternative excitation signal needed for nondestructive detection. This capability is illustrated in Figure 8, which shows the background-subtracted mass spectrum of argon recorded using broad-band SWIFT excitation. The SWIFT pulse covered a bandwidth of 500 kHz and the Ar+ peak (m/z 40) can be seen at 147 kHz. Tandem Mass Spectrometry. Tandem mass spectrometry can be performed in conjunction with ion remeasurement, as shown in Figure 9. The background-subtracted frequency domain spectra of a single population of acetophenone sample ions from three different transients are shown. Figure 9A shows the fullmass spectrum obtained after ionization and mass analysis using dc excitation. After the first excitation/detection event (i.e., when the ion population has cooled down) an rf-only isolation of the parent ions at m/z 120 is performed. The quality of the isolation was examined by re-exciting the ion population and acquiring an image transient. The FFT of this transient (Figure 9B) clearly shows the isolated m/z 120 parent ions and its 2fz harmonic. The isolated parent ions were then dissociated using resonance excitation to perform CID.27 In this case, a low-amplitude (2.4 V) supplementary ac signal of frequency (72 kHz) equal to the fz of m/z 120 was applied to the exit end cap electrode for 20 ms. This resonance excitation step was performed in the presence of (26) Doroshenko, V.; Cotter, R. J. Rapid Commun. Mass Spectrom. 1996, 10, 65. (27) Johnson, J.; Yost, R. A.; Kelley, P. E.; Bradford, D. C. Anal. Chem. 1990, 62, 2162.

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the same ion population, a feature that should also be useful in performing ion/molecule reactions, using a single-ion population.

Figure 9. Ability to remeasure a population of sample ions utilized in conjunction with a tandem mass spectrometry experiment using acetophenone. Spectra were recorded at various times during the experiment: (A) after ionization; (B), after parent ion isolation; and (C) after collision-induced dissociation. The three backgroundsubtracted frequency domain spectra show the characteristic acetophenone ions at m/z 120, 105, and 77 in the full-mass spectrum, the parent ions (m/z 120) after isolation, and parent ions and fragment ions (m/z 105) in the product ion spectrum.

added helium as collision gas and thus it caused fragmentation to yield product ions. Figure 9C shows the product ion spectrum recorded by impulsive excitation of the ion population after CID. The mild CID conditions used result in small energy deposition and principally yield the primary fragment ions at m/z 105. The ion excitation, ion isolation, and CID events can also be performed using other well-known methods, including the newer broad-band wave-form methods.14,26 Note that the present instrumental arrangement allows rapid collection of multiple transients from (28) Franzen, J. Int. J. Mass Spectrom. Ion Processes 1994, 130, 15. (29) Wang, Y. Rapid Commun. Mass Spectrom. 1993, 7, 920.

3320 Analytical Chemistry, Vol. 68, No. 19, October 1, 1996

CONCLUSIONS The feasibility of broad-band nondestructive Fourier transform image current detection using an ion trap mass spectrometer is demonstrated for the first time. An important innovation in the experiment is the small detector electrode that is used as an antenna to receive image currents from coherently oscillating ions. The detector electrode provides higher image signal magnitudes and lowers the rf noise pickup to provide a significant gain in S/N. Direct measurement of the image current using a relatively simple detection circuit consisting of a preamplifier, filter, and amplifier followed by real-time Fourier analysis is demonstrated. The procedures used here are easy to implement and allow rapid acquisition of broad-band mass spectra. The data shown were acquired using a detection bandwidth of 400 kHz and yield a resolution (fz/∆fz) better than 1000 in most experiments. The peak width is constant over the entire mass range, but since m/z and frequency are inversely related, resolution in terms of fz/∆fz increases with frequency. The operation of the ion trap at relatively higher pressures compared to other mass spectrometers provides a number of benefits which also apply to these experiments. Pumping requirements are simplified, and compatibility with external ionization methods should be increased. Higher dissociation efficiencies in tandem mass spectrometry and thermalizion of ion/molecule reaction products by the buffer gas are also made possible. The use of high pressures, however, limits mass resolution since collisions with the background neutrals shorten the length of the time domain transients. The presence of nonlinear fields in real traps is known to alter ion frequencies, and this also contributes to peak broadening in our experiments.28,29 We are currently evaluating these ion excitation and detection methods in terms of mass discrimination effects and re-excitation efficiency. Systematic studies of space charge effects on peak resolution and mass assignments are underway to further improve performance. More sophisticated FT data processing routines like summing, zero filling, and apodization are also being considered as a means to increase resolution and S/N ratios. A modification that uses dual-detector electrodes (detector electrodes in each of the end caps) is being implemented in order to provide differential detection as a means of further improving sensitivity. This modification should also remove the harmonics of even order. ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences and by Finnigan MAT. We thank W. Vaughn for technical assistance and Curt Cleven and Carsten Weil for helpful discussions.

Received for review June 12, 1996. Accepted July 30, 1996.X AC960577S X

Abstract published in Advance ACS Abstracts, August 15, 1996.