Journal of Thermal Analysis and Calorimetry, Vol. 91 (2008) 2, 419–426 STUDY ON THERMAL DECOMPOSITION OF POLYMERS BY EVOLVED GAS ANALYSIS USING PHOTOIONIZATION MASS SPECTROMETRY (EGA-PIMS) T. Arii* and S. Otake Strategic Business Unit Thermal Analysis Group, Rigaku Corporation, 3-9-12 Matsubara, Akishima, Tokyo 196-8666, Japan Photoionization mass spectrometry (PIMS) with vacuum ultraviolet (VUV) light source provides an efficient and fragmentation-free method for the soft ionization of gaseous compounds, in order to facilitate an understanding of thermal decomposition behavior and chemical composition of polymeric materials. The PIMS was applied to the evolved gas analysis (EGA) system equipped with a skimmer interface which is constituted based upon a jet separator principle between a vacuum MS chamber and an atmospheric sample chamber in a furnace. A photoionization source with a deuterium (D2) lamp was closely installed to the vacuum ionization chamber of a mass spectrometer to improve the ionization efficiency. The thermal decomposition of typical polymers in inert gas atmosphere was investigated by the EGA-PIMS and the resulting PI mass spectrum was characterized satisfactorily by only the parent ions with no contribution as a result of fragmentation during the ionization. The results suggested that the EGA-PIMS was an especially powerful and desirable in situ thermal analysis method for polymeric materials which evolve organic gases simultaneously and concurrently. The combination of EGA equipped with skimmer interface with no change of evolved gaseous species and PIMS with fragmentation-free during the ionization is described briefly, and the effective results are presented by comparing with EGA using conventional electron impact ionization mass spectrometry. Keywords: EGA-MS, fragmentation-free, photoionization, polymer decomposition, soft ionization, VUV Introduction Determination of the mass loss for the thermal decomposition of polymeric materials as function of temperature, i.e. thermogravimetry (TG), provides information on thermal events related to the structure and thermal behavior of the materials. However, TG provides no information on the chemistry of these thermal events or on the nature of the evolved gases. Typically, this can be achieved by the combination of TG and other techniques capable of providing both qualitative and quantitative information [1–5]. For instance, by coupling to gas analysis methods such as Fourier transform infrared spectroscopy (FTIR) and mass spectrometry (MS), the evolved molecular fragments can be detected and identified. Nowadays, these combinations are well-established thermal analytical techniques, and TG-FTIR and TG-MS interfaces are commercially available. In practice, the most popular EGA technique is combined with MS because of its sensitivity, versatility, and fast analysis time. In general, with these interfaces the evolved gases are led by a carrier gas flow via a heated transfer line to the EGA. A great variety of EGA-MS systems has been presented in literatures during the past decade and reviews [6–8] have been written on the ad* vantages, disadvantages, and applications. EGA-MS type is roughly classified into two kinds of gas interface systems from the difference in interface structure which connects between a sample chamber and a vacuum chamber : capillary type and skimmer type interface systems. Especially, because the gaseous species evolved by pyrolysis are not only stable species but also unstable species are existed, skimmer interface system based upon jet-separator principle offers principally significant advantages over capillary interface system [9–11]. In traditional, the gaseous species introduced into MS by EGA-MS are ionized by electron impact ionization (EI) technique of 70 eV. Although a gaseous molecule is ionized by colliding with an accelerated electron by EI method, a part of the generated molecular ion is further decomposed, and observed simultaneously as the fragmentation ions. When the multiple gases generate simultaneously and the resulting fragmentation ions overlap mutually, while the fragmentation ion gives the significant information about the structure of molecule, the apparent mass spectrum may be complicated. As mentioned above, the qualitative analysis of complex organic species formed by thermal decomposition of polymeric materials by conventional EGA using Author for correspondence: t-arii@rigaku.co.jp 1388–6150/$20.00 © 2007 Akadémiai Kiadó, Budapest Akadémiai Kiadó, Budapest, Hungary Springer, Dordrecht, The Netherlands ARII, OTAKE EI (EGA-EIMS) is made substantially more difficult because so many kinds of gaseous species may evolve simultaneously or continuously by heating. This means that the fragmentation due to higher ionization potential by EI often prevents the identification of the gaseous species formed by heating in a furnace. In order to differentiate such mixed multiple evolution gases, the use of separation and quantitative techniques such as gas chromatography (GC)/MS become effective [12]. However, this prevents the real-time monitoring and measuring the amount of evolved gases as a function of temperature. In addition, the thermal history of evolution gaseous species may change, which makes it difficult to confidently state that the gaseous components evolved from the sample are traced directly and correctly without any changes. The essential problem of differentiation of multiple organic gaseous species evolved simultaneously from the sample by conventional EGA-EIMS still remains. In order to solve this issue, one feasible approach is the use of MS with a selective and soft (fragment-free) ionization technique such as chemical ionization (CI), laser desorption ionization (LDI), ion attachment (IA) [13] or photoionization (PI) which controls fragmentation during the ionization. Single photon ionization with vacuum ultraviolet (VUV) light is a particularly soft and selective ionization method [14–16], well-suited for detection of both aromatic and aliphatic species. Traditionally, PI technology has been used in the elucidation of atmospheric chemical reaction processes and in monitorTable 1 Ionization potentials of typical organic components Compound E/eV Compound E/eV Argon 15.76 Benzene 9.25 Oxygen 12.07 Toluene 8.82 Nitrogen 15.57 Naphthalene 8.12 Carbone dioxide 13.78 Anthracene 7.40 Water 12.61 Phenanthrene 7.86 Methane 12.60 Biphenyl 8.27 Ethane 11.52 Biphenylene 7.53 Propane 10.97 Xylene 8.44 Butane 10.57 1-Methylnaphthalene 7.95 Acetone 9.71 Phenol 8.50 Hexane 10.18 Fluorobenzene 9.20 Ethylene 10.51 Chlorobenzene 9.07 Acetylene 11.40 Nitrobenzene 9.85 Methanol 10.80 Thiophene 8.87 Ethanol 10.49 Methylamine 8.90 Formaldehyde 10.90 Ethylamine 8.86 Tetrachloromethane 11.47 n-Butylamine 8.71 Tetrabromomethane 10.31 Aniline 7.72 420 ing for special toxic substances as part of research in the direct detection of free radicals and ion-molecule reaction clusters. In the present work, we have focused attention to the validity of a combination of skimmer interface with no change of evolved gaseous species and PIMS with fragmentation-free during the ionization. The instrumentation and experimental set-up are presented together with the representative results in polymer research. It is proposed that the skimmer interface type of EGA equipped with PIMS is powerful thermal analytical tool to differentiate real-timely the activated pyrolyzates formed during the complex decomposition process of polymers. Photoionization technology Photoionization process is the simplest electron transfer reaction induced by photoabsorption. The process of ionization occurs when a photon of sufficient energy is adsorbed by a molecule and results in the formation of an ion plus and electron. R+hv®R++e– where R=an ionizable species, hv=a photon with sufficient energy to ionize species R. In general, although a gaseous molecule is ionizable when the supplied photon energy is in the same levels as the ionization energy of the molecule, the molecule ion is dissociated if too larger than that, and if too small, it can not be ionized at all. The ionization potentials of typical components are listed in Table 1 [17]. As shown in the table, the ionization potentials of many organic compounds are in 8 to 11 eV and are smaller than those of inorganic compounds such as water vapor, nitrogen, oxygen, carbon monoxide and carbon dioxide. Therefore, when the energy of the irradiated photon is larger than ionization energy and is smaller than dissociation energy, ionization of almost all organic compounds becomes possible enough by the VUV lamp source of about 10 eV. Thereby, since only the parent ions of the gas molecule can be observed by the fragment-free mass spectrum, it is enabled to differentiate easily the multiple evolution gases, using the information of molecular ion. Additionally, these specific ionization characteristics are useful when observing selectively organic species even when ionizing components such as oxygen, nitrogen and water vapor, etc. are present in the measurement atmosphere. This is especially valuable for EGA carried out in various heating atmospheres including oxidative and humidity experimental conditions [18, 19]. J. Therm. Anal. Cal., 91, 2008