Anal. Chem. 1998, 70, 1208-1213
Organic Surface Analysis by Two-Laser Ion Trap Mass Spectrometry. 2. Improved Desorption/ Photoionization Configuration Oleg Kornienko, Earl T. Ada, Jillian Tinka, Muthu B. J. Wijesundara, and Luke Hanley*
Department of Chemistry, m/c 111, University of Illinois at Chicago, Chicago, Illinois 60607-7061
We previously described a two-laser ion trap mass spectrometer for molecular surface analysis of organic, biological, and polymeric surfaces (Kornienko, O.; et al. Anal. Chem. 1997, 69, 1536-1542). We have made several improvements in this instrument: a new 118-nm vacuum ultraviolet (VUV) photoionization source and an improved desorption/ionization geometry in the ion trap. The improved surface analysis capabilities of this instrument are demonstrated on C18 alkylsiloxane self-assembled monolayers on silicon, polypeptide thin films, mixed polystyrene thin films, and ion beam-modified polystyrene films. The new instrumental configuration has submonolayer sensitivity and the capability to perform tandem mass spectrometry on monolayer samples. The advantages of VUV photoionization for surface analysis studies are demonstrated to be lower fragmentation and relatively nonselective ionization when compared with multiphoton ionization with ultraviolet light. Laser desorption/photoionization, secondary neutral, and secondary ion mass spectrometries are increasingly being applied to molecular surface analysis of thin films, photomasks, and coatings.1-10 Many of these surface analytes are composed of complex mixtures of organic, polymeric, and/or biological compounds. While mass spectrometry of mixtures is usually preceded by chromatographic separation, this is generally not feasible for small amounts of complex surface mixtures. An alternative approach to molecular analysis of surface mixtures can be (1) Daolio, S.; Kristof, J.; Piccirillo, C.; Gelosi, S.; Facchin, B.; Pagura, C. Rap. Comm. Mass Spectrom. 1996, 10, 1769-1773. (2) Wruck, D.; Boyn, R.; Parthier, L.; Buhrow, T.; Henneberger, F. Mater. Sci. Eng. B Solid State Mater. Adv. Technol. 1997, 44, 395-399. (3) Gastel, M.; Breuer, U.; Holzbrecher, H.; Becker, J. S.; Dietze, H. J.; Wagner, H. Fresenius J. Anal. Chem. 1997, 358, 207-210. (4) Leeson, A. M.; Alexander, M. R.; Short, R. D.; Briggs, D.; Hearn, M. J. Surf. Interface Anal. 1997, 25, 261-274. (5) Semmache, B.; Lemiti, M.; Chaneliere, C.; Dubois, C.; Sibai, A.; Canut, B.; Laugier, A. Thin Solid Films 1997, 296, 32-36. (6) Meng, Z. G.; Jin, Z. H.; Gururaj, B. A.; Chu, P.; Kwok, H. S.; Wong, M. J. Electrochem. Soc. 1997, 144, 1423-1429. (7) Hwang, C. Y.; Schurman, M. J.; Mayo, W. E.; Lu, Y. C.; Stall, R. A.; Salagaj, T. J. Electron. Mater. 1997, 26, 243-251. (8) Kimizuka, M.; Ozaki, Y.; Watanabe, Y. J. Vac. Sci. Technol. B 1997, 15, 66-69. (9) Affrossman, S.; Henn, G.; Oneill, S. A.; Pethrick, R. A.; Stamm, M. Macromolecules 1996, 29, 5010-5016. (10) Zenobi, R. Int. J. Mass Spectrosc. Ion Processes 1995, 145, 51-77 and references therein.
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described in two steps. First, nonselective desorption and ionization followed by mass spectrometric detection can identify the masses and stoichiometry of all the components of a surface mixture. Next, tandem mass spectrometry (MS2) can be performed for a detailed structural analysis of the individual components of the mixture. We previously applied this two-step strategy to our development of two-laser ion trap mass spectrometry (L2ITMS) for surface analysis.11 L2ITMS uses two separate lasers for desorption of intact molecules and their subsequent photoionization in the gas phase.10-14 Ionized molecules are then analyzed in the ion trap by single or tandem MS. We previously demonstrated that L2ITMS can be used for the surface chemical analysis of organic and polymeric mixtures.11 In the present paper, we describe improvements to our L2ITMS apparatus that permits nonselective detection and higher sensitivity for molecular surface analysis. The previous configuration of our L2ITMS apparatus did not have sufficient sensitivity to permit MS2 of low-concentration surface species such as submonolayer coverages of adsorbates. This low sensitivity was due mostly to the large distance between the desorption and ionization regions which permitted only a small fraction of the desorbed neutrals to intersect with the irradiation volume of the photoionization laser. In addition, the ionization was done in the center of the ion trap, allowing additional photoadsorption events that might lead to the photofragmentation of stored ions during multiple laser shot experiments. Another limitation of our previous L2ITMS apparatus involved its use of the fixed 266 nm wavelength for photoionization. Only a limited number of species can be ionized at 266-nm via a (usually 1 + 1) multiphoton ionization process. We attempted to use 212nm photons from fifth harmonic generation, but this photoionization scheme often led to extensive fragmentation. A tunable wavelength laser for photoionization would have overcome some of the aforementioned problems, but it would have added considerable complexity and cost to the experimental apparatus. Even if a tunable laser had been installed, the maximum in multiphoton ionization cross section occurs at different wave(11) Kornienko, O.; Ada E. T.; Hanley, L. Anal. Chem. 1997, 69, 1536-1542. (12) Nicolussi, G. K.; Pellin, M. J.; Calaway, W. F.; Lewis, R. S.; Davis, A. M.; Amari, S.; Clayton, R. N. Anal. Chem. 1997, 69, 1140-1146. (13) Zhan, Q.; Zenobi, R.; Wright, S. J.; Langridgesmith, P. R. R. Macromolecules 1996, 29, 7865-7871. (14) Sugawara, K.; Miyawaki, J.; Nakanaga, T.; Takeo, H.; Lembach, G.; Djafari, S.; Barth, H. D.; Brutschy, B. J. Phys. Chem. 1996, 100, 17145-17147. S0003-2700(97)01116-5 CCC: $15.00
© 1998 American Chemical Society Published on Web 02/18/1998
lengths and varies in magnitude for different molecules.15 These limitations impart to multiphoton ionization, whether performed with fixed or variable wavelengths, a level of selectivity that is often not desirable in surface analysis. Furthermore, fragmentation often accompanies multiphoton ionization. This paper describes several major experimental improvements to address the aforementioned shortcomings in sensitivity and selectivity of our L2ITMS apparatus.11 The first improvement is the repositioning of the desorption and ionization regions so that they are three times closer. The photoionization laser beam was also shifted off-center of the ring electrode: this enables the photoionization process to occur outside the stored ion cloud and allows the use of an intact central portion of the ring electrode. The other experimental improvement to our L2ITMS is the implementation of vacuum ultraviolet (VUV) radiation for photoionization. VUV radiation is well established as a method for nonselective, single-photon ionization of gas-phase species.16 Recent work has shown that VUV photoionization with 118-nm radiation has several advantages over multiphoton ionization in two-laser mass spectrometric molecular surface analysis.17-19 VUV radiation appears to have similar single-photon ionization cross sections for a variety of species, provided their ionization potentials are below the 10.5-eV photon energy. Thus, VUV will not ionize background gases such as He, CO, or H2 due to their high ionization potentials. This fact is particularly important here since ion traps operate with a high background pressure of He. The small excess energy imparted to the ion during the photoionization event also minimizes the ion fragmentation. The 118-nm VUV wavelength is the most convenient available without a synchrotron light source since it can be produced by ninth harmonic generation from the Nd:YAG laser fundamental. Wavelengths shorter than 118 nm are limited by the transparency of common optical materials. The advantage of VUV single-photon ionization in twolaser mass spectrometry has recently been demonstrated by its ability to detect a wide range of organosulfur self-assembled monolayers with similar efficiencies and almost no fragmentation.17-19 Both laser-induced electron impact and femtosecond pulsed lasers have also been used for nonselective ionization of species in surface mass spectrometry;20,21 comparison of these various ionization methods is beyond the scope of this publication. We demonstrate here our L2ITMS with VUV photoionization and improved sensitive on a variety of samples. We compare and contrast VUV single-photon with UV multiphoton ionization for a monolayer-scale sample and a mixture of polymers. Finally, we chemically analyze a monolayer sample by MS2. The samples selected for this study include an alkylsiloxane self-assembled monolayer on silicon, nanometer-thick polymer films, a micrometerthick polypeptide film, and a polymer film whose top monolayer has been modified by a low-energy molecular ion beam. (15) Boesl, U. J. Phys. Chem. 1991, 95, 2949-2962. (16) Van Bramer, S. E.; Jonston, M. V. Appl. Spectrosc. 1992, 46, 255-261. (17) Trevor, J. L.; Hanley, L.; Lykke, K. R. Rapid Commun. Mass Spectrom. 1997, 11, 587-589. (18) Trevor, J. L.; Mencer, D. E.; Lykke, K. R.; Pellin, M. J.; Hanley, L. Anal. Chem. 1997, 69, 4331-4338. (19) Trevor, J. L.; Lykke, K. R.; Pellin, M. J.; Hanley, L., Langmuir, in press. (20) Schriemer, D. C.; Li, L. Anal. Chem. 1996, 69, 250-256. (21) Savina, M. R.; Lykke, K. R. Anal. Chem. 1997, 69, 3741-3746 and references therein.
Figure 1. Laser desorption/vacuum ultraviolet photoionization geometry in the two-laser ion trap mass spectrometer.
EXPERIMENTAL DETAILS Our L2ITMS instrument has been described in detail previously.11 Briefly, it consists of modified commercial ion trap electrodes that were placed in a new vacuum chamber and controlled by custom electronics and software. The ion trap electrodes were modified to allow laser desorption from a surface probe inserted inside of one of the end caps. The desorption laser was incident at 75° off the surface normal. Laser-desorbed neutrals passed through an array of holes in the end cap into the center of the ion trap. A second laser beam that passed through a hole in the ring electrode of the ion trap (normal to the principle z-axis) was used to photoionize these desorbed neutrals. The software allowed selective accumulation of ions by multiple laser shots in the trap and their subsequent tandem mass analysis, using the stored wave form inverse Fourier transform (SWIFT) method. Several changes were introduced to this configuration to improve sensitivity and permit nonselective ionization. Figure 1 shows a schematic diagram of the new geometry of the laser beams and the ion trap electrodes. The distance between the sample probe surface and the ionization region was reduced to 11 mm from its previous value of 34 mm. This was achieved by machining channels through the ring electrode, entrance end cap, and Teflon insulator ring to allow the desorption laser to hit the surface of the probe. This modification permitted ionization of a larger portion of the desorbed neutrals since the spreading of the desorbed neutrals increases with the distance traveled from the surface. However, this modification reduced the maximum allowable sample surface area. This modification also moved the photoionization beam off the center of the ring electrode, avoiding the possibility of fragmentation in multiple laser shot experiments. In the current configuration, the ions were formed off axis and then relaxed by He collisions into the center of the ion trap. Finally, the sample surface was rotated between laser shots to avoid desorption from a partially damaged, depleted, or decomposed area. Multiple laser shot experiments typically used two to five shots. No further increase in ion signal was observed for the storage of more than approximately five laser shots, due to the loss of ions stored for more than ∼0.5 s in the ion trap. One scan refers to a single full-range radio frequency voltage ramp on the ring electrode. The spectra presented here were typically the summation of five single scans to provide higher signal-to-noise (S/N) ratios. 1064-nm (8-12 mJ/pulse) infrared and 355-nm (4-7 mJ/pulse) ultraviolet (UV) wavelengths were used for desorption; 266- (0.5-3 mJ/pulse) and 118-nm (5-10 nJ/pulse estimated) wavelengths were used for photoionization. The time delay Analytical Chemistry, Vol. 70, No. 6, March 15, 1998
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between the desorption and ionization events was 15-20 µs in this new configuration. Figure 1 also depicts the specially designed Xe tripling gas cell that was used to generate 118-nm radiation by third harmonic generation from 355-nm Nd:YAG laser pulses.16-19 The major features of our design are its compactness to permit tight focusing of the 355-nm beam for increased conversion efficiency and its complete in-space separation of the 355- and 118-nm beams. The focus of the 355-nm (5-10 mJ/pulse) beam was placed at the back focal plane of a tilted (8°) MgF2 lens (plano-convex, d ) 25.4 mm, f ) 75 mm at 250 nm, Janos Technology, Inc.) which resulted in a parallel 355-nm beam between the lens and the ion trap. The design of the gas cell permitted variation of the lens tilt by (7°. The relatively short focal length and correspondingly highly curved convex surface of the lens also helped to angularly separate the 355- and 118-nm beams. The lens tilting resulted in different offset distances of the beams as they traveled through the 5-mmthick middle portion because of significant difference in refraction coefficients (n355 ) 1.39, n118 ) 1.68). The Xe (99.995% min, CGA) gas pressure was adjusted just below the discharge point to 3040 Torr. The 118-nm beam was focused to ∼50 µm immediately above (∼2 mm) the entrance holes of the end cap. The 355-nm residual beam was dumped at the outer surface of the ring electrode. The VUV intensity was optimized in a separate experiment where 5 × 10-7 Torr of acetone was leaked into the ion trap and the acetone molecular ion signal at m/z 58 was maximized. During this adjustment, the Fe+ and Cr+ signals that resulted from VUV laser desorption/ionization from the ion trap electrodes were minimized, ensuring the correct positioning of the VUV beam. The m/z 43 ion from acetone fragmentation was also minimized to ensure complete separation of the two beams, since the presence of 355-nm radiation induced photodissociation. The low-mass cutoff potential of the ion trap radio frequency was raised to a value corresponding to ejection of ions smaller than m/z 25, to ensure that no acetone ions penetrated the hole in the ring electrode from outside the trap. Various types of sample surfaces were prepared to test the instrument. C18 alkylsiloxane self-assembled monolayer samples were prepared by established methods from distilled C18H37SiCl3 on a piranha solution-cleaned, native oxide-covered Si(100) wafer.22 The formation of the C18 alkylsiloxane monolayer was verified by contact angle measurements and X-ray photoelectron spectroscopy. A 1:1 mixture (by number of monomers) of polystyrene and p-fluoropolystyrene was prepared as a thin film on a stainless steel surface, as previously described.11 A 50-eV SF5+ ion beamfluorinated sample of polystyrene on silicon was prepared as previously described.11,23 A tripeptide, glutathione (Sigma), was prepared as ∼1-µm-thick films by depositing the neutral pH solution onto a Ta foil and a native oxide-coated Si(100) wafer and then drying in air. A new multiple pulse stored inverse Fourier transform (SWIFT) sequence was designed for a tandem MS multiple laser shot analysis of m/z 142 ions from the ion modified polystyrene sample. Careful consideration was given to manual smoothing and the number of pulses/amplitude ratio in the SWIFT pulses to optimize (22) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 10741087. (23) Ada, E. T.; Hanley, L. Int. J. Mass Spectrom. Ion Processes, in press.
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Figure 2. Mass spectrum of a C18 alkylsiloxane self-assembled monolayer on a Si wafer using 355-nm desorption and 118-nm photoionization.
ion isolation and accumulation. The SWIFT pulse amplitude/ length ratio was also optimized for fragmentation. Each of the SWIFT pulses was 32K in length with amplitudes ranging from 1 to 5 Vp-p. Low-degree fast Fourier transform smoothing was applied to some of the spectra (Figure 5 and the bottom of Figure 6). This smoothing procedure did not alter the peak positions or their relative intensities and served only to improve the S/N ratio for the low intensity peaks. RESULTS AND DISCUSSION Molecular surface analysis with MS requires sufficient sensitivity to permit the analysis of submonolayer concentrations of adsorbates. The improvements in the experimental configuration allowed us to observe a C18 alkylsiloxane monolayer with our L2ITMS using 355 nm for desorption and 118 nm for photoionization. The alkylsiloxanes cannot be readily ionized via a multiphoton process because they do not possess any chromophores that strongly absorb in the UV. Figure 2 displays the L2ITMS of a C18 alkylsiloxane self-assembled monolayer on a SiO2/Si(100) surface. The C18H37+ ion at m/z 253 is the highest intensity peak in the mass spectrum: it is formed by C-Si bond cleavage at the surface during the laser desorption process. It was previously observed that aromatic siloxane monolayers on Si also undergo Si-C bond cleavage when exposed to 193- or 248-nm radiation: this bond cleavage was determined to be photochemical rather than thermal.24 The C18 desorption may also have occurred by photochemical Si-C bond cleavage. However, a thermal mechanism cannot be eliminated since we used both higher laser fluxes and lower energy photons for desorption. The similar bond strengths of surface C-C and C-Si bonds (∼380 and ∼370 kJ/mol, respectively) do not indicate any strong (24) Dulcey, C. S.; Georger, J. H., Jr.; Chen, M.-S.; McElvany, S. W.; O’Ferral, C. E.; Benezra, V. I.; Calvert, J. M. Langmuir 1996, 12, 1638-1650.
Figure 3. Mass spectra of a glutathione thin film. Top: 355-nm desorption + 118-nm photoionization of glutathione from a Si wafer. Bottom: 1064 + 118 nm from a Ta foil.
preference for thermal cleavage of the Si-C bond.25 Figure 2 also displays a m/z 254 peak with an intensity of 25% of the m/z 253 peak, within reasonable agreement of the expected value of 20% for the 13C18H37+ peak given the S/N ratio of this instrument. The peaks at m/z 239 and 268 correspond to the C17 and C19 chain lengths that likely contaminated the organic precursors used to prepare the monolayer. Similar chain length contamination has been identified in two-laser MS of alkanethiolate self-assembled monolayers.19 The ion trap was scanned up to m/z 400, but no strong signals were observed above m/z 268. The low-mass (
