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The kinetics of charge reduction and desorption of different species produced by soft-landing of mass-selected ions was studied using in situ secondar...
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18220

J. Phys. Chem. C 2007, 111, 18220-18225

First Observation of Charge Reduction and Desorption Kinetics of Multiply Protonated Peptides Soft Landed onto Self-Assembled Monolayer Surfaces Omar Hadjar, Jean H. Futrell, and Julia Laskin* Pacific Northwest National Laboratory, Fundamental Science Directorate, Richland, Washington 99352 ReceiVed: July 6, 2007; In Final Form: September 17, 2007

The kinetics of charge reduction and desorption of different species produced by soft-landing of mass-selected ions was studied using in situ secondary ion mass spectrometry (SIMS) in a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS). The improved SIMS capability described in this work utilizes an in-line 8 keV Cs+ ion gun and allows us to interrogate the surface both during the ion deposition and after the deposition is terminated. As a model system doubly protonated ions of Gramicidin S were deposited onto a fluorinated self-assembled monolayer (FSAM) surface. Our results demonstrate for the first time that various peptide-related peaks in FT-ICR SIMS spectra follow very different kinetics. We obtained unique kinetics signatures for doubly protonated, singly protonated and neutral peptides retained on the surface and followed their evolution as a function of time. The experimental results are in excellent agreement with a kinetic model that takes into account charge reduction and thermal desorption of different species from the surface.

Introduction Soft-landing (SL) of hyperthermal ions onto surfaces using a beam of mass-selected ions is a promising approach for highly selective preparation of novel substrates.1-8 In addition, controlled deposition of complex ions onto surfaces presents a new approach for obtaining molecular level understanding of interactions of large molecules and ions with a variety of substrates relevant for biology and catalysis research.9 Self-assembled monolayer surfaces (SAMs) provide a particularly convenient and flexible platform for tailoring the interfacial properties of metals and semiconductor surfaces.10,11 They have been also extensively utilized for studying ion-surface collisions12-15 and ion SL.1,2,9,16 In our first systematic study of factors that affect SL of peptide ions on inert SAM surfaces,9 we presented evidence that some or all peptide ions retain at least one proton after SL onto fluorinated self-assembled monolayer (FSAM) surfaces. In that study, we interrogated SL-modified surfaces mounted adjacent to the ICR cell of a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) using 2 keV Cs+ secondary ion mass spectrometry (SIMS) for in situ analysis. We found that FT-ICR SIMS spectra of all peptides were dominated by the singly protonated peptide ion peak, [M+H]+, regardless of the initial charge state of the mass-selected precursor ion. Since hardly any doubly or triply protonated ions were observed by SIMS analysis when multiply charged peptide ions were soft-landed on the SAM and mainly the singly protonated species were retained, we concluded that protons are readily transferred to the surface. More recently we compared the results obtained using in situ (FT-ICR SIMS) and ex situ time-of-flight secondary ion mass spectrometry (TOFSIMS) analysis of SAM surfaces following SL of peptide ions. This more detailed study demonstrated that a small but measurable fraction of multiply protonated peptide ions retain more than one proton following SL on the FSAM surface.17 * To whom correspondence should be addressed. Tel: (509)-376-4443. Fax: (509)-376-6066. E-mail: [email protected].

We concluded that the doubly protonated, [M+2H]2+, ions observed in FT-ICR SIMS spectra were produced by desorption of multiply charged ions from the surface, whereas reionization of singly protonated ions or neutral peptides was a principal source of [M+2H]2+ ions in TOF-SIMS spectra. Here we report a detailed study of the kinetics of charge reduction and desorption of peptide ions soft-landed on the FSAM surface using an improved in situ SIMS capability of our FT-ICR MS that utilizes an in-line 8 keV Cs+ ion gun to interrogate the surface. Our results demonstrate for the first time that various peptide-related peaks in FT-ICR SIMS spectra follow very different kinetics. We obtained unique signatures for the time evolution of doubly protonated, singly protonated, and neutral peptides retained on the surface during deposition and for several hours after deposition is complete. The experimental results are in excellent agreement with a simple kinetic model that takes into account charge reduction and thermal desorption of different species from the surface. Experimental Section Experiments were performed using a specially designed 6T FT-ICR instrument configured for studying ion-surface interactions.18 The experimental approach for SL experiments has been described elsewhere16 and is illustrated in Figure 1; it involves normal-incidence collision of externally produced ions with a SAM surface positioned at the rear trapping plate of the ICR cell. Ions are produced in a high-transmission electrospray source, efficiently thermalized in the collision quadrupole and mass-selected prior to acceleration and collision with the surface. Ion kinetic energy is controlled by varying the voltage difference between the collisional quadrupole of the ion source and the surface and was maintained at 40 eV in these experiments. During ion SL the surface is exposed to a continuous beam of mass-selected ions and the deposition time is varied between 20 and 150 min. Typical ion currents of 15 pA of mass-selected doubly protonated ions of Gramicidin S (GS) are delivered onto a ca. 3.5 mm diameter spot on the target. The spot size was determined ex situ using TOF-SIMS by performing a profile

10.1021/jp075293y CCC: $37.00 © 2007 American Chemical Society Published on Web 11/14/2007

Protonated Peptides Soft Landed onto SAMs

J. Phys. Chem. C, Vol. 111, No. 49, 2007 18221

Figure 2. FT-ICR-SIMS mass spectra of the a) FSAM surface and b) the FSAM surface after 58 min soft landing of 16 pA [GS+2H]2+. The insert is a zoom around m/z 572 showing the overlap between the symmetric fragment (LFPVO, m/z 571.4) and the retained [GS+2H]2+.

Figure 1. Schematic drawing of the instrument showing the electrospray interface used to generate the beam of mass-selected peptide ions and the in-line cesium gun used for in situ SIMS analysis of the surface.

scan through the peptide spot produced by soft landing of the ion beam.19 Our previous study demonstrated that the SIMS signal increases linearly with the ion dose as long as it does not exceed the equivalent of 30% of monolayer coverage, whereas at higher exposures, significant deviation from linearity (saturation) is observed.16,19 The maximum coverage of softlanded ions obtained in these experiments does not exceed 12% (15 pA [GS+2H]2+ for 158 min on a 3.5 mm diameter spot; 263 Å2 cross section) suggesting that the total ion dose is well below the saturation threshold.16 In situ analysis of surfaces following SL is performed by combining 8 keV Cs+ secondary ion mass spectrometry with FT-ICR detection of the sputtered ions (FT-ICR SIMS).9,16 Primary Cs+ ions are generated using a model 101502 HWIG250F cesium ion gun (HeatWave Labs Inc., Watsonville, CA) that is installed on-axis with the SL target. The Cs ion gun incorporates electrostatic deflection plates for pulsing and deflection of the Cs+ beam and an Einzel lens for additional focusing. As shown in Figure 1, the Cs+ beam is transmitted through the electrostatic bending quadrupole of the FT-ICR instrument and the remaining part of the electrostatic ion guide of the instrument into the surface mounted just behind the ICR cell. The bending quadrupole is located at the intersection of the peptide ion beam and the Cs+ beam. As indicated by SIMION trajectory calculations and confirmed experimentally the voltages applied to the quadrupole bending rods (Figure 1) result in negligible deflection of the 8 keV Cs+ ion beam, enabling simultaneous transmission of both ion beams to the surface for monitoring the soft landing process during and immediately following ion deposition. Static SIMS conditions with a total ion flux of about 1010 ions/cm2 (current 4 nA, duration 80 µs, spot diameter 4.6 mm, 10 shots per spectrum, ∼100 data points) were used in these experiments that typically lasted for 10-12 h. The following potentials were applied to various focusing elements: Cs+ gun floating voltage, +8 kV; extraction, +7 kV; lens, +5 kV; einzel lens, (L1 ) L2 ) -250 V; L3 ) +3 kV). The Cs+ ion beam was pulsed by alternating the potential applied to one of the deflection plates between 0 and -400 V the high value being used to block the Cs+ beam

from reaching the surface. Data acquisition was accomplished utilizing a MIDAS data station.20 Acquisition of SIMS spectra followed the procedure described in detail elsewhere.16 A typical event sequence included a quench pulse (51 ms); Cs+ pulse (80 µs); recoil (60 µs); Au substrate ions ejection (2.5 ms); delay (60 ms); Cs ejection (5.2 ms); gated trapping (100 ms); broadband chirp excitation (2.6 ms); delay (3 ms); and detection (102 ms). The peptide beam was deflected during SIMS analysis by switching the potential applied to one pair of rods of the electrostatic bending quadrupole. The time delay between switching off the Cs+ pulse and gated trapping of sputtered ions, the recoil time, determines the range of m/z values observed in the spectrum. Spectra obtained using relatively short recoil time are dominated by lowmass peaks, while long recoil time favors the detection of highermass ions.16 In this work we used recoil time of 60 µs that provides adequate representation of ions with m/z > 200. Each SIMS spectrum was averaged over 10 shots corresponding to an acquisition time of 10 s. The kinetics data was obtained by sampling the SAM surface every 2.5 min during the SL, every 1.2 min for the first 40 min after the SL and every 10 min for the remainder of the experiment. Fluorinated self-assembled monolayer (FSAM) surfaces were used as targets for these SL experiments. The surfaces were prepared following literature procedures.21 Gold coated silicon wafer (5 nm chromium adhesion layer and 100 nm of polycrystalline vapor-deposited gold) was purchased from SPI Supplies (Westchester, PA) and custom laser cut into 4.8 mm diameter substrates by Delaware Diamond Knives (Wilmington, DE). In this study, CF3(CF2)7(CH2)2SH (Fluorous Technologies Inc., Pittsburgh, PA) was used to form the FSAM surface by exposure of the gold surface to a 1 mM ethanol solution of the thiol for at least 12 h. The surface was removed from the SAM solution, ultrasonically washed in ethanol for 5 min to remove extra layers of the reagent, and dried under nitrogen gas before being introduced into the instrument. Gramicidin S (GS) was purchased from Sigma-Aldrich (St. Louis, MO) and used as received. The sample was dissolved in a 50:50 (v/v) methanol: water solution with 1% acetic acid to a final concentration of 0.1 mg/mL. A syringe pump (Cole Parmer, Vernon Hills, IL) was used for direct infusion of the electrospray samples at a flow rate of 20 µL/hr.

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Hadjar et al.

Figure 3. Kinetic plots obtained for (a) the major surface peak (Au2SH+, m/z 426.9) and (b-i) several peptide-related peaks normalized to the surface peak (m/z 426.9). Ions following the kinetics of the [GS+2H]2+ are shown as solid circles, [GS+H]+, as solid triangles, neutral GS - as open circles, and mixed behavior, as open triangles. Solid lines in panels (b-d) correspond to the kinetic curve obtained for the [GS+2H]2+ ion scaled to the maximum signal; solid lines in panels (f-i) correspond to the kinetic curve obtained for the PVO (m/z 311) fragment scaled to the maximum signal.

Results and Discussion Time Dependence of the SIMS Signal. Gramicidin S (GS), the cyclic peptide with a sequence c(-LFPVO-) was used as a model system in this study. Figure 2 shows typical 8 keV Cs+ FT-ICR SIMS spectra obtained before and after deposition of 16 pA of doubly protonated gramicidin S [GS + 2H]2+ on the FSAM surface for 58 min. In addition to common FSAM-related peaks, the SIMS spectrum of the modified surface (Figure 2b) contains an abundant peak of the singly protonated GS, [GS+H]+ (m/z 1141.7) and other peptide-related peaks at lower m/z values. These include internal fragments LFPVO (m/z 571.4), LFPV (m/z 457.3), FPVO-NH3 (m/z 441.3), LFPV-28 (m/z 438.3), PVO (m/z 311.2), LF (m/z 261.2), LF-28 (m/z, 233.2), and PV-28 (m/z 169.1), immonium ions of proline (P, m/z 70.07) and ornithine (O, m/z 115.09), along with several minor ions. The insert in Figure 2b shows an expanded region of the SIMS spectrum corresponding to the m/z of the doubly protonated GS (m/z 571.4). The observation of a regular series of ions nominally separated by half a mass unit is almost diagnostic for doubly charged ions; however, the distribution of peaks is not consistent with the predicted isotopic distribution of [GS + 2H]2+. This is readily rationalized by considering the major peak to consist of a mixture of the singly charged fragment ion LFPVO from GS and the doubly protonated [GS+2H]2+ ion. The second peak in this region (m/z 571.9) is the 13C1 of [GS+2H]2+, while the third peak (m/z 572.4) is a mixture of the 13C2 isotope of the [GS+2H]2+ ion and the 13C1 isotope of the LFPVO fragment ion. Consistent with this interpretation, the peak at m/z 571.9 is observed only in the SIMS spectrum obtained following deposition of the doubly protonated GS and not when singly protonated [GS+H]+ is deposited onto the FSAM surface. In addition, distinctly different time signatures were obtained for the peak at m/z 571.4 and m/z 571.9. Specifically, the first and the third peaks in the isotopic envelope exhibit a much slower decay with time than

the second component peak at m/z 571.9, unambiguously confirming the composite character of the isotopic peaks in this m/z range. We followed the evolution of the SIMS spectrum as a function of time during 58 min of SL deposition of the doubly protonated GS and for ca. 10 h after the ion beam was switched off. Kinetics plots were constructed from the time-dependent SIMS data by plotting the normalized abundance of each peak in the spectrum as a function of time. Characteristic plots for one of the major surface peaks (Au2SH+) and several peptiderelated peaks are shown in Figure 3. Note that there is no time dependence in the abundance of the Au2SH+ substrate peak except for small random fluctuation in the signal corresponding to variations in the intensity of the Cs+ ion beam over the course of the experiment. In contrast, peptide-related peaks showed very pronounced time dependence. Specifically, the SIMS signal for these peaks increased as a function of time during the SL and decreased after the end of the deposition. Random fluctuations in our SIMS experiment over the course of 10 h observation time are compensated by normalizing the signal of peptide peaks to the abundance of the surface peak Au2SH+ at m/z 426.9 in the discussion that follows. From the kinetic plots shown in Figure 3 it is clear that various peptide-related peaks exhibit distinctive time signatures. The signal of the [GS+2H]2+ ion shows an almost linear increase during the SL with a small but measurable negative deviation from linearity. The initial increase is followed by a relatively fast depletion of the [GS+2H]2+ signal after soft landing is terminated. The solid line is shown to guide the eye. The same solid line scaled to the maximum of the kinetic curve is also shown in Figure 3, panels c and d. Despite the significant scatter associated with low signal-to-noise ratio, the signal of the LF fragment ion follows similar time dependence as the doubly protonated ion, whereas the signal of the immonium ion (O, m/z 115) decreases somewhat more slowly with time. Substantially slower signal depletion was observed for most

Protonated Peptides Soft Landed onto SAMs SCHEME 1

J. Phys. Chem. C, Vol. 111, No. 49, 2007 18223 TABLE 1: Kinetic Parameters Obtained from the Best Fit of the Experimental Data parameter desorption rate constants (kD2+, kD1+, kD0 in min-1) proton loss rate constants (kPL2+, kPL1+ in min-1) efficiencies of instantaneous proton loss (F1+ and F0) reionization efficiencies (FRI2+ and FRI1+)

peptide fragment ions. This behavior is illustrated for LFPVO, LFPV, PVO, and PV fragments in Figure 3f-i. The normalized abundance of these fragment ions increases almost linearly during soft landing and slowly decays after the peptide ion beam is shut off. Some 10 h later the signal of the [GS+2H]2+ ion is depleted almost to baseline, whereas the abundance of the PVO fragment (Figure 3h) has decreased to about 50% of its maximum value. In contrast, the signal of the singly protonated GS ion, [GS+H]+, (Figure 3e) shows a slow growth for ca. 3 h after the SL beam is shut off and a very slow decay at longer times. The kinetic plots shown in Figure 3 can be classified into three distinct categories: (1) features that follow fast decay of the [GS+2H]2+ ion; (2) the unique kinetic signature of the [GS+H]+ ion; and (3) fragment ions that exhibit the same time dependence as the PVO fragment ion. In addition, several peptide-related peaks exhibit intermediate behavior. In a related study (to be published) we determined that PVO is one of the most abundant SIMS fragments obtained following deposition of GS on a COOH-terminated SAM surface (COOH-SAM). Because ions are efficiently neutralized on the COOH-SAM surface and because the kinetic plot obtained for this fragment ion is distinctly different from the kinetic plots obtained for the [GS+2H]2+and [GS+H]+ ions we conclude that this ion and all other fragments that follow the same time dependence originate from SIMS bombardment of neutral GS molecules on the surface. We therefore suggest that SL of doubly protonated GS generates three populations on the surface corresponding to the doubly charged ions that retained their charge, singly charged ions formed by a loss of one proton by the soft-landed ions, and neutral peptide molecules formed by complete neutralization of ions. Kinetic Model. The evolution of the three categories of species on the surface is described by the simple kinetic model shown in Scheme 1. Here we assume that the soft-landed [M+2H]2+ ion population is depleted by proton loss (kPL2+) and by natural desorption of intact ions from the surface at room temperature (kD2+). Our previous study of the slow decay of the singly protonated peptides from the FSAM surface is consistent with electrostatic binding energy between soft-landed ions and the FSAM surface of the order of 20 kcal/mol.9 The [M+H]+ ions produced by partial proton exchange from the doubly protonated species to the SAM surface are expected to exhibit similar decay characterized by rate constants kPL1+ and kD1+, respectively. The fifth rate constant, kD0, represents the rate of desorption of the neutral peptide molecules from the surface. We also consider in our kinetics scheme instantaneous proton loss or complete neutralization of the precursor ions during their collision with the surface. In particular, for SL rate for the [M+2H]2+ ion, R, the rate of formation of [M+H]+ ions and neutral molecules, M, by rapid proton loss during the collision is given by F1+R and F0R, respectively. Finally, we consider that a small fraction, FRI2+, of [M+2H]2+ ions observed

[GS+2H]2+

[GS+H]+

GS

e 10-4

6 × 10-4

1 × 10-3

1 × 10-2

2 × 10-5

1

0.62

0.77

0

0.09

0.009

in SIMS spectra are produced by re-ionization of singly protonated species in the plume along with the generation of [M+H]+, by re-ionization of neutral molecules, FRI1+. This kinetic model is described by the following set of coupled firstorder differential equations:

d[M + 2H]2+ ) -(kPL2+ + kD2+)[M + 2H]2+ + R dt d[M + H]+ ) -(kPL1+ + kD1+)[M + H]+ + dt kPL2+[M + 2H]2 + F1+R d[M] ) -kD0[M] + kPL1+[M + H]+ + F0R dt

(1)

where R ) 1 during the SL and R ) 0 after deposition is finished. The Laplace transform method was used to solve these equations analytically. Because the efficiency of desorption of retained ions and desorption/ionization of neutral GS from the surface is not known, the model was scaled to match the observed normalized abundances. The efficiencies of reionization, FRI2+ and FRI1+, and three scaling factors S2, S1, and S0 were included in the modeling using eq 2:

[M+2H]final2+ ) S2([M+2H]2+ + FRI2+ [M+H]+) [M+H]final+ ) S1 ([M+H]+ + FRI1+ [M]) [M]final ) S0 [M]

(2)

where [M+2H]2+, [M+H]+, and [M] are solutions of eq 1. The resulting expressions for the amount of [GS+2H]2+, [GS+H]+ ions, and neutral GS were used to fit the experimental data. The characteristic PVO fragment ion at m/z 311 was used as a surrogate for formation and decay of neutral GS. The results were compared to the experimental kinetic plots shown in Figure 3, and the fitting parameters varied until the best fit was obtained. The fitting parameters included five rate constants (kPL2+, kPL1+, kD2+, kD1+, and kD0), parameters describing instantaneous proton loss, F1+ and F0, the efficiencies of reionization, FRI2+ and FRI1+, and the three scaling factors. The parameters that provide the best fit to our experimental data are reported in Table 1, and the modeling results are shown in Figure 4. Despite the large number of fitting parameters, modeling all three kinetic curves of the doubly protonated, singly protonated and neutral GS simultaneously proved a difficult task, suggesting that the shapes and the relative position of the experimental curves provide significant constraints for the kinetic model. In particular, it is impossible to model the kinetic curve for the neutral species (Figure 4c) without taking into account both partial and complete charge loss during the collision event, i.e., the model does not adequately describe the experimental data if F1+ ) F0 ) 0. This indicates that the system in not underdetermined. Figure 4 also shows the best fit to the experimental data obtained by blocking the instanta-

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Hadjar et al. repulsion on the second proton affinity of GS resulting in substantially faster loss of the second proton by the peptide. The Scheme 1 kinetics model fully rationalizes the dramatic differences in the shapes of the experimental kinetic plots shown in Figure 4. While the decay of the singly protonated species is very slow, relatively fast proton transfer from the [GS+2H]2+ ion continually repopulates [GS+H]+ ions until the doubly protonated ion is fully depleted. This results in gradual increase in the SIMS signal of the [GS+H]+ ion that continues for more than 2 h after the SL is finished. Decay of the [GS+H]+ ion by desorption into the gas phase is substantially faster than the rate of proton loss. As a result, neutral GS on the surface is mainly produced by fast neutralization of the precursor ions during the collision and not by slow proton loss by the [GS+H]+ ion. Consequently, the time signature obtained from the neutral population is strikingly different from the kinetics observed for both [GS+2H]2+ and [GS+H]+. Conclusion

Figure 4. Kinetic plots obtained for the (a) [GS+2H]2+ ion, (b) [GS+H]+ ion, and (c) neutral GS molecules on the surface represented by the PVO fragment ion (points) and the results of the kinetic modeling with (solid lines) and without (red dashed lines) taking into account the instantaneous charge loss by ions upon collision.

neous charge loss during ion-surface collision (i.e., F1+ ) F0 ) 0). Although this model reproduces the time dependence of the [GS+H]+ signal reasonably well, it does not adequately describe the kinetic plots of the doubly protonated and neutral species. If the neutral GS molecules were produced only by a slow charge loss from the [GS+H]+ ion, the observed time dependence would follow the slow gradual increase as a function of time characteristic of the singly protonated ion. Clearly this simplified model does not account for our experimental results and the additional proton loss pathways included in our kinetic scheme are required. Finally, because the SIMS desorption/ ionization yields of different ions and neutral species are not known, little physical meaning can be attributed to the absolute values of the efficiencies of the proton loss during the collision and the re-ionization efficiencies listed in Table 1. Nevertheless, we can conclude from the model that charge loss during the collision is the major contribution to the formation of neutral GS molecules on the surface and has only a relatively minor effect on the population of singly protonated ions. Furthermore, the contribution of the reionization to the observed SIMS signal is very minor. Several interesting deductions follow from this analysis. The depletion of the [GS+2H]2+ ion mainly results from the proton loss (kPL2+) and the desorption rate (kD2+) is much smaller than kPL2+. Because the rate constant of the total decay of the [GS+2H]2+ signal is given by the sum of kD2+ and kPL2+ and kD2+ is much smaller than kPL2+, only an upper limit of 10-4 min-1 for kD2+ is estimated from the model. The rate constant for proton loss from the [GS+H]+ ion, kPL1+, is much smaller than the corresponding desorption rate constant, kD1+ suggesting that the singly protonated species mainly decays by desorption with only a small fraction (ca. 3%) depopulated by proton exchange. We note that kPL2+ is about 3 orders of magnitude larger than kPL1+ reflecting the strong influence of Coulomb

We report here the first detailed study of the kinetics of the ion loss following soft-landing of mass-selected doubly protonated ions of Gramicidin S onto an FSAM surface. We found that various peptide-related peaks observed in SIMS spectra follow very different kinetics. Specifically, the doubly protonated peptide deposited on the surface shows a relatively rapid decay after ion deposition is finished. In contrast, the peak corresponding to the singly protonated peptide ion formed on the surface by partial proton loss from the soft-landed [GS+2H]2+ ion, continues to increase for 2-3 h following ion deposition. Finally, a majority of peptide fragments follow a linear increase during ion deposition and an almost linear decrease after the deposition. These fragment ions originate from neutral GS molecules formed by complete neutralization of the soft-landed ions. Our results were rationalized using a kinetic model that incorporates charge reduction by the loss of proton, thermal desorption of ions and neutral GS molecules from the surface, re-ionization of neutral molecules and the singly protonated species and fast proton loss by soft-landed ions during ionsurface collision. This kinetic model demonstrated that the decay of the [GS+2H]2+ signal is mainly attributed to proton exchange to form the [GS+H]+ ion, while the singly protonated species mainly decays by thermal desorption. Fast population of the singly protonated species by charge reduction and slow depopulation by desorption are responsible for the unique time signature observed for this ion. Most of the neutral GS molecules and a relatively smaller fraction of [GS+H]+ ions are produced by instantaneous charge loss during the collision. Acknowledgment. This work was supported by the grant from the Chemical Sciences Division, Office of Basic Energy Sciences of the U.S. Department of Energy (DOE), and the Laboratory Directed Research and Development Program at the Pacific Northwest National Laboratory (PNNL). The work was performed at the W. R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the U.S. DOE of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle for the U.S. DOE. References and Notes (1) (a) Franchetti, V.; Solka, B. H.; Baitinger, W. E.; Amy, J. W.; Cooks, R. G. Int. J. Mass Spectrom Ion Processes 1977, 23, 29. (b) Miller, S. A.; Luo, H.; Pachuta, S. J.; Cooks, R. G. Science 1997, 275, 1447. (c) Luo, H.; Miller, S. A.; Cooks, R. G.; Pachuta, S. J. Int. J. Mass Spectrom. 1998, 174, 193. (d) Denault, J. W.; Evans, C.; Koch, K. J.; Cooks, R. G.

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