Anal. Chem. 1992, 64, 2206-2209
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Supersonic Jet Spectrometry of Chemical Species Resulting from Thermal Decomposition of Polystyrene and Polycarbonate Totaro Imasaka,' Masami Hozumi, and Nobuhiko Ishibashit Faculty of Engineering, Kyushu University, Hakozaki, Fukuoka 812, Japan
Polystyreneand polycarbonateare thermally decomposedby elevatingthe temperatureupto 400 OC. The resultingchemkal specles are measured by wpersonlc Jetspectrometry combined with excitatlon/fluorescence spectrometry and muC tlphoton lonlzatlon/mass spectrometry. A styrene monomer Is dominantly formed from polystyrene, and toluene Is a h found In the decomposed products; the toluene peak Is not clearly observed In the excitation spectrum by a spectral overlap of the congestedlines comlngfrom a styrene monomer but Is dlfferentlated In the resonance-enhanced multiphoton lonlzatlon spectrum. Many peaks are observed In a nonresonant multiphoton lonlzation/mass spectrum, correrpondlng to dimers and trlmers, and possible candidates are proposed In this study. Aiternatlvely, pcreclol Is clearly observed In the thermally decomposed products measuredby mass-solected muitlphoton Ionization spectrometry. Other possible candldates are also proposed, lncludlng the chemical specler correspondingto a monomer unit and an Inhibitor of the chaln radical reactlon.
Currently, many approaches have been developed for analysis of a polymer. A nondestructive method such as infrared absorption spectrometry or Raman spectrometry is widely used for characterization of the polymer. However, many chemical species are included as constituents and impurities, e.g. monomer, oligomer, catalyst, inhibitor, additive, solvent, etc. Therefore, a selective analytical means is essential for analysis of the polymer, especially for trace analysis of the components present as impurities. Pyrolysis gas chromatography combined with mass spectrometry is frequently used for the present purpose. However, the selectivity is not necessarily sufficient. For example, it is difficult to distinguish the isomer only by mass spectrometry, and many standard chemicals must be measured for each column to assign the components in chromatographic determination. Supersonic jet spectrometry provides us with a very selective analytical means, due to a narrow spectral line given by jet expansion and succeeding molecular cooling.1-4 Two spectrometric methods are most frequently used in this approach for sample detection, i.e. excitationJfluorescence spectrometryand multiphoton ionizationlmass spectrometry. These methods have many advantages and disadvantages as well. For example, multiphoton ionizationlmass spectrometry provides direct information concerned with a molecular weight (M,)of the sample. However, an ionization efficiencyis very low for a large polycyclic aromatic hydrocarbon in a onecolor ionization scheme and is not yet sufficient even in a two-color ionization scheme;5 two independent lasers and optimization of experimental conditions are requested in the
* To whom correspondence should be addressed.
+ Deceased.
(1)Hayes, J. M.; Small, G. J. Anal. Chem. 1983,55,565A. (2)Johnston, M.V. Trends Anal. Chem. 1984,3,58. ( 3 ) Lubman, D.M. Anal. Chem. 1987,59,31A. (4)Imasaka, T.;Ishibashi, N. h o g . Quantum Electron. 1990,14,131. 0003-2700/92/0364-2206.$03.00/0
latter scheme. On the other hand, excitation/fluorescence spectrometry provides a strong signal even for a large polycyclic aromatic hydrocarbon, but a database constructed by accumulating the spectral data for many standards is essential for assignment of the chemical species. Thus stateof-the-art supersonic jet spectrometry might be established by applying these two spectrometricmethods simultaneously. In this study we construct a supersonic jet spectrometer allowing measurements of both the excitation/fluorescence spectrum and the multiphoton ionization/mass spectrum simultaneously with a minor modification of the laser beam path. This spectrometric system is first applied to the chemical species resulting from thermal decomposition of the polymers; only a study is reported so far for a polymer sample which is measured by supersonicjet fluorometry after laser ablation? Polystyrene and polycarbonate are used as typical polymer samples in the present study, and the thermally decomposed products are determined by the above spectrometer. Many candidates are proposed as thermally decomposed products by mass spectrometry, some of which are confirmed spectrometrically by multiphoton ionization and excitationlfluorescence spectrometries.
EXPERIMENTAL SECTION Apparatus. The supersonic jet spectrometer constructed is shown in Figure 1. The sample is heated and decomposed in a gas chromatograph chamber (Shimadzu,GC-8A). The temperature is adjusted between 100 and 400 OC. The stainless steel tube for sample introductionis maintained at 250 O C . Thermally decomposd products are expanded from a pulsed nozzle maintained at 250 O C 7 into the first chamber, which is evacuated by an oil ejector pump (Ulvac, PBL-04) followed by a mechanical booster pump (Ulvac, PMB-001) and a rotary pump (Ulvac, D-330). The vacuum pressure is maintained below 3 X 10-3Torr during the experiment, which is measured by a pirani vacuum gauge (Ulvac, GP-2T). The center part of the jet is extracted as a molecular beam by a skimmer (Beam Dynamics, 1-mm i.d.) into the second chamber, which is evacuated by a cold trap (Dia Vacuum, L-type) and a diffusionpump (Ulvac,ULK-06)followed by a mechanical booster pump (Shimadzu, MB-30) and a rotary pump (Ulvac, PVD-180K). The vacuum pressure is maintained below 6 X 10-6 Torr during the experiment, which is measured by an ionization vacuum gauge (Ulvac, G1-TL2). The ions induced by multiphoton ionization are extracted by a repulsive potential into the third chamber, which is evacuated by a turbomolecular pump (Osaka Vacuum, OV-TH520C) followed by a rotary pump (Alcatel, T2012A). The vacuum pressure is maintained below 1 x 1PTorr during the experiment. Alaser beam (Quantel,YG581420, TDL-50,UVX-2;Lambda Physik, EMGlOZMSC, FL2002, homemade frequency-doubling/ auto-trackingsystem using &BaJ3204 crystal)is focused into the molecule in a supersonic jet 10 mm away from the nozzle. Fluorescence is measured by a monochromator (Jasco, CT-25CP) equipped with a photomultiplier (Hamamatsu, R1477). The signal is measured by a boxcar integrator (Stanford,SR-250). A quartz prism is placed to change the laser beam path for (5)Lin, C.H.; Hozumi, M.; Imasaka, T.; Ishibashi, N. Analyst 1991, 116,1037. (6)I m d a , T.; Tashiro, K.; Ishibashi, N. Anal. Chem. 1989,61,1530. (7)Imasaka, T.; Okamura, T.; Ishibashi, N. Anal. Chem. 1986, 58, 2152. (9 1992 Amerlcan Chemlcal Soclety
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