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Anal. Chem. 1996, 68, 250-256
Surface Analysis of Bulk Polymers Using Laser-Induced Photoelectron Ionization with Laser Desorption in a Time-of-Flight Mass Spectrometer David C. Schriemer and Liang Li*
Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
Laser desorption combined with photoelectron ionization in a time-of-flight mass spectrometer has been developed for surface analysis of bulk polymers. In this technique, a CO2 laser beam is focused onto a polymer surface to induce fast thermal dissociation and/or photodissociation of the polymeric materials. The resulting neutral species are then ionized using photoelectron ionization (PEI). The PEI process involves a laser-metal interaction in which a low-power, pulsed UV laser beam is directed to an appropriate metal surface exterior to the source region of a Wiley-McLaren time-of-flight mass spectrometer. The photoelectrons generated are accelerated as a narrowly distributed beam by the fringing fields of the source region. The electron beam, traveling in a direction almost parallel to the extraction grid, is used to ionize the desorbed neutrals that are entrained into the ionization region. A number of polymers of different molecular weight and monomer structure are examined using this technique. It is found that this technique can produce readily interpretable mass spectra for structural analysis and chemical identification. The application of this technique for terminal group analysis in chemically modified polymers is illustrated. A comparison of this technique with other mass spectrometric methods is presented. In addition, it is shown that the detection sensitivity can be very high, with a detection limit of 99%).4,5 These neutral species are less prone to surface matrix effects. Thus, analytical methods have been developed to decouple the desorption and ionization processes.4,5 In the twostep experiment, the neutrals generated by desorption are ionized by either an electron beam or lasers.3-5 Most of the two-step surface analysis methods have been primarily used for inorganic analysis.3-5 However, laser-induced postionization combined with laser desorption or particle beam sputtering has been recently applied for polymer surface analysis. The use of laser desorption/resonant-enhanced multiphoton ionization (REMPI) in supersonic beam mass spectrometry has been reported for the studies of aromatic polymers.6 However, because of its highly selective nature of ionization, REMPI can only be applied to certain types of polymers. Over the years, laserbased ionization schemes with reduced compound selectivity have been developed, most notably the single-photon ionization scheme using a vacuum UV laser beam. Becker and co-workers have developed a technique called surface analysis by laser ionization (SALI) in which a particle beam is used to desorb or sputter neutral species from the polymer surface, followed by ionization with the use of a laser beam at 118 nm (10.5 eV).5,7,8 This technique can be used to study both aromatic and nonaromatic polymers. It has been shown that it can provide more readily interpretable mass spectra from polymers compared to those obtained with the SIMS technique.7,8 Although the limit in detection sensitivity has not been firmly established for this technique, it is estimated to be about 10-16-10-17 mol for small organic molecules.5 Described herein is a simple alternative method for postionization based on laser-induced photoelectron ionization (PEI) for polymer analysis. In our previous work,9,10 it has been reported that when a low-power, pulsed UV laser beam is directed to the repeller plate of a Wiley-McLaren time-of-flight mass spectrometer (TOFMS), extensive ionization of gas phase species can take place. Similar results are obtained when the laser beam is directed to the extraction grid of the TOFMS. Classical EI-type mass spectra are observed. Further study indicated that the photoelectrons generated by the laser-metal interaction are responsible (4) Lubman, D. M. Lasers and Mass Spectrometry; Oxford: New York, 1990. (5) Becker, C. H. In Ion Spectroscopies for Surface Analysis; Czanderna, A. W., Hercules, D. M., Eds.; Plenum Press: New York, 1991; p 273. (6) Lustig, D. A.; Lubman, D. M. Int. J. Mass Spectrom. Ion Processes 1991, 107, 265. (7) Schuhle, U.; Pallix, J. B.; Becker, C. H. J. Vac. Sci. Technol. A 1988, 6, 936. (8) Pallix, J. B.; Schuhle, U.; Becker, C. H.; Huestis, D. L. Anal. Chem. 1989, 61, 805. (9) Schriemer, D. C.; Li, L. Rev. Sci. Instrum. 1995, 66, 55. (10) Schriemer, D. C.; Li, L. Instrum. Sci. Technol. 1995, 23, 4.
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for the ionization process in these experiments, in line with other studies using the photoelectric effect to induce electron impact ionization.11-15 It was demonstrated that this ionization technique can provide relatively uniform ionization efficiencies for both aromatic and nonaromatic compounds. The ionization efficiency can be quite high. For example, mass spectra of benzene with a signal/background noise ratio greater than 25 for the molecular ion peak can be generated with a sample consumption of
