ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979
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Pyrolysis Gas Chromat ographic-Mass Spectrometric Identification of Intractable Materials John L. Wuepper Whirlpool Corporation, Research & Engineering Center, Benton Harbor, Michigan 49022
Pyrolysis gas chromatography-mass spectrometry with data system is used to provide unequivocal identification of various intractable carbon black filled rubbers and is extended to the identification of paint resin, epithelial tissue, textile fibers, polymers, and gum.
While considerable work has been reported on various aspects of pyrolysis gas chromatography of polymers in general, the identification of various kinds of intractable samples is usually based on pyrogram fingerprinting techniques without the benefit of the unequivocal nature of mass spectra of the peaks and without the benefit of data system manipulation of data (1-4). Conventional pyrolysis gas chromatography does provide very useful fingerprint information ( 5 ) ,but we have found that reliance on pyrogram patterns and retention time alone can cause considerable uncertainty and lack of confidence in intractable sample identification when little or no sample history is available. Indirectly-related pyrolysis gas chromatography-mass spectrometry studies have been reported (6, 7 ) , but it appears that the problem solving and identification capabilities of pyrolysis gas chromatography-mass spectrometry-data system packages have been underutilized with regard to intractable samples. Pyrolysis infrared and pyrolysis mass spectrometry o f intractable organic solids without chromatography are both of considerable use in polymer characterization, but both suffer from the drawback of obtaining spectra of mixed compounds in the gas or liquid phases of the pyrolyzate (8, 9). Most spectrometry of polymer pyrolyzates have been performed with controlled off-line pyrolysis systems, which suffer from the disadvantage of added sample handling or transfer to the mass spectrometers (10, 11). T h e pyrolysis gas chromatographic-mass spectrometricd a t a system (PGCMSDS) procedure described below was initially used for fast characterization of unknown carbon-filled cured rubbers, b u t the ease of pyrogram peak identification with the quadrupole data system immediately suggested that the method be extended to other intractables such as paint, fibers, epithelial tissue, gum, and possibly others such as polymers. PGCMSDS seems to offer considerable potential where pyrogram profile matching is not sufficient for the intended result of t h e analysis such as might exist in other problem solving laboratories or in the forensic sciences and when independent data are needed to support other results.
EXPERIMENTAL The instrumentation used for the PGCMSDS identifications includes a Hewlett-Packard 18580A pyroprobe, a Hewlett-Packard 7620 gas chromatograph, a Finnigan 3000 quadrupole mass spectrometer with glass jet separator, a System Industries (Riber Data System) 150, and a Tektronix hard copy unit. A 30-m OV-101 glass column was employed for all the analyses. Quartz tubes, 2 mm X 26 mm, were used to contain the samples for pyrolysis. These sample tubes were always fired empty and in air at 1000 "C before sample analysis to ensure cleanliness and minimize extraneous peaks in the pyrograms. Pyrolyses were carried out with rise times in the 0.5 to 2 "C/ms range, an upper temperature limit of 700 "C, and a total pyrolysis time of 10 s. The column was held at ambient temperature for 2 min and then programmed to 180 "C at 10 "C per minute. A split ratio of about 1OO:l was used for the glass capillary and
a linear helium carrier gas velocity of 20 cm/s f 1 cm/s was employed. Twenty-five mL/min helium make-up gas flow was added at the exit end of the glass capillary column before the jet separator. Special preparation of the samples was not required in that the major diagnostic peaks of the pyrogram were well resolved in all cases. Sample sizes were approximately 0.1 mg to 2 mg in order to keep the column loading, after splitting 100:1, in the I- to 20-pg range.
RESULTS AND DISCUSSION An advantage of the PGCMSDS procedure, as with pyrolysis gas chromatography in general, is the capability to utilize the technique in spite of intractable matrices. For example, rubber vulcanizates are often highly filled with carbon black or the carbon black-filled vulcanizate itself can be used as a base for other fillers such as asbestos. T h e pyrolysis leaves the fillers behind and eliminates the need for sample pretreatment, and the small sample size required usually leaves a sufficient amount of sample available for characterization by other techniques. Several examples of intractable sample identification using PGCMSDS are shown below. Most examples show a computer reconstructed gas chromatogram with peaks identified with the data system and also a mass spectrum or a limited mass search of a peak that, for purposes of discussion, was chosen as diagnostic for that particular sample. Naturally, all the chromatographic peaks are diagnostic and can be easily examined with the data system, and several peaks are usually used to fix an identification. Other capabilities of most data systems are useful in locating specific peaks in the chromatograms. For example, if rubber vulcanizates are being analyzed, peaks with mass to charge ratios of 104 (styrene), 88 (2-chloro-l,3-butadiene), or 53 (acrylonitrile) might be chosen for limited mass chromatograms which would help to identify SBR, chloroprene, and nitrile rubbers, respectively. If little sample history is available on the sample, then mass spectra of the major peaks from the pyrogram are examined one by one until identification is complete. In all cases, the emphasis in the identification is based on mass spectral data while retention times and peak ratios are also noted. We have found t h a t t h e PGCMSDS procedure is useful for identifying intractable rubber vulcanizates, fibers, paint, epithelial tissue, chewing gum types, and polymers, and can undoubtedly be extended to other intractable pyrolyzable materials.
INTRACTABLE RUBBERS Cured carbon-filled rubber compounds that were characterized by PGCMSDS include styrene-butadiene, ethylene-propylene, acrylonitrile, synthetic polyisoprene, butyl, and neoprene. Table I lists the most useful diagnostic peaks for samples that have been routinely analyzed in this laboratory. All of the data in Table I are based on those obtained from known standards. A typical example PGCMSDS identification involved that of determining the type of rubber binder used in a friction pad. The friction pad consisted of an asbestos-like inorganic material that contained a cured carbon black-filled rubber. Figure 1 shows a computer reconstructed pyrogram and a limited mass chromatogram for a 104 mle ion, the molecular ion of styrene monomer. Styrene monomer is one of the chief
0003-2700/79/035 1-0997$01.0010 0 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979
Table I. Diagnostic Pyrogram Peaks for Intractable Rubbers material SBR
polychloroprene nitrile EPDM butyl synthetic polyisoprene
diagnostic peaks in pyrogram ( m / e ratio of molecular ion) Butadiene (54), benzene ( 7 8 ) , toluene ( 9 2 ) , styrene (104),azulene (128). Ratio of butadiene:styrene approximately 2-4:l. 2-Chloro-1, 3-butadiene ( 8 8 ) . Ratio of chloroprene to any other peak is about 1 O : l . Acrylonitrile (53), ratio of C,z-:acrylonitrile 2 : 1, Propylene (42),butene (56), benzene, and toluene can be similar to SBR but sytrene intensity is negligible. Isobutylene ( 5 6 ) dominates pyrogram. Ci diene ( 6 8 ) dominates gas chromatogram. Absence of diagnostic peaks such as styrene, acrylonitrile, and chloroprene. Aromatics are minor.
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rograms that are shown here. For example, the apparently unresolved envelopes of peaks, numbers A and B, in Figure 1 become resolved into several base-line resolved peaks when the data are expanded as shown in Figure 3. The expanded form of the chromatogram more accurately reflects the efficiency of the column.
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diagnostic pyrogram peaks for a SBR type of rubber. Additional diagnostic pyrolysis peaks for SBR rubber include butadiene, toluene, benzene, and azulene. Figure 2 shows a confirming mass spectrum for styrene monomer as the peak shown in the limited mass chromatogram in Figure 1. Similar mass searches and confirmatory mass spectra confirmed the rubber binder as a SBR rubber. It is worth noting that column resolution often is camouflaged but not lost in many computer reconstructed py-
PGCMSDS of hand epithelial tissue produced an interesting computer reconstructed pyrogram as shown in Figure 4. A limited mass search for toluene (mle 91, M - 1) is shown in Figure 4, bottom. Toluene appeared as the largest peak in the pyrogram for this type of sample. I t was found that the samples from several individuals produced qualitatively similar chromatograms with regard to the major diagnostic peaks which were shown to be toluene, phenol, p-cresol, carbon dioxide, and acetonitrile. This suggests that the combination of peaks is a qualitative basis for the identification for this type of material. T h e pyrogram shown in Figure 4 was from a control sample that was known to be free of hand creams, lotions, soaps, or detergent residues. Pyrograms from other sources did not show the presence of unusual peaks, but naturally the possibilities of the presence of externally applied materials would have to be included in the pyrogram interpretation.
PAINT RESINS Paint resin analysis has been shown to be important in forensic applications ( 5 )and has been of use in this laboratory for problem solving as in the identification of paints and in the identification of unknown materials. Figure 5 shows a
ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979 NE6
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Top: Pyrogram of Dynel fiber. (A) SO2; (B) acrylonitrile: (C) benzene; (D, E) C-4 cyanides; (F) cyanobenzene: (G, H) cyanoalkanes. Bottom: Limited mass chromatogram for m/e 103 (cyanobenzene M+) Figure 6.
computer reconstructed pyrogram and identification of an unknown associated with an appliance coatings problem. Mass spectra showed that the largest pyrogram peak was styrene. A limited mass search for mle ion 55, CH,=CHCO+, showed the possibility of ethyl acrylate. A positive identification of ethyl acrylate was obtained by the mass spectrum shown in Figure 5. Other characteristic peak identifications are included in the figure. This technique can undoubtedly be extended t o other unequivocal mass spectral identifications of intractable paint chips or resins.
TEXTILE FIBERS PGCMSDS is ideally suited to the identification of textile fiber polymers. Figure 6 shows a pyrogram of a Dynel fiber sample and a limited mass search for M+ (103),the molecular ion of cyanobenzene. T h e chief diagnostic peaks for this particular material were found t o be cyanobenzene, benzene, and acrylonitrile. The abundance of nitrogen-containing peaks in the pyrogram is to be expected since Dynel is a copolymer of ,dinyl chloride and acrylonitrile. Much of the chlorine in a polymer is often pyrolyzed to hydrochloric acid which explains the absence of chlorine-containing compounds in the pyrogram. Numerous other generic fibers are also amenable t o the same kind of unequivocal pyrogram peak characterization. Experience has shown t h a t oxygen-containing materials such as cotton or polyesters are less rich in hydrocarbon-type pyrolysis products in that carbon dioxide and/or carbon monoxide account for most of the original carbon
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content of the sample.
POLYMERS PGCMSDS is easily applicable to polymer identification and can serve as a complementary technique to infrared or other types of equally well suited procedures. While the strength of PGCMSDS is most obvious when sample preparation is difficult such as with carbon black-filled rubbers or with materials that are virtually insoluble in solvents, its utility is also readily apparent in polymer identification as, for example, in the differentiation of Nylon 6 and Nylon 66. Figure 7 shows a reconstructed pyrogram for Nylon 66 a n d a limited mass search for mle 84, which is the molecular ion of the main diagnostic peak (cyclopentanone) of Nylon 66.
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Figure 8 shows a similar reconstructed pyrogram for Nylon 6. Although cyclopentanone can be detected in the pyrogram of Nylon 6 using the limited mass search, the great difference in the relative amounts in the two pyrograms can be used to easily differentiate the two polymers.
LITERATURE CITED Cole, H. M.; Peterson, D. L.; Sijake, V. A,; Smith, D. S. Rubber Chem. Techno/. 1966, 39(2), 259. Hu, J. Chih-An. Anal. Chem. 1977, 49, 537. Vukovic, R.; Snjatovic, V. J. folym. Sci., Part A-7, 1970, 8, 139. Iglauer. N.; Bentley, F. J. Chromatogr. Sci. 1974, 12. 23. Williams, R. L. Anal. Chem. 1973, 45, 1076A. Eustache, H.; Robin, N.; Daniel, J. C.; Carrega, M. Europ. folym. J. 1978, 14, 239; Chem. Abstr. 1978, 89, 180458. Hrabak, F.; Mitera, J.; Kubelka, V.; Bezdek. M. Europ. folym. J. 1978, 14, 219; Chem. Abstr. 1978, 89, 180564. Hughes, J. C.; Wheals, 8. B.; Whitehouse, M. J. Forensic Sci. 1977, 10, 217. Pattacini, S. C. "The Identification of Cured Rubber Compounds Using Infrared Spectroscopy", ferkin-Elmer Bull. Oct 1975, No. 52. Happ, G. P.; Maier, D. P. Anal. Chem. 1964, 36, 1678. Futrell, J. H. "Pyrolysis Einhorn, I. N.; Chatfield, D. A.; Mickelson, R. W.; and Combustion Products of Nomex, Durette and Tedlar Polymers"; Flammability Research Center: University of Utah, 1974: FRC/UU-41, UTEC 75-022.
GUM Pyrolysis of a chewing gum sample produced a very characteristic pyrogram shown in Figure 9. T h e four most intense peaks according to mass spectra were shown to be butene, acetic acid, limonene, and cinnamaldehyde.
CONCLUSION PGCMSDS analysis provides unequivocal mass spectral information for the identification and characterization of a variety of intractable organic solids. An advantage over pyrolysis mass spectrometry is that pure spectra are obtained. Emphasis on mass spectra rather than retention time alone provides for a more positive identification of the parent materials.
RECEIVED for review February 6, 1979. Accepted March 27, 1979.
Determination of Conjugated Polyenes in Solid Poly(viny1 c hIor ide) by Selective Photooxidation P. Kohn, C.
Marechal, and J. Verdu"
Department Materiaux, ENSAM, 75640 Paris Cedex 13, France
The irradiation of a poiy(viny1 chloride) film in the near-uitravioiet range leads to the selective destruction of the preexisting conjugated polyenic sequences by oxidative processes. The evolution of the transmittance spectrum during the exposure can be used to determine the initial concentration of these sequences at the moi/L level.
Despite the high absorptivity of polyconjugated sequences, -(CH=CH),with n 2 4, in the electronic spectrum, low concentrations of these structures in poly(viny1 chloride) (PVC) are difficult t o determine by absorption spectrophotometry for two reasons: (1) It was difficult t o find solvents able to dissolve a large range of samples differing in their molecular weight or their tacticity and which do not give any interaction with the PVC macromolecules (highly polar solvents such as hexamethylphosphoramide) or with the proper conjugated sequences (forming charge-transfer complexes such as tetrahydrofuran (1)).
(2) I n solid state (films), the turbidity of the samples, depending on their morphological state, can mask weak absorption bands. T h e determination of these structures is of practical interest related to the problems of initial coloration and thermal or photochemical stability of commercial resins. A method based on the resonance Raman effect has recently been developed for the determination of large polyenes which absorb in the visible range (2, 3 ) . This method is very sensitive, but at present its extension to t h e shorter polyenes which absorb in near-UV (n = 4-7) presents some difficulties. The aim of this paper is to propose an inexpensive method based on the high reactivity of polyenes toward photo0003-2700/79/0351-1000$01 .OO/O
oxidation. Their disappearance in cast films exposed to near-UV irradiation was followed by UV-visible spectrophotometry.
EXPERIMENTAL Many samples, differing by their polymerization mode, are used in this study. @-Caroteneand thermal degradation experiments are performed with a "bulk" PVC having an average molecular weight: MN = 39 000. The films of thickness 50 to 200 pm are cast from 5 g/L tetrahydrofuran (THF) solution. The residual solvent and its antioxidant are extracted in soxhlet by the diethyl ether. The solvents are Baker BAR grade. The @-carotene (Serlabo used without purification) is introduced in the T H F solutions. The partially thermodegraded sample is obtained by extrusion of the polymer in the presence of dibutyltin dilaurate (2.5% in weight) at 150 "C in a Brabender extrusiograph. The polymer is recovered from the extrudate by THF-methanol reprecipitation. The photooxidation is performed in a reactor equipped with a fluorescent lamp-Osram L 40 W 70 emitting from 300 to 450 nm, a continuous, gauss-shaped spectrum, which peaked at 365 nm. The short-wavelength part is filtered off by a glass plate (cutoff at 310 nm, 50% transmittance at 340 nm). The evolution of the film transmittance during the UV exposure is followed with a Varian 635 D spectrophotometer. All measurements are made at 23 f 1 "C. The difference between the final (asymptotic) and the initial absorbance at a given wavelength is characteristic of the concentration of the polyene which absorbs at this wavelength. The absorptivities found in the literature ( 4 ) are used for the estimation of the concentrations.
RESULTS (1) Study of @-CaroteneDoped Films. We studied PVC films containing, respectively, 0.5, 1, and 2% by weight of @-carotene. During the UV exposure, we followed the evo0 1979
American Chemical Society
