Organic Surface Analysis by Two-Laser Ion Trap Mass Spectrometry

The number of laser shots is 1−5 for MS experiments and 1 for MS2 .... eV SF5+ is shown in Figure 6 on the upper left- and right-hand sides (“afte...
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Anal. Chem. 1997, 69, 1536-1542

Organic Surface Analysis by Two-Laser Ion Trap Mass Spectrometry Oleg Kornienko, Earl T. Ada, and Luke Hanley*

Department of Chemistry, M/C 111, University of Illinois at Chicago, Chicago, Illinois 60607-7061

This paper describes two-laser ion trap mass spectrometry (L2ITMS) for the analysis of organic, polymeric, and biological surfaces. L2ITMS uses one laser to desorb intact molecules from a surface, a second laser for photoionization, and an ion trap for single or tandem mass spectrometric analysis. We demonstrate the capabilities of this instrument by performing laser desorption/photoionization/tandem mass spectrometric analyses of amino acid and mixed polymer thin films. We then use L2ITMS in conjunction with X-ray photoelectron spectroscopy to chemically analyze the surface of an ion-bombarded polystyrene thin film. Low-energy SF5+ ion bombardment causes fluorination of intact polystyrene, partial destruction of the aromaticity of the polystyrene, fluorination of the resultant nonaromatic organic material, and sputtering of the polystyrene. The first of these dominates at 50 eV ion energy, while the latter three dominate at 250 eV. Chromatographic separation allows mass spectrometric methods to be used to analyze complex mixtures of organic, polymeric, and biological compounds. However, chromatography is usually not feasible for the separation of small amounts of organic mixtures on surfaces. Secondary ion, secondary neutral, and laser desorption/photoionization mass spectrometries are increasingly being applied to organic surface analysis.1-3 Tandem mass spectrometry (MS2) can be used for mixture analysis, but MS2 is difficult with the time-of-flight mass analyzers commonly used for surface analysis. This paper describes two-laser ion trap mass spectrometry (L2ITMS) for the analysis of organic surfaces and mixtures. L2ITMS uses one laser to desorb intact molecules from a surface, a second laser for their photoionization, and an ion trap for single or tandem MS analysis. We demonstrate the capabilities of this instrument by performing laser desorption/photoionization/MS2 analyses of amino acid and mixed polymer thin films. We then use L2ITMS in conjunction with X-ray photoelectron spectroscopy (XPS) to chemically analyze a polystyrene surface which has been modified by low-energy SF5+ ion bombardment. Many methods of organic surface analysis provide either insufficient or ambiguous data.4-6 XPS is, perhaps, the foremost method of organic surface analysis: it can identify and quantify

elements and functional groups on a surface. Unfortunately, XPS has limited chemical resolution and usually cannot distinguish the sequence of functional groups in a molecule. Surface vibrational techniques such as high-resolution electron energy loss, resonance Raman, and infrared reflection absorption spectroscopies can identify functional groups and determine their orientation on the surface. However, limitations imposed by molecular complexity, optical selection rules, film thickness, and electronic structure often make surface vibrational spectra difficult to interpret. Scanning tunneling and atomic force microscopy provide atomically resolved maps of surface morphology but cannot yet provide unambiguous surface chemical information. The limitations of photoemission, vibrational, and tunneling spectroscopies create an important role for mass spectrometry in the analysis of organic surfaces. The popular method known as time-of-flight secondary ion mass spectrometry (TOF-SIMS) generates spectral peaks which can often be identified with specific organic surface species.2 Primary ion beams can be readily focused and rastered, allowing TOF-SIMS spatial imaging of surface chemical compositions. However, TOF-SIMS ion yields are often low and/or matrix dependent. Ion fragmentation during the desorption/ionization process can also lead to complex mass spectra. These effects can combine to severely limit the utility of TOF-SIMS when analyzing an organic mixture on a surface. Photoionization of desorbed neutrals can overcome the problems of low and/or matrix-dependent ion yields which exist with TOF-SIMS.1 Chemical selectively in the analysis of mixtures can be partially achieved by the choice of photoionization method. Two-laser mass spectrometry (L2MS) is also used for the detection of microscopic amounts of organic materials on surfaces: one laser desorbs molecules from the surface, and a second laser selectively photoionizes these molecules.3,7-11 The best known application of this method is the recent analysis of Mars meteorites for chemical signs of life.12 Compared with TOF-SIMS, L2MS offers greater control over the ionization process by providing more abundant ion yields, especially for large molecular ions. Fragmentation of desorbed neutrals by kiloelectronvolt primary ion beams is known to occur in TOF-SIMS and secondary neutral MS.2,7 L2MS avoids the possibility of ion-induced fragmentation

* Author to whom correspondence should be addressed. E-mail: LHanley@ uic.edu. (1) Winograd, N. Anal. Chem. 1993, 65, 622A-629A. (2) Benninghoven, A.; Hagenhoff, B.; Niehuis, E. Anal. Chem. 1993, 65, 630A640A. (3) Zenobi, R. Int. J. Mass Spectrom. Ion Processes 1995, 145, 51-77 and references therein. (4) Woodruff, D. P.; Delchar, T. A. Modern Techniques of Surface Science, 2nd ed.; Cambridge University Press: Cambridge, 1994. (5) Gardella, J. A., Jr.; Pireaux, J.-J. Anal. Chem. 1990, 62, 645A-661A and references therein.

(6) Perry, S. S.; Somorjai, G. A. Anal. Chem. 1994, 66, 403A-415A. (7) Ayre, C. R.; Moro, L.; Becker, C. H. Anal. Chem. 1994, 66, 1610-1619. (8) Aicher, K. P.; Wilhelm, U.; Grotemeyer, J. J. Am. Soc. Mass Spectrom. 1995, 6, 1059-1068. (9) Lee, I.; Calcott, T. A.; Arakawa, E. T. Anal. Chem. 1992, 64, 476-478. (10) Behm, J. M.; Hemminger, J. C.; Lykke, K. R. Anal. Chem. 1996, 68, 713719. (11) Zhan, Q.; Zenobi, R.; Wright, S. J.; Langridge-Smith, P. R. R. Macromolecules 1996, 29, 7865-7871. (12) McKay, D. S.; Gibson, E. K., Jr.; Thomas-Keprta, K. L.; Vali, H.; Romanek, C. S.; Clemett, S. J.; Chillier, X. D. F.; Maechling, C. R.; Zare, R. N. Science 1996, 273, 924-930.

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but itself can introduce thermal or photolytic fragmentation during the laser desorption process. The relative merits of ion versus laser desorption of neutrals is a point of ongoing debate which cannot be resolved here. Introduction of optical microscopic projection of the desorption laser in L2MS allows a simple and low-cost implementation of molecular spatial imaging capabilities with spatial resolution similar to that possible with TOF-SIMS and ion desorption/photoionization.10,13 An ion trap mass spectrometer is no more sensitive than the time-of-flight analyzers most commonly used in L2MS and other surface MS methods. The advantage of the ion trap is its tandem mass spectrometry (MS2) capability, which can serve the same purpose as chromatographic sample introduction by isolating peaks for subsequent fragmentation analysis.14 A Fourier transform ion cyclotron resonance mass spectrometer could also be used to perform MS2 for surface analysis. However, its large magnet impedes the introduction of two lasers into the sample area and can interfere with the electron optical components of multiprobe surface analysis systems. Furthermore, the relative simplicity and low cost of the ion trap are particularly attractive for surface analysis. Ion trap mass spectrometers have been utilized in a variety of configurations which allow ion or laser desorption for the analysis of solids.15-19 Several ion trap instruments have also been designed for the introduction of ions formed by matrix-assisted laser desorption.20,21 We have chosen L-tryptophan, L-tyrosine, polystyrene, and p-fluoropolystyrene to test out the surface analysis capabilities of our instrument because they serve as simple models of biological and synthetic polymeric surfaces. The amino acid L-tryptophan is often used as a test case for desorption/photoionization methods and is known to be efficiently ionized by a resonant two-photon process at 266 nm.7,8 The amino acid L-tyrosine is expected to be similarly ionized and fragmented. The polymer polystyrene and p-fluoropolystyrene monomers both have a benzene ring in their structures, which makes them suitable for 266 nm resonant two-photon ionization. Polystyrene films have been examined previously by TOF-SIMS13,22 and laser desorption/photoelectron ionization mass spectrometry.14 A mixture of these two polymers is used as a model of organic mixtures on surfaces and to calibrate the L2ITMS experiment for the subsequent ion-induced fluorination of polystyrene. Our research interests in ion-surface interactions have led to the study of the low-energy ion modification of organic films.23-25 Low-energy atomic and polyatomic ion-surface collisions are relevant to materials science since they can be used directly or (13) Benninghoven, A. Angew. Chem., Int. Ed. Engl. 1994, 33, 1023-1043. (14) Schriemer, D. C.; Li, L. Anal. Chem. 1996, 68, 250-256. (15) Heller, D. N.; Lys, I.; Cotter, R. J.; Uy, O. M. Anal. Chem. 1989, 61, 10831086. (16) McIntosh, A.; Donovan, T.; Brodbelt, J. Anal. Chem. 1992, 64, 2079-2083. (17) Chambers, D. M.; Goeringer, D. E.; McLuckey, S. A.; Glish, G. L. Anal. Chem. 1993, 65, 14-20. (18) Jonscher, K.; Currie, G.; McCormack, A. L.; Yates, J. R., III. Rapid. Commun. Mass Spectrom. 1993, 7, 20-26. (19) Alexander, M. L.; Hemberger, P. H.; Cisper, M. E.; Nogar, N. S. Anal. Chem. 1993, 65, 1609-1614. (20) Qin, J.; Chait, B. T. Anal. Chem. 1996, 68, 2108-2112. (21) Doroshenko, V. M.; Cotter, R. J. Anal. Chem. 1996, 68, 463-472. (22) Muddiman, D. C.; Brochman, A. H.; Proctor, A.; Houalla, M.; Hercules, D. M. J. Phys. Chem. 1994, 98, 11570-11575. (23) Ada, E. T.; Hanley, L.; Etchin, S.; Melngailis, J.; Dressick, W. J.; Chen, M.J.; Calvert, J. M. J. Vac. Sci. Technol. B 1995, 13, 2189-2196. (24) Burroughs, J. A.; Hanley, L. Anal. Chem. 1994, 66, 3644-3650. (25) Wainhaus, S. B.; Burroughs J. A.; Wu, Q.; Hanley, L. Anal. Chem. 1994, 66, 1038-1043.

Figure 1. Schematic diagram of the experimental apparatus used for two-laser ion trap mass spectrometry.

in plasmas to chemically modify, deposit, or etch surfaces. The technological applications of polyatomic ion-surface collisions include semiconductor processing, polymer film modification, solar collectors, wear resistant coatings, and fusion containment.26 Several groups have demonstrated that polymer films and alkanethiolate self-assembled monolayers can be modified by bombardment with low-energy polyatomic ions.24,25,27,28 These modified organic films present a unique challenge to surface analysis because the ion beam appears to create a distribution of different chemical species on the surface. XPS can be quite effective for analyzing ion-modified organic films, but even XPS cannot fully describe the nature of the chemical modification.23 Low-energy SIMS experiments indicated that several chemical species are formed on the surface by ion bombardment, but SIMS did not provide unequivocal identification or quantification of these species.24 We demonstrate here that L2ITMS can be utilized to observe ion-induced chemical changes in polymer films. We modify polystyrene thin films on Si(100) by mass-selected ion beam bombardment with 50 and 250 eV SF5+, and then we analyze these films by both XPS and L2ITMS. EXPERIMENTAL DETAILS The schematic diagram of the experimental apparatus is shown in Figure 1. We previously described a prototype in which an ion trap detector (Finnigan MAT 700) had been modified by the addition of two data acquisition boards and remounted in a new (26) See, for example: Low Energy Ion-Surface Interactions; Rabalais, J. W., Ed.; Wiley: Chichester, 1994, and references therein. (27) Hu, H.-K.; Schultz, J. A.; Rabalais, J. W. J. Phys. Chem. 1982, 86, 33643367. (28) Pradeep, T.; Feng, B.; Ast, T.; Patrick, J. S.; Cooks, R. G.; Pachuta, S. J. J. Am. Soc. Mass Spectrom. 1995, 6, 187.

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vacuum chamber.29,30 The ion trap electrodes are in the standard stretched geometry for instruments produced in the mid-1980s. The current experimental configuration has been expanded considerably since this previous description and is described in detail here. The new apparatus has separate lasers for desorption and postionization, the capability of MS2 analysis, and a 170 L/s turbomolecular pump. Solid samples are introduced into the chamber on the end of a solids probe which is mechanically rotated so that desorption occurs from a fresh spot with every laser shot. Sample desorption is achieved with the 1064 or 355 nm pulses of a Nd:YAG laser (Lumonics HY400, 9 ns pulse length). The distance between the probe’s surface and the first end cap is 29 ( 2 mm. The end cap is a standard seven-hole detection electrode (Finnigan) which has had six additional 1 mm diameter holes drilled into it to enhance neutrals transmission. By desorbing neutrals outside of the trap and requiring them to traverse ∼34 mm prior to ionization, we have considerably reduced the sensititivity of the instrument relative to desorbing from a sample inserted through the ion trap electrodes. However, our geometry allows us to analyze large area samples and is thus better suited to surface analysis applications. Laser ionization is achieved with 266 nm pulses (Continuum Surelite, 7 ns pulse length). The 266 nm photoionization laser is focused into the center of the trap through a 1 mm conical hole drilled in the ring electrode. An arbitrary waveform generator (LeCroy LW 410) is used to apply auxiliary frequency pulses to the end cap electrodes of the ion trap. The stored waveform inverse Fourier transform (SWIFT) method is used here to design the auxiliary pulses.31,32 The basic control software is written in-house in the C language using the LabWindows programming environment (National Instruments). The ion trap experimental conditions are optimized to obtain maximum ion current at minimum peak broadening. The ion trap is operated in the rf-only mode, with the occasional addition of SWIFT pulses to eject unwanted low-mass ions or to select and collisionally excite ions of interest. The inset of Figure 2 displays the timing sequence used for the ion trap radio frequency (rf) amplitude, the desorption laser, the photoionization laser and the SWIFT pulses. A 1.1 MHz rf voltage of 189 V0-p, corresponding to a low-mass cut off of m/z 15, is first applied to the ring electrode of the ion trap. Next, the desorption laser pulse is fired, followed 40-60 µs later by the ionization laser pulse. The SWIFT pulse trains are then applied to the end caps of the ion trap. The SWIFT pulses are applied in a dipolar fashion with a 180° phase difference using a home-built amplifier and a phase inverter. This amplifier employs two OPA-603 operational amplifiers and has the advantage over a transformer-based device of an extremely wide dynamic range. Previous transformerless designs have been described elsewhere.21 No attenuation of the stored pulse is seen in the 5-3000 kHz range when using this amplifier. The SWIFT pulses are designed by a program written in-house using Mathcad (Mathsoft) and then downloaded to the arbitrary waveform generator. Mathcad provides for graphical display of the SWIFT pulses, allowing further refinement during the pulse design step. Typical SWIFT pulse lengths are 32 kB, with sampling rates that range from 1.5 to 2.0 MHz. The notch width in the SWIFT pulses (29) Burroughs, J. A.; Hanley, L. J. Am. Soc. Mass Spectrom. 1993, 4, 968-970. (30) Burroughs, J. A. Ph.D. Thesis, University of Illinois at Chicago, 1995. (31) Chen, L.; Wang, T.-C. L.; Ricca, T. L.; Marshall, A. G. Anal. Chem. 1987, 59, 449-454. (32) Julian, R. K.; Cooks, R. G. Anal. Chem. 1993, 65, 1827-1833.

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Figure 2. Mass spectrum of laser-ablated Mo+ (top) and SWIFTisolated 96Mo+ (bottom). Inset: Timing sequence used for the ion trap radio frequency amplitude, the desorption laser, the photoionization laser, and the SWIFT pulses.

varies, depending on the mass and mass window to be selected and isolated. The number of SWIFT pulses in a typical pulse train is varied from 1 to 5, depending on the abundance of low-mass ions and the mass distribution of the ion packet to be isolated. Finally, the ions are ejected from the trap by ramping the amplitude of the ring electrode rf and detected by the channeltron/dynode detector. The He buffer gas pressure inside the ion trap is adjusted to ∼5 × 10-4 Torr. The dynode voltage is 6.06.5 kV, and the channeltron voltage is 1.2-1.6 kV. The desorption laser intensity is dependent on the wavelength used. Typical values are 8-12 mJ/pulse for 1064 nm and 4-6 mJ/pulse for 355 nm, corresponding to surface power densities of 108-109 W/cm2. The 266 nm photoionization laser intensity is varied from 0.5 to 2 mJ/pulse. These power densities represents typical threshold values for laser desorption and ionization of organic ions in our apparatus. The number of laser shots is 1-5 for MS experiments and 1 for MS2 experiments. The two spectra in Figure 2 demonstrate the effect of SWIFT pulses on the ion trap mass spectra. The top of Figure 2 depicts the mass spectrum of the several naturally occuring Mo+ isotopes formed by 1064 nm laser ablation of a contaminated Mo foil. The bottom spectrum in Figure 2 shows the isolation of the 96Mo+ isotope with no loss in signal. Due to improved SWIFT design code and improvements in the amplifier (see above), it is possible to isolate any single mass packet of ions within the full mass range of the ion trap without resorting to increasing the rf amplitude. Different methods are used to prepare the thin films of amino acids and polymers. Solutions of L-typtophan and L-tyrosine at pH 9 are dried onto a stainless steel (SS 302) surface in open air at room temperature. Acetone is used to completely dissolve the powdered polystyrene and p-fluoropolystyrene. Droplets of these

polymer solutions are then dried onto stainless steel in open air at room temperature. The SiO2 native oxide covered surfaces of Si(100) wafers are spin-coated with a 0.3% solution of polystyrene in dichloromethane at 6000 rpm. The initial thickness of polystyrene films prepared by this method has been previously measured to be ∼15 nm.33 The polystyrene films are ion bombarded in a separate chamber which is also equipped with an X-ray photoelectron spectrometer (XPS). The details of this apparatus are described elsewhere and will only be summarized here.23,34 The SF5+ ions are generated by electron impact of SF6, mass-selected by a Wien filter (Colutron), and then guided by a series of ion lenses to the surface. Typical ion currents are in the 10 nA range, requiring ion bombardment times of several hours to give total ion fluences of 1.5 × 1015 ions/cm2. Duplicate samples of polystyrene on Si are bombarded in a separate chamber with 50 and 250 eV SF5+ ions at normal incidence and at room temperature. Survey and core-level X-ray photoelectron spectra of the polystyrene films are taken before and after ion bombardment using a hemispherical electron energy analyzer operated in the constant analyzer energy mode and a nonmonochromatic Mg KR X-ray source (Fisons VG CLAM2).23 C(1s), F(1s), S(2p), Si(2p), and O(1s) core-level spectra are taken at 50 and 250 eV pass energies and a 45° photoemission angle off the surface normal. To correct for surface charging, all peaks are referenced to the C(1s) main peak (-C-C-) envelope centered at 284.6 eV.27 Deconvolution and peak fitting procedures are performed with commercial software (VG Microtech VGX900) by assuming constant fwhm and Gaussian line shapes for all component peaks. Peak areas are calculated after Shirley background subtraction.35 Following XPS analysis, the ion beam-modified polystyrene films are transported in air from the ion beam/XPS chamber to the L2ITMS chamber. Total air exposure is estimated to be ∼30 min. RESULTS AND DISCUSSION Demonstration of Laser Desorption/Photoionization/MS2. Our instrument combines several methods which have been previously developed for an ion trap.16,19,21,32 However, our instrument is unique in its combination of laser desorption/ ionization schemes and SWIFT ion manipulation, which have been implemented here using new electronics and software. We demonstrate here how this combination expands the capabilities of the ion trap to include surface analysis of organic, polymeric, and biological materials. Figure 3 depicts the results of laser desorption/photoionization/MS and MS2 experiments on L-tryptophan using our L2ITMS instrument: at the top is the full mass spectrum obtained with 1064 nm desorption and 266 nm photoionization. The highest intensity fragment, with m/z 130, corresponds to the cleavage of the bond between the first (R) and the second (β) carbon atoms (counting from the aromatic rings), as indicated by the cartoon in Figure 3. Ions formed by the loss of the entire aliphatic chain (m/z 117) and additional loss of nitrogen or CH2 (m/z 103) and the bare benzene ring (m/z 77) are the assignments for the remaining major peaks. Previously studies of L-tryptophan observed all these peaks and also found that m/z 130 was the major ion formed for different desorption methods followed by 266 nm (33) Nowak, P.; McIntyre, N. S.; Hunter, D. H.; Bello, I.; Lau, W. M. Surf. Interface Anal. 1995, 23, 873-878. (34) Ada, E. T.; Hanley, L. Manuscript in preparation. (35) Shirley, D. A. Phys. Rev. B 1972, 5, 4709.

Figure 3. Tandem mass spectra of L-tryptophan. Full laser desorption/photoionization spectrum of L-tryptophan (top), SWIFTisolated m/z 130 ion (middle), and collision-induced dissociation of m/z 130 (bottom).

photoionization.7,8 Becker and co-workers previously found that the m/z 204/130 ratio could be used as a qualitative measure of the internal energy of the tryptophan ion following desorption/ionization: a small or nonexistent m/z 204 peak was typically observed for relatively energetic desorption/ionization schemes.7 The relatively intense m/z 204 peak of Figure 3 indicates that our desorption/ionization scheme is gentle compared to many other schemes reported previously.7,8 We do not observe any peaks resulting from rearrangement or ion attachment. The middle spectrum of Figure 3 depicts the SWIFT isolated peak of m/z 130 from tryptophan. This spectrum is taken following the application of a single low-power SWIFT isolation pulse (2 MHz sampling rate) to all ions formed by laser desorption/photoionization. This SWIFT pulse is designed to eject ions with masses lower than m/z 120 and greater than m/z 140. The bottom spectrum of Figure 3 is the final tandem mass spectrum (MS2) of the m/z 130 peak following collision-induced dissociation with the background He buffer gas, achieved by applying an excitation SWIFT pulse after the isolation SWIFT pulse. The daughter ion with m/z 117 is easily formed by collision-induced dissociation of the m/z 130 via loss of CH2. The light He buffer gas does not allow further cleavage of the stable bicyclic ring structure of the m/z 117 ion. Further cleavage could probably be accomplished by introduction of a mixture of He with heavier buffer gases, as described elsewhere.21 Figure 3 shows that our instrument can perform MS2 on the most intense peak of an ion packet. Figure 4 shows the results of a MS2 experiment with L-tyrosine, where we demonstrate that MS2 can also be performed on a weaker peak in an ion packet. L-Tyrosine is structurally similar to L-tryptophan and, therefore, is expected to undergo similar photoionization and collisionAnalytical Chemistry, Vol. 69, No. 8, April 15, 1997

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Figure 4. Tandem mass spectra of L-tyrosine. Full laser desorption/ photoionization spectrum of L-tyrosine (top), SWIFT-isolated m/z 180 ion (middle), and collision-induced dissociation of m/z 180 (bottom).

induced dissociation. However, it should be more difficult to form an intact molecular ion for L-tyrosine since it has two different fragments attached to the aromatic ring. The full mass spectrum of L-tryrosine shown at the top of Figure 4 confirms these arguments. There are three important peaks in Figure 4: m/z 180 corresponding to the molecular ion, m/z 107 displaying the elimination of OOCCHNH2, and m/z 77 representing the benzene ring. C6H5OH+ does not appear in the spectrum. The middle spectrum of Figure 4 shows the SWIFT isolation of the weak molecular ion peak at m/z 180 with a single low-power (1.5 MHz sampling rate) SWIFT pulse. This isolation is difficult to achieve since there is only a 300 Hz difference in the oscillatory frequencies between m/z 180 and 178. Nevertheless, the m/z 178 is almost completely eliminated by SWIFT isolation of m/z 180. The major fragment formed during MS2 of m/z 180 is m/z 107 (bottom of Figure 4): this corresponds to the same R,β bond cleavage observed for L-tyrosine. Potentially, our L2ITMS instrument can select and subsequently fragment any ion peak with a reasonable signal-to-noise ratio from a surface mass spectrum. L2ITMS can also be used to analyze mixtures of organics or polymers on surfaces. The mass spectra of mixtures are often difficult to assign, since it is difficult to determine which peaks are fragments, which are molecular ions, and how these are related to one another. The top of Figure 5 shows the desorption/ ionization MS of a 1:1 mixture (by the total number of monomer units) of polystyrene (PS) and p-fluoropolystyrene (fluoro-PS) on stainless steel. MS and MS2 of pure PS and fluoro-PS thin films have also been obtained from both the Si wafer and stainless steel surfaces (not shown). Both silicon and stainless steel surfaces give identical mass spectra by the methods described here. The top spectrum in Figure 5 is obtained using 355 nm laser desorption and 266 nm ionization of the two polymers. The ultraviolet 1540 Analytical Chemistry, Vol. 69, No. 8, April 15, 1997

Figure 5. Tandem mass spectra of p-fluoropolystyrene (monomer 2) isolated from a 1:1 mixture with polystyrene (monomer 1). Full spectrum of mixture (top), SWIFT-isolated p-fluoropolystyrene monomer (middle), and collision-induced dissociation of p-fluoropolystyrene (bottom).

desorption wavelength is used to intentionally depolymerize the PS and fluoro-PS, although depolymerization can also be achieved with infrared laser pulses.11 The two highest intensity peaks are assigned as the monomers with m/z 122 and 104, corresponding to fluoro-PS and PS, respectively. The spectrum in the middle of Figure 5 is the SWIFT isolation of the fluoro-PS monomer (m/z 122), and the bottom spectrum is the MS2 of the isolated fluoroPS monomer. The highest intensity fragment in the MS2 is m/z 96, which corresponds to the elimination of the ethylene group. The m/z 91 ion represents the detachment of both CH2 and F from the fluoro-PS peak (bottom, Figure 5). The MS2 of fluoroPS from the mixture is in complete agreement with the MS2 spectrum of pure fluoro-PS obtained during a separate run (not shown). By comparing the relative intensity of the two monomer peaks in the MS of the PS/fluoro-PS mixture (top of Figure 5), we can obtain a crude calibration which takes into account their different desorption yields and two photon ionization cross sections.36 The total Gaussian peak-fitted areas are related as 1:2.5 for PS/fluoroPS (m/z 104:m/z 122), indicating a 2.5× higher detection efficiency for the fluoro-PS. This calibration performed is used below to estimate the ratio of fluorinated to non-fluorinated PS following ion bombardment. Single-laser desorption/ionization is a simpler and less costly alternative to two-laser experiments. For that reason, we initially attempted to use a single laser to desorb and ionize all the samples described above. At best, we found that single laser desorption/ ionization led to extremely weak signals resulting from light decomposition products of the analyte molecule. At worst, either (36) Boesl, U. J. Phys. Chem. 1991, 95, 2949-2962.

Figure 6. C(1s) (left) and F(1s) (right) X-ray photoelectron spectra of a polystyrene thin film on a Si(100) wafer before (bottom) and after bombardment (top) with 50 eV SF5+ ions.

no signal or only atomic ions were observed. Our attempts to use a single-laser desorption/ionization scheme for many other samples have often failed to provide useful data. There are certainly cases where single-laser desorption has proven useful for surface analysis. For example, negative ion formation has been used to detect alkanethiolate self-assembled monolayers on gold surfaces.37 Our own attempts to utilize this method met with limited success, presumably due either to our use of a different wavelength laser or electron detachment resulting from collisions with the buffer gas.29 Matrix-assisted laser desorption/ionization is generally not useful for surface analysis since the matrix preparation method usually involves severe modification of the surface. Two-laser mass spectrometry is the most reliable approach to surface analysis, since the desorption of neutrals is much more reproducible and efficient that the direct formation of ions.3,11 Ion Beam Modification and Analysis of Polystyrene Films. We demonstrate the utility of two-laser ion trap mass spectrometry (L2ITMS) on the analysis of SF5+ ion-modified polystyrene (PS) thin films. X-ray photoelectron spectroscopy (XPS) is also used since it can quantitatively discriminate fluorinated from nonfluorinated carbon atoms. The lower left-hand spectrum in Figure 6 shows the C(1s) XPS peak of untreated polystyrene on SiO2/ Si(100) (“before”), which is assigned a binding energy of 284.6 eV27 to account for potential charging effects. This C(1s) core level peak is symmetric, with fwhm ) 1.6 eV, compared with an expected 1.0 eV fwhm for a single peak using this instrument.34 This additional peak broadening is assigned to a convolution of the aliphatic and aromatic carbon environments.33 The π-π* shake-up transition due to the aromatic ring in PS is observed at 291.5 eV.33,38 The lower right-hand spectrum in Figure 6 displays the F(1s) XPS peak of the untreated PS film (“before”). The absence of the Si(2p) and O(1s) peaks of the substrate in the XPS survey scan (not shown) indicates that the initial PS film thickness is greater than ∼5 nm.27 The C(1s) and F(1s) XPS of the PS film exposed to 50 eV SF5+ is shown in Figure 6 on the upper left- and right-hand sides (37) Huang, J.; Hemminger, J. C. J. Am. Chem. Soc. 1993, 115, 3342-3343. (38) Moulder, J. F.; Sticle, W. F.; Sobol, P. E.; Bomden, K. D. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corp.: Minneapolis, 1992.

Figure 7. Two-laser ion trap mass spectrum of a polystyrene spincoated Si(100) wafer after bombardment with 50 eV SF5+ ions.

(“after”), respectively. Assuming that one monolayer equals 3 × 1014 ions/cm2, the SF5+ ion fluence of 1.5 × 1015 ions/cm2 corresponds to a dose of approximately 25 monolayers of F atoms. Compared to the untreated film, the C(1s) peak does not show any significant loss in intensity of the main carbon peak at 284.6 eV but displays a shoulder at ∼287 eV, assigned to a single fluorinated carbon (CF). The relative amount of the ∼287 eV CF peak is estimated to be 6% of the main C(1s) line at 284.6 eV. The π-π* shake-up transition at 291.5 eV is reduced in intensity by ion bombardment, indicating some loss in the aromaticity of the film. The symmetric F(1s) core level peak appears at 687.8 eV. The XPS survey scan also shows a small S(2p) peak at 164 eV and a F(KLL) Auger peak at 699 eV. Based on peak-fitted areas and sensitivity factors, the surface atomic composition is estimated as 87% C, 11% F, and 3% S. The fluorinated film thickness is estimated from the XPS data to be ∼0.1 nm,27 implying that only the top layer of the PS film is fluorinated by 50 eV SF5+ ion bombardment. The presence of the CF environments in the C(1s) spectra shows that the F atoms are covalently bound to the PS film rather than simply trapped by the film. Figure 7 depicts the L2ITMS spectrum of 50 eV SF5+ ionmodified PS film on the Si(100) surface, the same samples which are analyzed by XPS. Here, 355 nm laser pulses are used for desorption and 266 nm wavelength for ionization, as described above for the polymer mixture (Figure 5). The mass spectrum in Figure 7 shows three new peaks in addition to the native PS peaks: m/z 19, 121, and 142. The m/z 19 peak is F+. The m/z 121 peak is singly fluorinated PS or C8H6F+. The m/z 142 peak is doubly fluorinated PS or C8H8F2+. These latter two peaks indicate the covalent attachment of fluorine atoms to the PS film resulting from SF5+ bombardment. The C8H8F2+ peak at m/z 142 does not come from PS with two F atoms on the same carbon atom, since the XPS spectrum does not show a C(1s) peak at Analytical Chemistry, Vol. 69, No. 8, April 15, 1997

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∼289.5 eV indicative of a CF2 group.27 The fact that the ion-fluorinated PS can be photoionized by (presumably) two 266 nm photons indicates that the aromatic nature of the desorbed/ ionized species is preserved during ion bombardment. The p-fluoro-PS standard shows a monomer peak at m/z 122 (Figure 5), while the ion-fluorinated-PS shows a monomer peak at m/z 121; this difference is likely due to the formation of a different monomer or to partial dehydrogenation of the PS during ion bombardment. Analysis of the m/z 104 and 121 peak areas in Figure 7 shows a ratio of 12:1. The same ratio is observed for the peak area ratio of m/z 104-142. To estimate the ratio of fluorinated to nonfluorinated species on the surface, we multiply the 1:2.5 PS-tofluoro-PS sensitivity factor obtained from the calibration experiment (Figure 5) by the peak fitted areas of 1:12 (Figure 7). The result is 1:30, or 3.3% for each fluorinated peak, or 7% for the total of the two fluorinated peaks in the L2ITMS spectrum. This agrees surprisingly well with the XPS estimate that 6% of the PS film is fluorinated by the ion beam, given the limitation of the calibration method. The use of p-fluoro-PS as a calibration standard for these experiments is limited since the location of the F atom on PS probably affects the desorption/photoionization yield and the 50 eV kinetic energy of the incident SF5+ ion is sufficient to form a distribution of different monofluorinated PS species.24 The signal-to-noise ratio in the L2ITMS of ion-fluorinated PS is too low to permit MS2 experiments on the fluorinated peaks. This is unfortunate, since MS2 could determine whether the F is attached to the benzene ring or the ethyl side chain of the PS monomer. The low L2ITMS signal for the ion-fluorinated PS relative to the untreated PS film indicates that the ion bombardment process also leads to destruction of some of the aromatic nature of the film. Some of the ion modification probably leads to sputtering of PS off the surface and some to the formation of a nonaromatic component on the surface.23 The formation of a nonaromatic component is consistent with the reduction in the π-π* shake-up transition observed at 291.5 eV by XPS (Figure 6). This nonaromatic component is probably not two-photon ionized by 266 nm, since photoionization likely occurs through the benzene ring. The similarity in the estimated percentages of the fluorinated-PS from XPS and L2ITMS initially seems inconsistent with the inability of L2ITMS to detect the nonaromatic component. However, this inconsistency can be resolved if the nonaromatic component also has a fluorinated fraction of 6-7%. XPS data for 250 eV SF5+ ions bombardment at similar ion fluences (not shown) indicate that fluorinated species constituted 13% of the total of the ion-modified PS, roughly twice that of the 50 eV ion-modified PS. The absence of the π-π* transition in the XPS indicates a significant reduction in the PS film aromaticity. The appearance of Si(2p) and O(1s) peaks for the 250 eV ion-

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bombarded film indicates that the film thickness is reduced to