C60 Secondary Ion Fourier Transform Ion Cyclotron Resonance Mass

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C60 Secondary Ion Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Donald F. Smith,† Errol W. Robinson,† Aleksey V. Tolmachev,‡ Ron M. A. Heeren,§ and Ljiljana Pasa-Tolic*,† †

Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, United States Fundamental and Computational Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352, United States § FOM Institute for Atomic and Molecular Physics (AMOLF), Science Park 104, 1098 XG Amsterdam, The Netherlands ‡

bS Supporting Information ABSTRACT: Secondary ion mass spectrometry (SIMS) has seen increased application for high spatial resolution chemical imaging of complex biological surfaces. The advent and commercial availability of cluster and polyatomic primary ion sources (e.g., Au and Bi cluster and buckminsterfullerene (C60)) provide improved secondary ion yield and decreased fragmentation of surface species, thus improving accessibility of intact molecular ions for SIMS analysis. However, full exploitation of the advantages of these new primary ion sources has been limited, due to the use of low mass resolution mass spectrometers without tandem MS to enable enhanced structural identification capabilities. Similarly, high mass resolution and high mass measurement accuracy would greatly improve the chemical specificity of SIMS. Here we combine, for the first time, the advantages of a C60 primary ion source with the ultrahigh mass resolving power and high mass measurement accuracy of Fourier transform ion cyclotron resonance mass spectrometry. Mass resolving power in excess of 100 000 (m/Δm50%) is demonstrated, with a root-meansquare mass measurement accuracy below 1 part-per-million. Imaging of mouse brain tissue at 40 μm pixel size is shown. Tandem mass spectrometry of ions from biological tissue is demonstrated and molecular formulas were assigned for fragment ion identification.

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ass spectrometry (MS) imaging allows for the spatial mapping of biomolecules directly from complex surfaces.1 Matrix-assisted laser desorption ionization (MALDI) MS imaging has seen much advancement for the localization of drugs/ metabolites, lipids, peptides, and proteins on biological surfaces. However, the spatial resolution of MALDI is limited by the focus of the laser beam and typically reaches a minimum of ∼20 μm. However, secondary ion mass spectrometry (SIMS)2 has long been used for high spatial resolution (micrometer and submicrometer) chemical imaging of surfaces.3,4 SIMS uses a highly focused beam of primary ions to impact the surface, which causes sputtering and secondary ionization of surface molecules. As compared to the laser focus for MALDI, micrometer or submicrometer primary ion beam spot sizes are routinely achieved. Although traditionally used for nonbiological applications, growth in the area of soft-matter analysis by SIMS has been on the rise. The commercial availability of cluster ion and polyatomic primary-ion sources (e.g., gold cluster,5 bismuth cluster,6 and C607) has greatly extended the capabilities of SIMS for analysis of biological samples.8,9 These primary-ion sources alleviate some of the drawbacks of SIMS for biological applications, as they reduce the fragmentation of surface molecules, but still must be operated under the SIMS static limit. As a result, r 2011 American Chemical Society

molecular ions (rather than fragments) of biomolecules can now be spatially mapped on a submicrometer scale. Despite the success of TOF-SIMS,10 identification of secondary ions has been limited. This is due, in part, to the inherent lack of tandem MS (MS/MS) capabilities of TOF-SIMS instrumentation and low mass resolving power and mass accuracy. Tandem MS of secondary ions has been shown in a triple quadrupole,11 13 an ion-trap14 and by TOF-SIMS,15 however, without the added benefit of high mass resolving power or mass measurement accuracy. Recently, the Winograd group successfully coupled a C60 primary-ion source with a commercial quadrupole orthogonal TOF MS.16,17 This system provides mass resolving power (m/ Δm50%) of ∼14 000, mass measurement accuracy (MMA) in the low parts-per-million (ppm) range, and MS/MS capabilities with an imaging spatial resolution (for biological samples) down to ∼30 μm.18,19 Similarly, Vickerman et al. have developed a buncher TOF instrument fitted with a C60 primary ion gun that allows SIMS imaging at 1 μm pixel size with mass resolving powers of ∼6 000 (at m/z 500)20 and MS/MS capabilities in a collision cell.21,22 Received: September 2, 2011 Accepted: November 7, 2011 Published: November 07, 2011 9552

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Analytical Chemistry The high MMA and high mass resolving power of Fouriertransform ion cyclotron resonance mass spectrometry (FTICR MS)23 make it attractive for identification of SIMS generated ions. The high mass resolving power of FTICR insures that isobaric ions, unresolved by lower performance mass spectrometers, can be resolved for ultimate specificity of ion selected images. Further, the high MMA of FTICR allows chemical formula assignment to low mass ions (100 000 are shown, with root-mean-square (rms) MMA below 1 ppm. We illustrate the MS/MS capabilities of the platform on a standard peptide as well as ions directly from a biological tissue section. SIMS FTICR MS imaging with high mass accuracy and high mass resolving power is presented at a spatial resolution of 40 μm. The results represent the first steps toward an imaging system for high specificity chemical imaging at high spatial resolution for the analysis of complex surfaces.

’ EXPERIMENTAL SECTION Instrumentation. Figure S-1 in the Supporting Information shows a schematic of the C60 SIMS source that has been coupled to a 12 T solariX FTICR mass spectrometer (Bruker Daltonics, Billerica, MA). A 40 keV C60 primary ion gun (Ionoptika, Chandlers Ford, Hampshire, U.K.)27 was fitted to a solariX source chamber via a home-built housing at an incidence angle of 45°. The end nose cone of the primary ion gun was 20 mm from the sample surface. A compound turbo-molecular drag pump (510 L/s; Pfeiffer Vacuum GmbH, Asslar, Germany) maintains a pressure of 2  10 7 mbar in the C60 source region. All experiments were performed with C60+ primary ions at 40 keV. Secondary ions are injected into a radio frequency (rf) only octopole and transferred through a stage of differential pumping (255 L/s; Edwards, Tewksbury, MA) via an rf only quadrupole. The commercial solariX source typically operates at intermediate pressure (∼3 mbar) and employs electrodynamic ion funnels for ion collection and focusing. These ion funnels were replaced with a long rf only quadrupole that traverses the original solariX source chamber and extends to the first rf multipole of the solariX system. The solariX pumping scheme was modified to allow this chamber to be backed by the first turbo-molecular drag pump of the Bruker system for a pressure of 3.0  10 5 mbar. The rest of the solariX system was unmodified, except for the optimization of external trapping in the hexapole collision cell. The X Y Z translation stage and CCD camera from the solariX FTICR MS system were used to allow the use of the commercial software for instrument control and MS imaging experiments. The FTICR pulse program was modified to send a TTL trigger to the C60 primary ion gun to unblank the C60 beam during primary ion-surface interaction, synchronously with the

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ion accumulation events in the experimental sequence. Secondary ion populations were accumulated in the external hexapole and subsequently sent to the ICR cell for excitation and detection. Similarly for tandem-MS experiments, collision-induced dissociation (CID) was performed in the hexapole collision cell after which product ions were sent to the ICR cell. For gramicidin S, the molecular ion ([M + H]+, 1041.7 m/z) was quadrupole isolated (20 m/z window) and fragmented with 15 V collision energy. For cholesterol, the [M OH + H]+ fragment (369.3 m/z) was isolated (4 m/z window) and fragmented with 5 V collision energy. Sample Preparation. Polyethylene glycol 1000 (PEG; SigmaAldrich, St. Louis, MO) was dissolved in methanol at a concentration of 5 μM, and 1.5 pmol were deposited on a Bruker AnchorChip MALDI target. Gramicidin S (Sigma-Aldrich, St. Louis, MO) was prepared at a concentration of ∼1.99 mM in 1:1 methanol/water. This solution was desalted using two 100 μL C18 OMIX tips (Agilent Technologies, Santa Clara, CA) using the manufacturer suggested protocol. The desalted solution was diluted to ∼20 μM in 1:1 methanol/water, and ∼5 pmol were deposited on a silicon wafer. Mouse brain tissue (mouse female type 9 CFW-1; Harlan Laboratories, Boxmeer, The Netherlands) was coronally sectioned at 12 μm on a cryomicrotome (Microm International, Walldorf, Germany), deposited on indium tin-oxide coated glass slides (ITO; Delta Technologies, Stillwater, MN), and stored at 80 °C before use (no further sample preparation). An adjacent 12 μm brain section was stained with hematoxylin and eosin (H&E; BBC Biochemical, Mount Vernon, WA) for comparison with ion selected images. Data Analysis. Spectra were analyzed using Bruker DataAnalysis version 4.0. Transients were zero-filled once and apodized using a Sine-Bell multiplication function. The “FTMS” peak picking method was used for peaks above a signal-to-noise ratio (S/N) of 5:1. MS imaging data was converted to “Datacube” format with a mass bin size of both 0.001 and 0.002 Da using AMOLF developed software.28 Transients were zero-filled once and apodized with an exponential function. Peaks were picked above a threshold corresponding to a S/N of 2 at m/z 400 using an apex algorithm. Peak lists were read into MATLAB (R2010a, MathWorks, Natick, MA) for construction of ppm error histograms. Cyclic peptide CID fragments were annotated as described by Ngoka and Gross using tools provided online by the University of California San Diego (http://rofl.ucsd.edu/nrp/ index.html).29,30

’ RESULTS AND DISCUSSION The aim of the new instrument platform was to combine the high mass spectral performance of FTICR MS with the high spatial resolution capabilities of SIMS for high spatial resolution chemical imaging and identification of biomolecules. The system is operated with a direct-current (dc) primary ion beam for fast generation of secondary ion populations, on the order of 1 20 s.16,21 In this way, ion populations sufficient for FTICR detection can be accumulated in the external collision cell in a relatively short amount of time. All experiments described herein used a primary ion beam spot size of ∼35 μm with 45 pA current measured on the target in the dc mode. Thus, the primary ion beam dose (or fluence) is determined by the accumulation time for each experiment. The primary ion spot size is currently limited by instrumental sensitivity, and current work is focused 9553

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Figure 1. Mass spectrum of 1.5 pmol of PEG 1000. Inset shows the measured mass and the mass measurement accuracy (in ppm) of the sodiated oligomers (indicated by an asterisk) after internal calibration. Average m/Δm50% = 84 000.

Figure 3. C60 SIMS FTICR MS imaging of cholesterol in mouse brain. (a) left, H&E stain of adjacent mouse brain section; right, ion selected image of cholesterol ([M OH + H]+, 369.3515 ( 0.0005 m/z, in green) overlaid with H&E stained adjacent section. Scale bar = 120 μm. (b) Mass measurement error histogram of all observations of the ion shown in part a with a signal-to-noise greater than 4 (Npeaks = 441). The distribution is centered close to zero, with a rms error of 1.82 ppm.

Figure 2. CID MS/MS spectrum of gramicidin S ([M + H]+; 1141.7 m/z). Good fragmentation coverage is obtained and fragment ion mass measurement accuracies are below 1.5 ppm. Average m/Δm50% = 111 000. *Half of the cyclodecapeptide; other possible annotations: b5LO, b5FL, b5PF, b5VP, b5OV, b5FL, b5PF, b5VP, or b5OV. It is indicated here as b5LO due to the identification of the other fragment ions, where the cyclic peptide is broken at LO to form a decomposing linear ion.

on optimization of the ion transfer optics for ultimate transfer efficiency of secondary ions. An organic polymer (PEG 1000) was analyzed to access performance characteristics of the imaging platform. Figure 1 shows C60 SIMS FTICR MS of PEG 1000 with an average m/Δm50% = 84 000. The peaks with an asterisk were used for internal calibration and the inset table includes the measured mass and MMA (in ppm) of these sodiated PEG oligomers, where the rms mass accuracy is 0.65 ppm. Here, the stage was rastered randomly across the sample area (∼1 mm diameter sample well) for 10 s (primary ion dose = 2.92  1013 ions/cm2) in order to ensure sufficient sampling of the sample surface for good ion signal. Higher S/N was observed with larger primary ion beam spot sizes and an increased primary ion beam current on the target (data not shown). In addition to high mass resolving power and high mass accuracy, FTICR MS offers a number of tandem MS methods31

for fragmentation of secondary ions, such as collision induced dissociation (CID, both external and in-cell methods),32 35 infrared multiple-photon dissociation (IRMPD),36 and electron ionization dissociation (EID).37 Other electron-based dissociation methods are amenable to FTICR but not favorable with regards to the predominantly singly charged ions generated by SIMS. Combined with the polyatomic C60 primary ion beam, FTICR MS and MS/MS present an unparalleled capability for highly confident identification of secondary ions from biomolecules such as metabolites, lipids, and peptides. Figure 2 shows a hexapole CID MS/MS spectrum of gramicidin S, a cyclic-peptide from Bacillus brevis. The sodiated pseudomolecular ion at 1163.6931 m/z had the highest abundance in the precursor scan and fragmentation was minimal (see Figure S-2 in the Supporting Information). When externally calibrated (with respect to the spectrum shown in Figure 2), the rms MMA of the ions in Figure 2 is 1.6 ppm. Upon internal calibration, the rms MMA is 0.69 ppm (see Table S-1 in the Supporting Information) and thus allows confident identification of the amino acid sequence of the cyclodecapeptide gramicidin S. An area of a mouse brain section was analyzed to test the imaging capabilities of the new platform. Figure 3a shows an ion selected image of cholesterol ([M OH + H]+; green) overlaid with a H&E stained adjacent brain section. The accumulation time per pixel was 15 s, with a primary ion dose of 4.38  1014 ions/cm2 and a stage step size of 40 μm. The long accumulation time (and thus high ion primary ion dose) was necessary for quality mass spectra with good S/N. The section analyzed is from the rear of the mouse brain (Bregma index ≈ 6 mm) and contains 9554

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measured for Figures 3 and 4, with an accumulation time of 20 s (primary ion dose = 5.84  1014 ions/cm2) while rastering the stage over the cholesterol-rich area (as shown in Figure 3a). The high mass accuracy of the FTICR MS allows unequivocal molecular formula assignment to the fragment ions shown in Figure 5. As Fletcher and Vickerman suggest,38 the difficulty of SIMS MS/MS analysis is compounded by low secondary ion yield and the need for sufficiently high product ion intensity for good tandem MS spectra (thus the longer accumulation time needed here). While efforts continue to improve sensitivity, the current SIMS platform with high mass resolution, high mass measurement accuracy, and tandem MS capabilities has proven to be a powerful tool for SIMS analysis. Figure 4. Zoom of the average mass spectrum from a C60 SIMS FTICR MS imaging run of mouse brain. This 0.4 m/z mass window shows no less than nine distinct peaks and illustrates the complexity of the mass spectra. Ion selected images are shown with a m/z bin size of 0.002. Scale bar = 120 μm.

Figure 5. CID MS/MS of cholesterol measured directly from a mouse brain section (section adjacent to that measured for Figure 4). High mass measurement accuracy allows assignment of elemental compositions to all observed fragment ions. Average m/Δm50% = 385 000.

a large majority of the cerebellum (as seen in the histological features on the right and left side of the brain). Cholesterol was observed with high abundance in the inferior cerebellar peduncle and along the lobules. The MMA of all observations of cholesterol had a rms error of 1.82 ppm, see Figure 3b. The ppm error histogram was constructed from all cholesterol peaks observed in Figure 4a with S/N > 4 (Npeaks = 441). No peaks for this cholesterol species were observed with a mass error greater than (3 ppm. This high mass accuracy and precision allows very small bin widths for ion selected image generation, as shown in Figure 4a where a bin width of 1 mDa was used. Combined with the high mass resolving power of FTICR, this ensures ultimate specificity for generation of ion selected images. Figure 4 shows a mass zoom of the average mass spectrum at 276 277 m/z and ion selected images of four ions in a 0.4 Da window. Figure 4 clearly demonstrates the complexity of the mass spectra revealed by the high mass resolving power of FTICR MS. Over 600 unique ions were observed in this experiment, and we are currently working to identify tissue specific secondary ions. Figure 5 shows a CID MS/MS spectrum of cholesterol directly from a mouse brain tissue section. The spectrum was collected from an immediately adjacent section to the one

’ CONCLUSIONS SIMS shows much potential for high spatial resolution chemical imaging of biomolecules such as metabolites, lipids, and peptides. For the first time, we show the realization of a platform for high mass resolution, high mass measurement accuracy, along with tandem MS capabilities for SIMS. However, challenges still remain for an instrument with these performance characteristics, along with the sensitivity to allow the submicrometer chemical imaging achievable with modern SIMS primary ion sources. External accumulation of secondary ions (i.e., decoupling of sputtering/ionization and mass spectrometry) allows the primary ion source to be operated in direct-current mode for optimum FTICR MS performance and acquisition speed. The high mass resolution yields hundreds of unique peaks from SIMS FTICR MS imaging experiments and results in the need for advanced data handling and analysis tools. The spatial resolution achievable with this platform is currently limited by sensitivity. Optimized ion transfer optics will result in improved spatial resolution and reduce the acquisition time per pixel. The improved ion transfer efficiency will also increase the sensitivity of this approach for lower abundance species. The high performance of this platform offers data that is highly complementary to TOF-SIMS and is already in use for “profiling” of small molecules for comparison with other imaging and MS imaging techniques. ’ ASSOCIATED CONTENT

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Supporting Information. Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: 509-371-6555.

’ ACKNOWLEDGMENT Portions of this research were supported by the American Reinvestment and Recovery Act of 2009 and the U.S. Department of Energy (DOE) Office of Biological and Environmental Research. The research described in this article was performed at the W. R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated by Battelle for the U.S. Department 9555

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Analytical Chemistry of Energy under Contract DE-AC05-76RLO 1830. In addition, work done at AMOLF is part of the research program of the “Stichting voor Fundamenteel Onderzoek der Materie (FOM)”, which is financially supported by the “Nederlands organisatie voor Wetenschappelijk Onderzoek (NWO)”. R.M.A.H. would like to acknowledge the support of the EMSL Wiley Visiting Scientist Fellowship program for portions of this work. We thank Dan Meili and Christian Berg from Bruker Daltonics for assistance with the solariX FTICR and R. James Ewing, the EMSL Instrument Development Laboratory, and the EMSL machine shop for technical support.

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