Anal. Chem. 1986, 58,2935-2938
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Extension of Dynamic Range in Fourier Transform Ion Cyclotron Resonance Mass Spectrometry via Stored Waveform Inverse Fourier Transform Excitation Tao-Chin Lin Wang,' Tom L. Ricca,2and Alan G . Marshall*'v3 Department of Chemistry, T h e Ohio State University, 140 West 18th Avenue, Columbus, Ohio 43210
The relatively limited dynamic range In a Fourler transform Ion cyclotron resonance (FTIICR) mass spectrum can be extended by selective ejection of the most abundant ions, followed by normal excltatlon and detectlon of the remaining Ions. Unfortunately, frequency-sweep ejectlon does not provlde sufflclent mass selectivity. I n this paper, we propose stored waveform Inverse Fourier transform (SWIFT) excltatlon for ejection of all Ions above a speclfled intensity threshold. The SWIFT technique accompllshes simultaneous rapid muitlple-Ion ejection, wRh high mass selectlvlty, so that the less abundant ions can subsequently be detected with enhanced signal-to-noise ratio and enhanced mass resolution. The theoretical advantages of SWIFT multiple-Ion ejection are demonstrated experlmentally for low-abundance (e.g., carbon-13 Isotope) Ions in the FT/ICR mass spectrum of perfluorotrlbutylamlne.
Fourier transform ion cyclotron resonance (FT/ICR) mass spectrometry ( I ) offers several analytically useful features which have recently been reviewed (2-7): ultrahigh mass resolution (>lOOOOOO a t m / z 1200); accurate mass measurement for gas chromatography with FT/ICR detection; facilitated detection of low-volatility samples due to lo00 times lower source pressure than in other mass spectrometers; versatile ion sources (electron impact (EI), self chemical ionization (self-CI), laser desorption (LD), secondary ionization, and fast atom bombardment (FAB)); trapped-ion capability for study of ion/molecule reaction kinetics, equilibria, and energetics; mass spectrometry/mass spectrometry (MS/MS) with a single sample chamber and mass analyzer. However, a major current limitation of FT/ICR mass spectrometry is its relatively small dynamic range (ca. 103:1) compared to other mass spectrometers (L106:1).At the lower limit of ICR signal strength ( B ) , detector noise limits the minimum number of ions required to give an observable signal to ca. 100 or so. This limit can be extended somewhat by: accumulating several time-domain transients, in order to build up the signal-to-noise ratio; lowering the source pressure, in order to give a narrower (and therefore higher) peak of the same area for a given number of ions; or cooling the detector to reduce Johnson noise. At the upper limit, more than ca. 100000 ions in the detector cell will shift and broaden the FT/ICR spectral peaks due to space charge (9) and ion-ion Coulomb repulsion (10)effects. Therefore, the effective analog dynamic range in FT/ICR is limited to ca. 1OOO:l. In addition, the dynamic range of an analog-to-digital converter at typical ICR bandwidths limits the digital dynamic range to about 1 2 bits in the absence of noise. When noise is present, signal averaging can extend the digital dynamic range up to the word length of the host computer (2C-32 bits) 'Department of Chemistry. Campus Chemical Instrument Center, Department of Biochemistry.
or even higher with block averaging. Thus, in the presence of a strong ICR signal, the receiver gain must be reduced in order to avoid truncating the time-domain signal. However, a reduced receiver gain results in reduced signal-to-noise ratio for low-abundance ions. In principle, one method for extending FT/ICR dynamic range is to eject the more abundant ions (large peaks) in order to better detect the remaining less abundant ions (small peaks). Ions can be ejected by increasing either their trapping oscillation amplitude parallel to the applied magnetic field direction or their cyclotron radius transverse to the applied magnetic field direction, until the ions are removed from the trapped-ion cell (11). Because the ICR frequency is so much larger than the trapping oscillation frequency for a given mass-to-charge ratio ( m / z ) ,transverse ejection can be performed with much higher mass resolution than longitudinal ejection. Transverse ejection of ions of a single m / z value is readily accomplished via irradiation with a resonant sinusoidal radio frequency transverse electric field of large amplitude and/or long duration. However, if ions of several m / z values are to be ejected, then one must irradiate different m / z values at different times, either via a succession of sinusoidal pulses whose frequencies match the ICR frequencies of the m / z values to be ejected or via frequency sweeps (12,13) over one or more m/z ranges to be ejected. In other words, it becomes necessary to eject ions of different m / z values at different times, leading to two undesirable consequences: (a) high mass resolution for multiple-ion ejection can require so long an ejection period that much of the desired remaining ICR signal may have disappeared (via ion/molecule collisions and/or reactions) before it can be detected; and (b) attempts to shorten the ejection period reduce unacceptably the mass selectivity of the ejection. For example, in the example to be discussed below, a frequency-sweep ejection of ions of 23 different m / z values would take 0.39 s to accomplish ejected-mass resolution equivalent to single-ion frequency-sweep ejection of 17 ms. Fortunately, we have previously shown that the ion cyclotron is highly linear; Le., the amplitude of the detected response signal is proportional to the amplitude of the transmitter excitation signal (14). For such a linear system, there exists a Fourier transform relationship between the time- and frequency-domain representations. Furthermore, an ICR frequency-domain spectrum can easily be converted to a mass-domain display. Therefore, we can produce any desired mass-domain excitation or ejection profile by first converting to its frequency-domain spectrum, and then performing an inverse Fourier transform to give the equivalent time-domain excitation waveform which is stored in the computer until we are ready to send it to the transmitter plate(s) of the trapped-ion cell (15). This stored waveform inverse fourier transform (SWIFT) technique can excite and/or eject ions with almost any desired mass dependence: e.g., uniform power over a specified mass range; multiple-ion excitation/detection; multiple-ion ejection,
0003-2700/86/0358-2935$01.50/00 1986 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
a
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Figure 2. Functional diagram for a module that adds SWIFT capability to an existing FT/ICR mass spectrometer. The time-domain waveform stored in the computer is clocked out through a digital-to-analog converter, sampled and held, amplified, (optionally) filtered, and then mixed (or not) with a radio frequency carrier before being conditioned and sent to the transmitter cell plates. A mode selector allows for SWIFT direct mode, SWIFT heterodyne mode, or frequency-sweep direct or heterodyne mode.
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an applied magnetic field strength of 3.0 T. Experiments followed
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Figure 1. Experimental event sequence for (a) frequency-sweep excitation and detection and (b) tailored ejection followed by frequencysweep excitation and detection.
simultaneous ejection of some m / z values and excitation of others; ejection with a narrow-mass window (for the first stage in an MS/MS experiment); etc. In particular, it is possible to excite simultaneously, a t high mass resolution, ions having an arbitrary number of m/z values (15). Therefore, we propose that FT/ICR dynamic range can be extended as follows. First, a "normal" frequency-sweep excitation spectrum, in which ions of all m / z values are present, is acquired (Figure la). Next, a SWIFT multiple-ion ejection event (Figure l b ) is introduced before the broad-band excitation/detection procedure in the normal experimental sequence. The SWIFT multiple-ion ejection is tailored to eject ions of all m / z values found to have a magnitude-mode peak height greater than a specified threshold in the original spectrum, thereby leaving a smaller number of ions of other m / z values. Because most of the originally formed ions are removed in the tailored ejection event, ions of the remaining m / z values are much less abundant and can be detected a t higher receiver gain. Alternatively, more ions can be generated in the ion-formation step (e.g., E1 with higher filament current or electron beam duration), for the same receiver gain. In either case, dynamic range is enhanced.
EXPERIMENTAL SECTION All theoretical SWIFT waveforms and experimental spectra were produced on a Nicolet FTMS-1000 instrument, equipped with a home-built module whose functional block diagram is shown in Figure 2. The common mass calibrant, perfluorotri-n-butylamine, purchased from Chemresearch, Inc., was introduced at a pressure of 1 X lo-* torr via a Varian 501 leak valve into a Fourier transform mass spectrometer operating at a cell temperature of 100 "C and
operation, frequency-sweepexcitation was used to bring ions into coherent cyclotron motion so that they could be detected by virtue of the induced macroscopic charge on the receiver plates. In order to extend the dynamic range in FT/ICR, a tailored multiple-ion ejection event was introduced prior to the normal frequency-sweep excitation and direct-mode detection. The tailored ejection procedure starts by defining a desired ejection spectrum in the frequency-domain followed by an inverse Fourier transform computation to obtain the corresponding time-domain waveform, which is stored in a buffer memory. When the ejection event is triggered by the experimental event sequence gate pulse, the time-domain ejection stored waveform is clocked out at a preset rate to a digital-to-analogconverter and then amplified and routed to the transmitter plates of the trapped-ion cell. In these experiments, the desired tailored multiple-ion ejection waveform was defined intially by 16K frequency-domain data points. In order to avoid any power spillover to nearby m / z values during the ejection, each of the resonant ejection ranges was less than 400 Hz wide. The corresponding time-domain waveform was clocked out to the digital-to-analogconverter at 2 MHz during the ejection event. For both normal mode and tailored multiple-ion ejection mode of operation, 32K discrete time-domain data points were acquired at a 2-MHz sampling rate. Five thousand time-domain transients were accumulated and filled with another 32K of zeros before Fourier transformation, in order to increase digital resolution. The receiver gains were set just low enough to prevent clipping of the time-domain signal by the analog-todigital converter. All other experimental parameters, except the electron beam durations in Figure 3, were the same for all experiments.
RESULTS AND DISCUSSION Normal FT/ICR mass spectra of PFTBA, for electron beam durations of 15 ms and 50 ms, are shown in Figure 3, parts a and b, respectively. More ions are generated with longer beam, and the signal-to-noise ratio is thereby enhanced. However, space charge and ion-ion Coulomb repulsion effects become noticeable when more ions are present in the cell, and the peaks are broadened and distorted (compare parts a and b of Figure 3). Figure 3c shows the magnitude-mode spectrum of an excitation tailored to eject all ions whose FT/ICR mass spectral peak heights are greater than a threshold of 1.6% of the largest peak in Figure 3b. In this case, excitation power in 23 different narrow radio frequency ranges was applied to the transmitter plates in the ejection event. The advantages of SWIFT over frequency-sweep for multiple-ion ejection are seen clearly in Figure 4, in which only six frequency regions are shown for purposes of illustration. Figure 4a is the desired ideal ion ejection magnitude-mode
ANALYTICAL CHEMISTRY, VOL. 58, NO 14, DECEMBER 1986
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200 300 400 MASS (A. M. U.) Figure 3. FT/ICR mass spectra of PFTBA, generated via electron ionization and frequency-sweep excitation: (a) 15-ms electron beam duration; (b) 50-ms electron beam duration (With longer beam duration, more ions are produced and the signal-to-noiseratio is enhanced, but the mass spectral peaks are broadened and distorted as well.); (c) SWIFT excitation spectrum for multiple-ion ejection of the 23 ions
whose magnitude-mode peak heights exceed a threshold of 1.6% of the largest peak in Figure 3b.
Figure 4. Multiple-ion ejection profiles, for ejection of ions of six different m /zvalues: (a) ideal magnitude-mode excitation spectrum: (b) theoretical magnitude-mode excitation spectrum obtained by successive multiple frequency sweeps: (c) theoretical magnitude-mode spectrum resulting from SWIFT multiple-ion excitation. The total time period for ejections is the same (17 ms) in (b) and (c).
profie. Figure 4b is the theoretical magnitude-mode spectrum that would be produced by successive frequency sweeps over the six specified narrow mass ranges. Figure 4c is the magnitude-mode ejection profile for SWIFT excitation, which
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300 400 500 MASS (A.M.U.) Figure 5. FT/ICR mass spectra of PFTBA, in which m / z 503 is full scale: (a)verticaly scalaexpanded plot of the normal mass spectrum of Figure 3b; (b) mass spectrum obtained following prior SWIFT
multiple-ion ejection of all of the 23 peaks whose magnitude-mode peak heights exceed a threshold of 1.6% of the largest peak in Figure 3b. exhibits three major advantages over frequency-sweep multiple-ion ejection. First, because the SWIFT time-domain waveform is turned on and off only once, whereas the frequency sweep must be turned on and off N times for ejection over N distinct m / z ranges, the average SWIFT mass selectivity will be N times better than for frequency sweeps of the same total period of ejection. Higher mass selectivity ensures that ions with m / z values outside the ejection ranges will not be affected by the ejection process. For example, the SWIFT ejection profile of Figure 3c was generated with a total ejection period of 17 ms, yielding a theoretical frequency selectivity of 58.8 Hz, for an arbitrary number of ejection bands. In contrast, use of multiple frequency sweeps (with the same total ejection period of 17 ms) to eject the 23 most abundant ions in the PFTBA example would give frequency a selectivity of 1350 Hz. Second, comparison of parts a and b of Figure 4 shows that the ejection power within each ejected mass range will be much more uniform with SWIFT than with frequency-sweep multiple ejection, thereby ensuring that all ions with m / z values within each desired ejection range will be ejected. Third, ions in all of the specified m / z ranges are ejected simultaneously via SWIFT but are ejected sequentially with successive frequency sweeps. The simultaneity aspect will be important when (as in MS/MS experiments) ion/molecule reactions are occurring during the experiment. Other benefits from SWIFT multiple-ion ejection are revealed in Figure 5. A vertically scale-expanded plot of the normal frequency-sweep spectrum (Figure 5a) is compared to the spectrum (Figure 5b) obtained after SWIFT ejection of ions corresponding to the 23 largest peaks in the normal spectrum. In the scale-expanded normal spectrum, the nonflat base line creates great difficulty in measuring the relative intensities of small peaks, especially the carbon-13 isotope peaks that appear on the shoulders of their more abundant carbon-12 relatives. Because SWIFT multiple-ion ejection removes the abundant ions (and their broad peak shoulders), the resulting flatter base line greatly facilitates the mea-
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Table I. FT/ICR Mass Spectral P e a k Relative Intensities of Low-Abundance Ions (Mostly Carbon-13 Isotopic Species) Formed by Electron Ionization of P F T B A
chemical formula
m,/z 70 120 170 231 314 377 415 427 465 503
re1 peak intens predicted from Figure 3a Figure 3h 85.5 13.7 16.2 100.Oh 17.8 61.6 28.5 37.7 88.5
51.2 15.0 16.2 83.3h 70.5' 16.0 61.5 27.6 43.0 100.0
re1 peak intens from Figure Fih 68.0 13.8 17'5 87.6 73.6 16.5 59.1 28.7 39.4 100.0
a The rightmost column lists intensities measured directly from a spectrum from which ions of the 23 most abundant m / z values had previously been ejected via the SWIFT technique. Alternatively, these relative intensities can be predicted from the relative intensities of the same ion with all carbon-12 isotopes in the normal spectrum (Figure 3, parts a and 3b). *The relative intensities of these peaks were measured directly (with poor precision. because the peaks were so small) from the spectra of Figure 3, parts a and b.
surement of peak heights and areas. It is worth noting that the carbon-13 isotope peak at mlz 503 becomes the base peak in Figure 5b, whereas the m / z 502 carbon-12 peak is eliminated completely, even though the two peaks are only 183 Hz apart at a magnetic field strength of 3.0 T. An obvious test of the multiple-ion ejection experiment is to see if the postejection relative intensities of the (lowabundance) ions containing a single carbon-13 atom are the same as those predicted from the preejection relative intensities of the (high-abundance) ions of the same chemical formula with all carbon-12 atoms (see Table I). The relatively close agreement (to within ca. 5%) between the predicted and observed relative intensities is good evidence for highly selective ejection of the abundant 12C,-containingions without affecting the nearby 13C12C,-1-containingions. The relatively minor inconsistencies are likely due in part to slight fluctuations in the numbers of ions produced from scan to scan. (The 12C6F9+ and I2C6F5N+ values have poor precision, because of limited vertical digital resolution for such weak peaks in the "normal" spectra of parts a and b of Figure 3.) Because ejection of the most abundant ions greatly reduces broadening arising from ion-ion repulsions, the line widths of the remaining peaks are much narrower (Figure 5b) than in a scale-expanded plot (Figure 5a) of the normal mass spectrum. The ejection process also reduces space charge induced frequency shifts, as confirmed by the larger peak center frequencies in Figure 5b than in Figure 3b, facilitating mass calibration and accurate-mass determination. Because most of the ions have been ejected, the remaining ions can be detected with higher receiver gain, so that the weak signals fill the analog-to-digital converter to give improved accuracy in peak intensity measurement. Finally, because most of the initial ions are ejected, more ions can be introduced into the cell in the first place by using longer electron beam duration or higher emission current. Thus, more of the less abundant ions are formed, and the dynamic range of the final mass spectrum is enhanced. For cases in which extremely high dynamic range is needed, one could project a series of experiments in which successively weaker peaks are removed while the number of initially formed ions increases: e.g., normal spectrum, then a spectrum with all peaks above a 1% threshold removed, then a spectrum with all peaks above a 0.1% threshold removed. etc.
The problem of ICR excitation of ions of a given m/z value (with ICR frequency, w,) by single-frequency irradiation for a period, T , by an rf electric field of amplitude, E,, centered at a nearby frequency (a1)has been treated theoretically by Beauchamp (16) and examined experimentally by Castro and Russell (17). The translational energy, E,,, imparted to the ion is given by
Equation 1 was derived under the same assumptions as the present treatment, namely, that no ion/molecule collisions occur during the excitation period and that the system is linear (Le., response amplitude is proportional to excitation amplitude). The right-hand factor in eq 1 can be thought of simply as the normalized power spectrum of the excitation. Thus, some of the excitation power centered at w1 will appear to "spill over" to w,, because the Fourier transform of a time-domain rf pulse of duration, T , is a sinc function centered at w1 (18). The result of this "classical uncertainty principle" is that frequency-domain excitation power can be no more selective than ca. (1/7')Hz,for a time-domain excitation of duration T. Equation 1 therefore becomes another way to understand the advantage of SWIFT over frequency sweep for multiple-ion excitation or ejection. For a frequency sweep, it is necessary to turn the excitation on and off N times, where N is the number of different m / z ranges to be excited; the equivalent SWIFT excitation is turned on and off only once. In the limit that each frequency sweep is simply a constant-frequency rf pulse, eq 1 then reveals that the frequency-(or mass-)resolution of the excitation is N times worse for N separate pulses of duration TIN each, than for the single SWIFT irradiation of duration T. A similar result holds for swept rather than constant frequency during each irradiation interval.
ACKNOWLEDGMENT The authors wish to thank Duane Littlejohn for helpful suggestions. Registry No. PFTBA, 311-89-7. LITERATURE CITED Comisarow, M. B.; Marshall, A. G. Chem. Phys. Lett. 1974, 2 5 , 282-283. Marshall, A. G. A c c . Chem. Res. 1985, 18,316-322. Gross, M. L.; Rempel, D. L. Science (Washington, D.C.)1984, 226, 261-268. Wanczek, K. P. I n t . J. Mass Spectrom. Ion Processes 1984, 6 0 , 11-60. Freiser. B. S. Talanta 1985, 32, 697-708. Laude, D. A., Jr.; Johlman, C. L.; Brown, R. S.; Weil. D. A , ; Wilkins, C. L. Mass Spectrom. Rev. 1988, 5 , 107-166. Russell D. H. Mass Spectrom. Rev. 1986, 5 , 167-189. Comisarow, M. B. J. Chem. Phys. 1978. 6 9 , 4097-4104. Jeffries, J. B.; BarLow, S. E.;Dum, G. H. I n t . J. Mass Spectrom. Ion Processes 1983, 54, 169-187. Wang, T.-C. L.; Marshall, A. G. I n t . J. Mass Spectrom. Ion Processes 1988, 6 8 , 287-301. McIver, R. T., Jr. Rev Sci. lnstrum. 1970, 41,555-558. Comisarow. M. B.; Marshall, A. G. Chem. Phys. Lett. 1974, 26, 489-490. Marshall, A. G.; Roe, D. C. J. Chem. Phys. 1980, 7 3 , 1581-1590. Marshall, A. G.; Wang, T.-C. L.;Ricca, T. L. Chem. Phys. Lett. 1984, 105,233-236. Marshall. A. G.: Wana. T . 4 . L.: Ricca, T. L. J , A m . Chem. SOC. iga5, 107, 7a93-78f7. Beauchamp, J. L. Annu. Rev. Phys. Chem. 1971, 2 Z 3 527. Castro, M. E.; Russell, D. H. Anal. Chem. 1985, 57, 2290-2293. Marshall. A. G.. I n Fourier. Hadamard. and Hilbert Transforms in Chemistiy; Marshall, A. G., Ed.; Plenum. New York, 1982; pp 1-43.
RECEIVED for review June 4,1986. Accepted July 9,1986. This work was supported by grants (to A.G.M.) from the USA Public Health Service (N.I.H. 1R 0 1 GM-31683) and The Ohio State University.
