Anal. Chem. 1989, 61, 489-491
489
CORRESPONDENCE Impulse Excitation for Fourier Transform Mass Spectrometry Sir: Fourier transform mass spectrometry(FTMS) has been used for many years to study ion-molecule reactions in the gas phase (1-4). More recently, it has been used in conjunction with pulsed lasers for the characterization of polymers (51, high molecular weight biomolecules (6-8), and trace levels of molecular species on surfaces (9-11). FTMS is especially suited for these experiments because it is intrinsically a pulsed technique. A burst of ions, such as might be produced by a laser pulse, can be trapped efficiently in an analyzer cell and then mass-analyzed at very high mass resolution. Most FTMS experiments have been performed by using a rapid radio frequency (rf) sweep, or “chirp”, to accelerate ions which are trapped in an ion cyclotron resonance (ICR) analyzer cell (12). Typically, the chirp excitation signal is generated by a digitally controlled frequency synthesizer, which is scanned from 20 kHz to 2 MHz in a period of about 1ms. This signal is applied differentially to two transmitter plates of the analyzer cell to produce an oscillating electric field perpendicular to the magnetic field. As the frequency of the oscillating electric field approaches the cyclotron resonance frequency of an ion, the ion absorbs energy from the field and is accelerated along a spiral path to a larger radius orbit. One of the limitations of chirp excitation is its nonuniform power spectrum (13). Even if the amplitude of the chirp signal is constant as the frequency is scanned, all ions in the scan range do not receive the same excitation power. This results in mass discrimination that can cause isotope ratios to be in error by as much as 20%. Another problem with chirp excitation is that ions can be inadvertently ejected from the cell in the z direction (parallel to the magnetic field) (14-16). Since the chirp period is relatively long (milliseconds) compared to the oscillations of the ions between the trapping plates, the z component of the chirp electric field can accelerate the ions in the trapping well and cause mass-dependent ejection. These problems with chirp excitation have stimulated interest in alternative excitation methods. The first FTMS experiments were performed by using a fixed-frequency rf pulse, or “burst”,just as is currently done with FT-NMR (17). Although this method works well over a narrow mass range, it is not practical for exciting the full range of cyclotron frequencies (10 kHz to 2 MHz) that are normally encountered in FTMS (13).In another approach, FTMS experimenb have been performed by applying a near critically damped sinusoidal pulse between the end caps and the ring electrode of a Penning ion trap (18, 19). A limitation of this method, however, is that the tuning behavior of the Penning trap is strongly dependent on the trapping voltage and the amplitude of the applied pulse. A promising new approach is the stored waveform inverse Fourier transform ( S W ” ) method (20-22). With SWIFT excitation, the desired spectral excitation frequency profile is specified as a frequency-domain spectrum, and then an inverse discrete Fourier transform is used to generate the equivalent discrete time-domain waveform. The time-domain waveform is strobed from a memory buffer to a digital-to-analog converter and applied to the transmitter plates of the cell. In this paper we describe our first experiments with an impulse excitation method for FTMS that utilizes a new 0003-2700/89/036 1-0489$01.50/0
waveform for ion excitation. The principle of impulse excitation is illustrated in Figure 1, which shows the waveform used and the equipotentials that are formed when it is applied differentially to the transmitter plates of an ICR cell. Initially, ions are formed at the center of the cell and are trapped by a strong magnetic field (1-7 T) and a weak electric field (0.25 V/cm) that is produced by direct current (dc) voltages applied to a pair of trapping plates. Under these conditions, the ions have low translational energies and the radius of their cyclotron orbits is only a few tenths of a millimeter. When the impulse waveform is applied to the transmitter plates, all ions in the cell are abruptly accelerated by the intense electric field. For example, in Figure 1,positive ions are accelerated toward transmitter plate 2 for a time T when the impulse waveform is applied. When the pulse turns off, the accelerated ions move in circular cyclotron orbits having a larger radius of gyration. Since all the ions are accelerated simultaneously in the same direction, the excited cyclotron motion is phase coherent and the coherently moving ions produce a transient image current signal that is a composite of all the various cyclotron frequencies of each different mass. A mass spectrum of the ions in the analyzer cell is obtained by digitizing this signal and subjecting it to a Fourier transform analysis to separate the individual cyclotron frequency components. In terms of a physical model, impulse excitation is conceptually the same as when a piano is hit hard with a hammer, causing all the strings to vibrate simultaneously. The basic features of impulse excitation are illustrated by the following simple model. The equation of motion for an ion in a magnetic field B and an electric field E is
m do/dt = qE
+ q ( D X B)
(1)
where m is the mass of the ion, q is its electric charge, 0 is its velocity, and t is time. When the impulse is off, the electric field term E contains only the dc voltages which are applied to the electrodes of the cell to trap the ions. Under these conditions, solutions to eq 1 are well-known and show that the ions undergo cyclotron motion in the plane perpendicular to the magnetic field, oscillate between the trapping plates, and drift slowly around the cell (23,24). However, when the impulse excitation waveform is applied to the transmitter plates, as shown in Figure 1,an intense electric field is created perpendicular to the magnetic field. The waveform E, that we use for impulse excitation is For 0 It 5 For t
T, E,
= 2(U/L)(1 - e-kt)
(2)
> T, E, = 2 ( U / L ) ( 1- e-kT)e-k(t-T) (3)
where U is the peak voltage applied to the transmitter plate, L is the spacing between the plates, and (l/lz)is the rise time of the pulse. Thus, the pulse rises rapidly to a peak value of 2U/L (volts per meter), remains on for a short time until t = T, and then decays exponentially to zero, as specified by eq 3. The motion of ions in response to the impulse waveform can be most easily seen by making two approximations. First, it is assumed that the rise time of the pulse (l/k) is very fast compared’tothe pulse duration T and the period of a cyclotron 0 1989 American Chemical Society
490
ANALYTICAL CHEMISTRY, VOL. 61, NO. 5, MARCH 1 , 1989
Transmitter plate 1
li
1’ 154
I I
20
8
do
60
1 li
\
8b
100
140
IAO
160
do
260
mi=
+lv c--
Flgwe 2. Electron ionization mass spectrum of bromobenzene obtained with impulse excitation.
-+lV
Tra ing
Trawing date 1
daP2
13
I 6
(CSl),CS+ 22
Transmitter plate 2
~9
1
4
,
1
37
0
Flgure 1. Schematic drawing of the equipotentials inside a cubic ICR analyzer cell when the impulse excitation waveform is applied differentially to the two transmitter plates. M A S S SCALE,
orbit. Essentially, the ions are subjected to a “voltage step,” and eq 2 and eq 3 can be rewritten For 0 It IT , E, = 2(U/L) (4) For t > T , E, = 0 (5) The second approximation is that the force caused by the impulse is so intense that it overpowers the effects of the magnetic field and the dc trapping voltages. Under these conditions, the equation of motion (eq 1) can be rewritten m dv/dt = qE (6) Combining this with eq 4 for the electric field gives m dv/dt = q(2U)/L (7) for the velocity in the direction of the impulse electric field. Since the peak voltage U is approximately constant, eq 7 can be integrated to give an expression for the ion’s velocity u after it has been accelerated for a time T. u = 2qUT/mL
(8)
After the pulse turns off, the force of the magnetic field again becomes important, and the accelerated ions move in circular cyclotron orbits. The radius of their circular motion, called the radius of gyration, is given by r = u / w , where w = qB/m is the cyclotron frequency. Combining this with eq 8 for the ion’s velocity gives the following final result for the radius of gyration: r = 2UT/BL (9) This result shows that all ions in the cell, regardless of mass or charge, are accelerated by impulse excitation to the same radius of gyration. This is an important conclusion because the image current signal produced by coherently moving ions is directly proportional to their radius of gyration (25,26). As a result, an FTMS instrument that uses impulse excitation is expected to have minimal mass discrimination. Substitution of typical operating parameters for impulse excitation (U = 120 V, T = 5 X lo-’ s, B = 1 T, and L = 0.03 m) into eq 9 gives a radius of gyration of 0.40 cm, which is quite sufficient to produce detectable FTMS signals. The derivation above does not take into consideration the finite turn-on and turn-off time of the impulse and does not apply if the duration T of the pulse is comparable to the period of an ion’s cyclotron orbit. For example, a 0.5-ps pulse works
mh
Figure 3. Mass spectrum of cesium iodide cluster ions produced by impulse excitation with an external ion source FTMS instrument and a 6-T superconducting magnet. The numbers over the peaks correspond to the values of n for the cesium iodide clusters, (CsI),Cs+.
best for ions having cyclotron frequencies less than about 1 MHz. A t a magnetic field strength of B = 1T, this corresponds to a lower mass limit of about mlz 15. Shorter pulses and higher voltagea must be used to decrease this cut-off point and accelerate ions of lower mass. Figures 2 and 3 show FTMS mass spectra obtained with impulse excitation. Figure 2 is a mass spectrum of bromobenzene taken with a 1.25-T electromagnet-based FTMS instrument. Ions were formed by electron ionization (30-eV acceleration voltage), and the impulse waveform had an amplitude of U = 120 V and a pulse duration of 0.5 ja.The ions were stored in the analyzer cell for 1 s, and during this time the ion at mlz 154 forms by an ion-molecule reaction between the mlz 77 fragment ion and neutral bromobenzene. When chirp excitation is used, the signal-to-noise ratio is essentially the same, but the isotope ratio for the two bromine-containing ions, mlz 156 and 158, is quite variable and unstable. Figure 3 shows that impulse excitation can also be used with FTMS instruments that utilize a high-field (6 T) superconducting magnet. Under about the same conditions for the impulse amplifier, Figure 3 is a mass spectrum from mlz lo00 to 13 OOO showing cesium’iodide cluster ions generated with an external ion source FTMS instrument by 9-keV bombardment of CsI coated on a copper probe tip. The ions were transferred from the source region to an ICR cell by an rf-only quadrupole lens that has been described previously (27-29). The highest mass ion observed is CsdIII)+ at mlz 12863. In conclusion, it appears that impulse excitation has the following desirable features. First, the new waveform for impulse excitation (eq 2 and 3) is so simple mathematically that complete analytical solutions can be obtained (30). We are using these solutions to calculate ion trajectories and predict how the response varies with the various parameters of the impulse. In contrast, with chirp excitation the ion trajectories are very complex because ions of different mass are accelerated at different times as the frequency of the oscillating electric field is scanned. Second, it is our experience that FTMS signals obtained with impulse excitation are more
Anal. Chem. 1989, 61, 491-493
stable and easier to tune than those obtained with chirp excitation. Generally, the same pulse width and height can be used for all ions, and isotope ratios are more stable and precise than with chirp excitation experiments performed under the same conditions. This may result because with impulse excitation all ions in the cell are accelerated simultaneously in a very short period of time (less than a microsecond). Another feature of impulse excitation is that the electronic circuitry and computer software needed for impulse excitation are far simpler than are needed for chirp excitation or SWIFT. Chirp excitation requires a computer-controlled frequency synthesizer that can scan a frequency range of several megahertz in a time period of just a few milliseconds. The chirp excitation signal must be highly stable and reproducible from scan to scan so that repetitive scans can be summed together coherently to improve the signal-to-noise ratio of the measurement. In contrast, an impulse excitation amplifier needs only a trigger pulse from the computer and simple adjustments for the amplitude and duration of the pulse. Impulse excitation is fundamentally different from the rf burst and rf chirp excitation methods used previously in FTMS because it is a nonselective, nonoscillatory excitation means. As a result, one of the limitations of impulse excitation is that double resonance ejection and sweep-out experiments, which are so useful for studying ion-molecule reactions, cannot be done because it lacks the necessary mass selectivity. However, there is no reason why these selective, resonance experiments cannot be done in the conventional manner while impulse excitation is used for the FTMS detection of the ions.
ACKNOWLEDGMENT We thank Dr. Carlito Lebrilla for assistance in using the external ion source FTMS instrument a t the University of California, Irvine. LITERATURE CITED Gross, M. L.; Rempel, D. L. Sclence (Washlngton, L X ) 1984, 226, 261-268. Freiser, 8. S. Talenta 1985,3 2 , 697-708. Laude, D. A,, Jr.; Johlman, C. L.; Brown, R. S.; Weil. D. A.; Wilkins, C. L. Mass Spectrom. Rev. 1986,5 , 107-166. Russell, D. H. Mass Spect”. Rev. 1986,5 , 167-189. Ijames. C. F.; Wilkins, C. L. J. Am. Chem. SOC. 1988. 110, 2687-2688. - - -. - - - -. Hunt, D. F.; Shabanowitz, J.; McIver, R. T., Jr.; Hunter, R. L.; Syka, J. E. P. Anal. Chem. 1985,57, 765-768. Hunt, D. F.; Shabanowk, J.; Yates, J. R.. 111; McIver, R. T., Jr.; Hunter, R. L.; Syka, J. E. P.; Amy, J. Anal. Chem. 1985, 57, 2733-2735.
49 1
(8) McCrery, D. A.; Ledford. E. D., Jr.; Gross, M. L. Anal. Chem. 1982, 108, 1435-1437. (9) Sherman, M. G.; Kingstey, J. R.; Hemminger, J. C.; McIver, R. T., Jr. Anal. Chlm. Acta 1985, 178, 79089. (10) Sherman, M. G.; Land, D. P.; Hemminger. J. C.; McIver, R. T., Jr. Chem. Phys. Lett. 1987, 137, 298-300. (11) Land, D. P.; Tai. T . I . ; Llndquist. J. M.; Hemminger, J. C.; McIver, R. T., Jr. Anal. Chem. 1987. 5 9 , 2924-2927. (12) Comisarow, M. B.; Marshall, A. G. Chem. Phys. Lett. 1974, 26, 489-490. (13) Marshall. A. G.; Roe, D. C. J. Chem. Phys. 1980, 7 3 , 1581-1590. (14) Huang, S. K.; Rempel, D. L.; Gross, M. L. Int. J. Mass Spectrom. Ion Processes 1986, 72, 15-31. (15) Kopel. P.; Allemann, M.; Kellerhals. H.P.; Wancrek, K. P. Int. J. Mass Spectrom. Ion Processes 1986, 74, 1-12. (16) van der Hart, W. J.; van de Guchte. W. J. Int. J. Mass Spect”. Ion Processes 1988p8 2 , 17-31. (17) Comisarow, M. B.; Marshall, A. G. Chem. Phys. Lett. 1974, 2 5 , 282-283. (18) Ledford, E. B., Jr.; Rempel, D. L.; Gross, M. L. Anal. Chem. 1984,5 6 , 2744-2748. (19) Rempel, D. L.; Ledford, E. B., Jr.; Gross, M. L. Anal. Chem. 1987,5 9 , 2527-2532. (20) Marshall, A. 0.; Wana. T.C. L.; Rlcca. T. L. J. Am. Chem. SOC. 1985,107, 7893-7asi7. (21) Wang, T.C. L.; Ricca, R. L.; Marshall, A. 0. Anal. Chem. 1986,5 8 , 2935-2938 -- - - - - - -. (22) Chen, L.; Wang, T.-C. L.; Rlcca, T. L.; Marshall, A. G. Anal. Chem. 1987. 5 9 , 449-454. (23) Sharp, T. E.; Eyler, J. R.; Li, E. I n t . J. Mass Spectrom. Ion Phys. 1972. 9 , 421. (24) Hunter, R. L.; Sherman, M. G.; McIver, R. T., Jr. Int. J. Mass Spectrom. Ion Phys. 1983,50, 259-274. (25) Comisarow, M. B. J. Chem. Phys. 1978, 6 9 , 4097-4104. (26) McIver, R. T., Jr.; Hunter, R. L.; Ledford. E. B., Jr.; Locke, M. J.; Francl, T. J. Int. J. Mass Spectrom. Ion Phys. 1981, 3 9 , 65-84. (27) McIver, R. T.. Jr.; Hunter, R. L.; Story, M. S.; Syka. J.; Labunsky. M. Paper presented at the 31st Annual Conference on Mass Spectrometry and Allied Topics, Boston, MA, May 6-13, 1983. (28) McIver, R. T., Jr.; Apparatus and Method for Injectlon of Ions into an Ion Cyclotron Resonance Cell. US. Patent 4,535.235. Aug 13, 1985. (29) McIver. R. T., Jr.; Hunter, R. L.; Bowers, W. D. I n t . J. Mass Spectrom. Ion Processes 1985. 6 4 , 67-77. (30) McIver, R. T., Jr.; Baykut, G., Hunter, R. L., unpubilshed results.
Robert T. McIver, Jr.* Department of Chemistry University of California Irvine, California 92717
Richard L. Hunter Gokhan Baykut IonSpec Corporation 17951 Skypark Circle, Suite K Irvine, California 92714 RECEIVED for review August 22, 1988. Accepted November 11, 1988.
Separation of Dansylated Methylamine and Dansylated Methyl-&-amine by Micellar Electrokinetic Capillary Chromatography with Methanol-Modified Mobile Phase Sir: Recent papers have expanded the versatility of capillary electrophoresis by utilizing micellar solutions in micellar electrokinetic capillary chromatography (MECC) (I,2). The technique has been applied to such separations as phenylthiohydantoin amino acids (3),B6Vitamers ( 4 ) , nucleic acid constituents (5), neurotransmitters (6),and amino acid enantiomers (7). The present report describes a system that yields near base line resolution for dansylated methylamine and danyslated methyl-d3-amine. The system uses a phosphateborate buffer system containing 25 mM sodium dodecyl sulfate (SDS)and 20% methanol (MeOH). The two labeled amines, shown in Figure 1, differ by only one trideuterated methyl group. 0003-2700/89/0361-0491$01.50/0
EXPERIMENTAL SECTION Materials. A fused silica capillary with dimensions 50 pm i.d. and 150 pm 0.d. was obtained from Polymicro Technologies (Phoenix, AZ). Electrophoresis grade SDS was obtained from Bethesda Research Labs (Gaithersburg, MD). Methylamine, methyl-d3-aminehydrochloride, and octylamine were obtained from Aldrich (Milwaukee,WI).Dansyl chloride and n-hexylamine were obtained from Sigma (St. Louis, MO). Dodecylaminewas obtained from Eastman Kodak (Rochester,NY), and formamide was obtained from Fisher Scientific (Raleigh, NC). Deionized water was further purified with a Barnstead Nanopure system (Boston, MA). Equipment. Two detectors were used in this work. Dansylated compounds were detected by using a variable wavelength 0 1989 American Chemical Society
