Rapid Polymer Identification by In-Source Direct Pyrolysis Mass

Anal. Chem. 1996, 68, 1019-1027

Rapid Polymer Identification by In-Source Direct Pyrolysis Mass Spectrometry and Library Searching Techniques Kuangnan Qian,* William E. Killinger, and Melissa Casey†

Washington Research Center, W.R. Grace & Co.sConn., 7379 Route 32, Columbia, Maryland 21044 Gordon R. Nicol

Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716

In-source direct pyrolysis mass spectrometry is studied for rapid identification of polymers in industrial products. In this technique, polymers are pyrolyzed within the ion source of a mass spectrometer by a coiled filament designed for desorption chemical ionization/desorption electron ionization applications. Pyrolysis products are ionized by 70 eV electron impact, yielding highly reproducible mass spectra characteristic of the polymer composition. Using this technique and a commercial library editor, a comprehensive mass spectral library containing more than 150 polymer entries was developed. The impacts of experimental and polymer variables on the reproducibility of polymer mass spectra were evaluated. The technique was successfully applied to the analysis of polymers in industrial products, showing advantages of short analysis time, high confidence level, and low matrix interference. This study suggests that a robust standard polymer library, independent of laboratory and mass spectrometer type, may be developed. However, more rigorous interlaboratory tests are needed to achieve this goal. Pyrolysis mass spectrometry (Py-MS) has been extensively studied as a means for polymer analysis and polymer thermal degradation over the past 40 years.1-14 The technique is often used in combination with other spectroscopic techniques, such † Present address: Department of Chemistry, The Pennsylvania State University, University Park, PA 16802. (1) Zemany, P. Anal. Chem. 1952, 24, 1709-1713. (2) Jones, C. E. R.; Cramer, C. A. Analytical Pyrolysis; Elsevier: New York, 1977. (3) Alekseeva, K. V. J. Anal. Appl. Pyrolysis 1980, 2, 19-34. (4) Montaudo, G.; Garozzo, D. J. Anal. Appl. Pyrolysis 1985, 9, 1-17. (5) Casanovas, A. M.; Rovira, X. J. Anal. Appl. Pyrolysis 1987, 11, 227-232. (6) Wronka, J. W.; Vouros, P. In Modern Methods of Polymer Characterization; Barth, H. G., Mays, J. W., Eds.; John Wiley & Sons, Inc.: New York, 1991. (7) Meuzelaar, H. L. C.; Kistemaker, P. G. Anal. Chem. 1973, 45, 587-590. (8) Boon, J. J. Int. J. Mass Spectrom. Ion Processes 1992, 118/119, 755-787 and references therein. (9) Meuzelaar, H. L. C. Proceedings of the 26th Annual ASMS Conference on Mass Spectrometry and Applied Topics, St. Louis, MO, May 28-June 2, 1978; pp 29-40. (10) Irwin, W. J. Analytical Pyrolysis; Marcel Dekker, Inc.: New York, 1982. (11) Liebman, S. A.; Levy, E. J. Pyrolysis and GC in polymer analysis; Marcel Dekker, Inc.: New York, 1984. (12) Chatfield, D. A.; Hileman, F. D.; Voorhees, K. J.; Einhorn, I. N.; Futrell, J. H. Appl. Polym. Spectrosc. 1978, 241-256. (13) Wells, G.; Futrell, J. H.; Voohees, K. J. Rev. Sci. Instrum. 1981, 52, 735740.

0003-2700/96/0368-1019$12.00/0

© 1996 American Chemical Society

as nuclear magnetic resonance (NMR) spectroscopy and infrared (IR) spectroscopy,15-17 to provide detailed information on polymer composition and structure. The high selectivity, specificity, and sensitivity of mass spectrometry make Py-MS particularly valuable in situations where matrix interference and detection limit may present challenges to the application of other techniques. Two types of Py-MS techniques, known as pyrolysis gas chromatography/mass spectrometry (Py-GC/MS) and direct pyrolysis mass spectrometry (DP-MS), have been used in industrial labs for polymer analysis. In Py-GC/MS, pyrolysis takes palce near the GC injection port, generating pyrolysis products that are characterized by standard GC/MS analyses. Polymer composition is determined on the basis of the identification of the pyrolysis products and the knowledge of polymer pyrolysis chemistry. PyGC/MS is very useful in determining monomer types, end groups, and grafting functionalities.2,10-12 The technique is particularly valued for its ability to separate complex pyrolysis products and to utilize well-established mass spectral libraries for confident identification of organic compounds. Although the GC separation facilitates the identification of the pyrolysate, it prevents the detection of certain pyrolysis products, such as polar and highboiling compounds, due to incompatible chromatographic conditions in certain circumstances. In situ derivatization reduces this problem to some extent by converting polar species into nonpolar/ volatile compounds.18,19 DP-MS offers an alternative for polymer composition analysis. The technique directly analyzes thermal degradation products without prior separation. Conceivably, it has a short analysis time and a high sample throughput, features that are highly desired by industrial laboratories. It has been recognized that DP-MS produces mass spectra feasible for fingerprinting and pattern recognition.4,7,20-22 When combined with “soft” ionization meth(14) Chatfield, D. A.; Einhorn, I. M.; Mickelson, R. W.; Futrell, J. H. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 1367-1381. (15) Pastorova, I.; Botto, R. E.; Arisz, P. W.; Boon, J. J. Carbohydr. Res. 1994, 262, 27-47. (16) Menescal, R.; Eveland, J.; West, R.; Blazso, M. Macromolecules 1994, 27, 5893-5899. (17) Cho, W.-J.; Choi, C.-H.; Ha, C.-S. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 2301-2309. (18) Van der Peyl, G. J. Q.; Linnartz, T. C. T.; Van Rossum, C. A. J. J.; Zeelenberg, M. J. Anal. Appl. Pyrolysis 1991, 19, 279-283. (19) Anderson, K. B.; Winans, R. E. Anal. Chem. 1991, 63, 2901-2908. (20) Statheropoulos, M.; Georgakopoulos, K.; Montaudo, G. J. Anal. Appl. Pyrolysis 1992, 23, 15-32.

Analytical Chemistry, Vol. 68, No. 6, March 15, 1996 1019

Figure 1. Comparison of polymer (in-source DP-MS) and monomer (NIST library) mass spectra. (a) A case of depolymerization. (b) A case of mixed random cleavage and depolymerization.

ods, such as chemical ionization (CI),23-26 field ionization FI,27,28 fast atom bombardment (FAB),29-31 etc., DP-MS also provides information on polymer microstructure. The routine application of the technique, however, has suffered from the lack of a standard library to facilitate the data interpretation and the lack of literature demonstrating the reproducibility of polymer spectra generated by various pyrolysis methods and instrument designs. The difficulty in standardization of pyrolysis devices is apparently a major hurdle in the development of a standard mass spectral library for polymers. Pyrolysis can be conducted within an ion source of a mass spectrometer8 using a heated insertion filament, as described in desorption chemical ionization/desorption electron ionization (DCI/DEI) studies.25,32-35 With in-source pyrolysis, secondary reactions and condensation of pyrolysate can be largely avoided (21) Statheropoulos, M.; Georgakopoulos, K.; Montaudo, G. J. Anal. Appl. Pyrolysis 1991, 20, 65-71. (22) Pidduck, A. J. J. Anal. Appl. Phys. 1985, 7, 215-229. (23) Majumdar, T. K.; Eberlin, M. N.; Cooks, R. G.; Green, M. M.; Munoz, B.; Reidy, M. P. J. Am. Soc. Mass Spectrom. 1991, 2, 130-148. (24) Shimizu, Y.; Munson, B. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 1991. (25) Cotter, R. J. Anal. Chem. 1980, 52, 1589A-1604A. (26) Reinhold, V. N.; Carr, S. A. Anal. Chem. 1982, 54, 499-503. (27) Hummel, D. O.; Dussel, H. J.; Rubenacker, K. Makromol. Chem. 1971, 145, 267-287. (28) Schulten, H. R.; Simmleit, N.; Muller, R. Anal. Chem. 1987, 59, 29032908. (29) Ballistrei, A.; Garozzo, D.; Giuffrida, M.; Montaudo, G. Anal. Chem 1987, 59, 2024-2027. (30) Montaudo, M. S.; Montaudo, G. Macromolecules 1992, 25, 4264-4280. (31) Montaudo, M. S.; Montaudo, G. Makromol. Chem., Macromol. Symp. 1993, 65, 269-278. (32) Baldwin, M. A.; McLafferty, F. W. Org. Mass Spectrom. 1973, 7, 13531356.

1020

Analytical Chemistry, Vol. 68, No. 6, March 15, 1996

because of the high-vacuum environment and the very short time gap between the pyrolysis and the ionization events. Consequently, the mass spectra thus generated are simpler and more reproducible and have fewer experimental variables. By scanning the temperature of the filament pyrolyzer, the technique allows the separation of nonpolymeric components from the polymer and avoids the necessity of sample fractionation. The use of a DCI/ DEI probe for pyrolysis has another important advantage. The probe has become a standard component of most of the mass spectrometers presently available on the market. The polymer mass spectra can therefore be obtained without specialized pyrolysis devices, which facilitates interlaboratory comparison of polymer spectra and provides an opportunity for the development of a standard polymer mass spectral library similar to those used in monomer analysis. This article describes our recent study of the in-source DPMS technique and the development of a mass spectral library for quick identification of polymers. The library currently contains more than 150 entries of standard and specialty polymers, copolymers, and terpolymerssit is still in expansion as this article goes into print.36 The impacts of heating rate, sample loading, ion source temperature, and polymer molecular weight on the polymer spectral pattern were investigated to evaluate the robustness and reproducibility of the technique. Polymer spectra generated by two different laboratories on quadrupole and sector (33) Dell, A.; Williams, D. H.; Morris, H. R.; Smith, G. A.; Feeney, J.; Roberts, G. C. K. J. Am. Chem. Soc. 1975, 97, 2497-2502. (34) Adams, R. E. Anal. Chem. 1983, 55, 414-416. (35) Hunt, D. F.; Shabanowitz, J.; Botz, F. K.; Brent, D. A. Anal. Chem. 1977, 49, 1160-1163. (36) Polymer reference spectra can be obtained by contacting the authors.

Figure 2. Total ion current pyrogram and mass spectrum of poly(acrylic acid), a two-step pyrolysis via side-chain elimination.

mass spectrometers were compared. A number of applications are presented to demonstrate the effectiveness of the technique. EXPERIMENTAL SECTION Most of the in-source DP-MS experiments were carried out on Finnigan MAT TSQ 700 and SSQ 700 mass spectrometers, both equipped with a direct exposure probe (DEP) designed for DCI/DEI applications. The probe is a coiled rhenium filament which can be inserted into the ion source of a mass spectrometer and heated to 1000 °C. Comparative experiments were conducted on a VG-Autospec mass spectrometer at the University of Delaware to evaluate the reproducibility of the mass spectra generated by different instrument configurations. The temperature of the filament pyrolyzer was controlled by the current applied to the filament. The current-temperature relation varies with filaments. Since the temperature of the filament cannot be directly measured, we use filament current to indicate the pyrolysis temperature throughout our discussions. Unless otherwise indicated, the heating rate of the DEP filament was controlled at a constant value of 75 mA/min. The initial current was set at 250 mA to remove any solvent and other nonpolymeric components in the sample that could interfere with the analysis. Depending on the filament and polymer types, a majority of polymers pyrolyze between 400 and 600 mA. Pyrolysis involving side-chain eliminations could occur at a lower filament current. The filament was cleaned at the end of each analysis by setting the filament current to 1000 mA for roughly 1 min. One analysis is normally completed within 7 min. Liquid samples were directly depostied onto rhenium filament by using a syringe. Solid samples were ground into fine particles

under liquid nitrogen cooling; these could also be directly grafted to the filament or mixed with proper organic solvents to form a solution or an emulsion and then deposited onto the filament. A sample loading between 0.1 and 1 µg is recommended. The polymer library was created using a library editor37 provided by the Finnigan MAT 700 software. Polymer standards were obtained from various sources, including Aldrich, Scientific Polymer Standards, and Dow Chemical Co. The electron energy was fixed at 70 eV to maximize the reproducibility of the polymer spectra. The mass analyzer was scanned from m/z 33 to 600 at a scan rate of 1 s/scan. A total of 300-400 spectra were normally collected during a DP-MS experiment. The total ion current (TIC) pyrogram of a polymer often shows one or two peaks, corresponding to one- or two-step pyrolysis. The library spectrum was otained by averaging mass spectra across the TIC peak top (90% of the peak height, 4-5 scans). Polymers with two TIC peaks were introduced as two separate entries in the library. The Py-GC/MS experiments were conducted on a Finnigan MAT SSQ 710 equipped with a CDS Pyroprobe 2000 and a Varian 3400 gas chromatograph. A 20 m Rtx DB-5 column was used for the chromatographic separation. The polymer was pyrolyzed by a pyroprobe (a coiled filament), which was heated from 200 °C to 800 °C in 6 ms and held at 800 °C for 10 s. RESULTS AND DISCUSSION Polymer Spectra by In-Source Direct Pyrolysis Mass Spectrometry. Direct pyrolysis mass spectrometry can be considered a three-step process: (a) thermal degradation of (37) Finnigan MAT 700, ICIS, User’s Manual.


1021

Table 1. Six Representative Pyrolysate Ions of Polystyrene m/z

relative abundancea

78

36

91

45

104

100

117

17

207

7

CH

CHCH

312

2

CH

CH CHCH2CHCH3•+

assignment •+

CH2•+

CH

CH2•+

CH2CH

CH2•+

CH•+

a Relative to ion m/z 104. Measured at a heating rate of 75 mA/ min, a sample loading of 0.04 µg, and a source temperature of 150 °C.

polymers, (b) ionization of the thermal degradation products, and (c) further fragmentation of the ionized species by excess ionization energies. The mass spectrum of a polymer is therefore more complicated than that of its monomer due to the convolution of the multiple processes. For synthetic polymers, four major degradation pathways have been widely recognized:10,38 (1) Depolymerization. This process corresponds to depropagating polymer chain radicals, which results in the unzipping of the polymer chain. (2) Statistical or Random Cleavage of the Polymer Chain. This process occurs only when the bonding energies are similar along the polymer chain and no intramolecular rearrangement occurs. (3) Elimination of Thermally Labile Side Chains. This is often followed by condensation reactions, such as cyclization and dehydrogenation, of the polymer backbone. This is normally a two-step pyrolysis. (4) Decomposition via Cyclic Oligomers. This is a non-freeradical process involving intramolecular exchange. Polymer pyrolysis may follow one or a combination of these mechanisms, depending on the polymer type and structure. In the temperature-controlled pyrolysis, polymer normally degrades across a temperature range, generating a TIC profile (pyrogram) of the polymer. The shape of a TIC pyrogram is determined by many polymer and experimental variables, such as thermal degradation mechanism, molecular weight distribution, sample loading, heating rate, etc. For example, polymers degrading via the depolymerization mechanism always show a single pyrolysis peak, while those degrading via the side-chain elimination mechanism give two or more pyrolysis peaks. A higher heating rate results in a narrower peak. At a heating rate of 75 mA/min, the peak width (fwhm) of a single pyrolysis peak ranges from 20 to (38) Luderwald, J. Pure Appl. Chem. 1982, 54, 255-265.

1022


30 s. The pyrolysis peak position is affected by the pyrolysis temperatures of a polymer and the filament. The change in filament could cause a peak position shift by almost 40 s in the pyrogram. However, for the same filament, the shift in the peak centroid normally does not exceed 5 s. When the pyrolysis and ionization conditions are fixed, the mass spectrum of a polymer is largely determined by its thermal degradation mechanism. In this study, we have generated more than 150 polymer mass spectra and found that they can be roughly classified into three categories on the basis of the pattern of their TIC pyrograms and mass spectra. In the first category, the polymers degrade via the depolymerization mechanism, which produces mostly monomers and low molecular weight oligomers. The mass spectra of these polymers are similar to those of their monomers and are normally easy to interpret. An example given in Figure 1a compares the mass spectrum of poly(methyl methacrylate) (PMMA) obtained by insource DP-MS and its monomer mass spectrum from an NIST library. The similarity between the polymer and the monomer spectra is evident. Polymers in the first category normally follow a one-step pyrolysis and shows a single peak in the TIC pyrogram. They often contain tertiary carbon atoms in the skeleton polymer structure, such as those in PMMA, poly(R-methyl styrene), poly(tetrafluoroethylene), etc. Polymers in the second category, although also showing a single peak in the pyrogram, degrade via random cleavage or a mixed random cleavage and unzipping mechanism, resulting in polymer spectra that are different from their monomer spectra. As an example of this, Figure 1b compares the mass spectra of poly(methyl acrylate) (PMA) and its monomer. Although PMA has only one methyl group less than PMMA, its mass spectrum is much more complicated than the latter. In contrast to PMMA, there is little similarity between the monomer and the polymer spectra of PMA. The polymer spectrum shows a repeating mass of 86 Da (monomer mass of PMA) between certain thermal fragments, indicating that the degradation of PMA is partially selective. Some hydrocarbon polymers, such as polyethylene, and certain condensation polymers degrade predominantly via the random cleavage mechanism. Most polymers in this category, however, follow mixed degradation pathways. The third category of polymers contains thermally labile side chains and pyrolyzes via two consecutive steps. In the first step, the pyrolysis removes the thermally labile side chains, such as the carboxyl group from poly(acrylic acid) (PA) and HCl from poly(vinyl chloride). In the second step, the backbone decomposes and forms more stable and condensed structures, such as aromatic hydrocarbons in the pyrolysis of vinyl polymers. The mass spectra of this polymer type are normally very complicated and less characteristic of their monomer structures. Their TIC pyrograms often show two peaks corresponding to the consecutive decomposition steps. Figure 2 shows the TIC pyrogram and the mass spectra of PA. The side-chain elimination starting at 350 mA yields both carbon dioxide and water as the early pyrolysates (the water peak was not shown because of the low mass limit setting). The backbone starts to pyrolyze at about 400 mA, which generates a highly complex spectrum, carrying very few oligomer features. Interpretation of these spectra can be simplified via the library searching. Impacts of Experimental and Polymer Variables on the Polymer Mass Spectra. Mass spectrometry is well recognized

Figure 3. Effects of (a) heating rate and (b) molecular weight on the relative abundances of the six fragment ions of polystyrene (2, m/z 104; 9, m/z 91; [, m/z 78; O, m/z 117; 4, m/z 217; b, m/z 312).

for its fingerprinting and library searching capabilities. The application of the technique in organic analysis has greatly benefited from the establishment of a comprehensive mass spectral library containing over 50 000 spectra of organic compounds and isomers. The development of a standard library for pyrolysis mass spectrometry, however, is much more complicated due to variations in pyrolysis techniques, such as Py-GC/MS versus DP-MS, flash pyrolysis versus temperature-controlled pyrolysis, pyrolysis inside the ion source versus pyrolysis outside the ion source, etc. Apparently, the more experimental variables of a technique, the more complicated it is to control the reproducibility. With in-source DP-MS, the experimental variables are considerably reduced. For example, when pyrolysis is conducted outside the ion source of a mass spectrometer, the temperature and distance of the transferring line have shown significant impacts on the pyrolysate ion distribution.39 This is less of a problem with in-source DP-MS, where pyrolysis and ionization steps are not clearly separated. In the case of flash pyrolysis (where the temperature is rapidly increased to a fixed value), the final pyrolysis temperature is also known to be an important parameter affecting the mass spectrum.10 This variable, again, is not present with in-source DP-MS, where the pyrolysis mass spectrum is collected over the entire pyrolysis process and the mass spectrum is averaged across a fixed pyrolysis region. For the same reason, (39) Flowers, W. T.; Haszeldine, R. N.; Henderson, E.; Sedgwick, R. D. Trans. Faraday Soc. 1966, 62, 1120-1128.

the change of a pyrolyzer (the DCI/DEI filament in this case), although it affects the shape and position of the TIC profile, has no impact on the pyrolysis mass spectrum. We examined the impact of four experimental and polymer variables, heating rate, sample loading, ion source temperature, and polymer molecular weight, on the reproducibility of polystyrene mass spectra by in-source DP-MS. The parameters were selected because they can vary significantly among laboratories, operators, and samples types. One expects that the conclusions on the reproducibility of polystyrene mass spectra apply to other polymers. Table 1 lists six representative ions of polystyrene and the assigned structures used in the evaluation. The relative abundane of the six ions was averaged across the peak top (90% of the peak height). The strong monomer peak (m/z 104) suggests that the polymer degrades mainly via the depolymerization mechanism. Among the four variables we examined, the effect of heating rate is more visible than those of the other three parameters. As shown in Figure 3a, the relative abundance of the monomer ion (m/z 104) was increased by about 35% as the heating rate increased from 25 to 2000 mA/min, suggesting that the thermal degradation is more selective at high heating rates than at low heating rates. Interestingly, the relative abundances of all the even-mass ion (m/z 78, 104, and 312) increase with the heating rate, while those of all the odd-mass ions (m/z 91, 117, and 207) decrease with the heating rate. Since the odd-mass ions can only be produced by further bond cleavage of ionized molecules, the Analytical Chemistry, Vol. 68, No. 6, March 15, 1996

1023

Figure 4. Comparison of mass spectra of polystyrene collected by a VG-Autospec at the University of Delaware and a Finnigan TSQ-700 at W.R. Grace, Washington Research Center (inset).

Figure 5. Identification of trace poly(vinylidene fluoride) on a surface of a commercial battery electrode. (a) Total ion current pyrogram of the sample. (b) Mass spectrum and the best library matches.

results suggest that fast heating generates less excited molecules than does the slow heating. As a result, there is a crossover in the relative abundance of ions m/z 78 and 91 as the heating rate 1024


proceeds from low to high. Although ideally the heating rate should be strictly controlled to ensure the reproducibility, it was found that a moderate variation in heating rate ((50 mA/min)

Figure 6. Differentiation of copolymers. (a) Total ion current pyrogram of a styrene/isoprene/styrene (SIS) copolymer. (b, c) Mass spectra of the styrene/isoprene copolymer after background subtraction and their library matches.

has little impact on the library search results. The change in heating rate has a significant impact on the profile of the TIC program. As the heating rate was increased from 25 to 2000 mA/min, the TIC peak width (fwhm) of polystyrene was dramatically reduced from 40 to 2 s, while the peak height was increased by ∼20-fold. Clearly, a high heating rate forced the polymer to be pyrolyzed within a short time frame, resulting in an improved sensitivity and a reduced analysis time. A slow heating rate, on the other hand, offers a better temperatureresolving capability, which was found to be critical in the analysis of multicomponent polymers and polymer samples with complicated matrices. It also improves the reproducibility by providing sufficient scans for spectrum averaging. In this study, a heating rate of 75 mA/min was used in the library creation to compromise the needs for sensitivity, temperature-resolving capability, and stability of the mass spectra. Three sample loadings, 0.04, 0.4, and 4 µg, were examined in this experiment. The variations in the relative ion abundance are within the experimental errors (5%). This result is important since we could not control the sample loading in the analysis of an unknown sample. We found that 0.1-1 µg of sample loading can give sufficient signal for confident library searching without increasing the efforts in source cleaning and maintenance. Within a normal ion source temperature range (150 °C-200 °C), the impact of changing source temperature on pyrolysate ions was found to be insignificant. However, at a low source temper-

ature (100 °C), an increase in the ratio of ions 104/91 (m/z) was observed, suggesting a possible relaxation of pyrolysate ions in internal energy by the low source temperature. Molecular weight is also an unpredictable parameter in an unknown analysis. Our study showed that the impact of molecular weight on the polymer mass spectra is minimal over a broad molecular weight range (3600-325 000), as shown in Figure 3b. The monomer ion abundance decreased only slightly (about 8%) as the molecular weight changed from 325 000 to 3600. However, as the molecular weight became even lower (below 1500), significant changes in the mass spectrum were observed due to desorption electron ionization (DEI) of the low molecular weight oligomers prior to the pyrolysis. For polymers with very low molecular weights, separate library entries have to be created. Polymer spectra generated by in-source DP-MS were compared on different mass spectrometer types. Figure 4 shows the mass spectrum of polystyrene collected by a VG-Autospec at the University of Delaware (UD) in comparison with the spectrum collected by a Finnigan TSQ-700 in our laboratory. The heating rates used in UD and our laboratory are 100 and 75 mA/min, respectively. Despite the differences in the configurations of the DCI/DEI probes, ion sources, and mass analyzers, the two mass spectra are very similar. The sector instrument yields a higher abundance of larger oligomer ions (m/z 207, 312, etc.) than does the quadrupole mass spectrometer. However, we believe that this difference is largely due to the different ion transmissions of the Analytical Chemistry, Vol. 68, No. 6, March 15, 1996

1025

Figure 7. (a) Py-GC/MS analysis of a “polymer fraction” isolated from an industrial latex product. (b) In-source DP-MS analysis of the same product prior to polymer fractionation.

two instruments. Good spectral matches were also observed for a number of other polymers, including nylon 6, poly(ethylene glycol), and poly(acrylic acid). The study suggests that a standard polymer library, which must be independent of laboratories and instrumentation, may be generated by in-source DP-MS. Applications. Identification of polymers at low levels, such as polymers on a surface, has always been challenging, especially if the polymers are not separable or extractable from their matrices. In-source DP-MS shows obvious advantages in these situations. Figure 5 demonstrates the identification of trace poly(vinylidene fluoride) (PVDF) on the surface of a commercial battery electrode. The electrode was composed mostly of carbon and was coated with PVDF as a polymer binder. The low concentration of the polymer and the matrix interference prevented confident identification of the material by other spectroscopic techniques. It was also difficult to extract or physically remove the polymer from the electrode for further analysis. In the in-source DP-MS analysis, a tiny piece of the sample, cut from the electrode, was directly placed on the filament pyrolyzer. The polymer (PVDF) was confidently identified within 7 min. The good quality of the library match is indicated by the library fit value (891) shown in Figure 5. The library fit is a similarity index calculation provided by the Finnigan MAT library software that compares the spectrum of a sample with that of the polymer library. The calculation has a scale from 0 to 999, where 999 is the best fit. With the temperature-resolving capability, in-source DP-MS has the potential to differentiate multicomponent polymers, such as copolymers, terpolymers, and polymer blends, given that the polymer components have different pyrolysis temperatures. Figure 6a illustrates the TIC program of Kraton 1107, a styrene/ isoprene/styrene (SIS) block copolymer. The isoprene component, which has a lower pyrolyzing temperature, starts to pyrolyze at 380 mA and reaches its peak at 400 mA. As the temperature 1026


prceeds higher, the pyrolysis of polyisoprene is joined by that of polystyrene. The latter reaches its peak at 440 mA. The two partially resolved peaks in the pyrogram suggest a certain degree of “blockiness” in the monomer distribution. Because of the separation in the pyrolysis peaks, a good-quality mass spectrum can be obtained for each component by subtracting the overlapping scans. This is demonstrated by the library matches shown in Figure 6b,c. In most cases, however, the coexisting polymer components may not be differentiable by the pyrolysis temperatures. Library searching often identifies several possible candidates. Spectrum subtraction has been found to be useful for differentiating the copolymeric constituents. Upon subtracting the library spectrum of one identified candidate, the residual mass spectrum often showed an improved match with the library spectrum of the second component. However, it was difficult to apply the strategy to systems containing more than three polymers. In addition, copolymers containing similar characteristic ions (e.g., nylon 6/nylon 6/12) cannot be differentiated by the spectrum subtraction. Other techniques, such as DP-MS using chemical ionization and Py-GC/MS, are more effective in these identifications. An important advantage of the in-source DP-MS technique is its low matrix interference, because nonpolymeric materials are vaporized prior to polymer pyrolysis. As an example of the matrix complexity in industrial products, Figure 7a shows a Py-GC/MS chromatogram of a “polymer fraction” isolated from a commercial latex product. Since some nonpolymer compounds are not completely separated, it is difficult to differentiate the pyrolysate of a polymer from the nonpolymeric ingredients in the sample. The identification of acetic acid and unsaturated cyclic hydrocarbons suggests the possible presence of poly(vinyl acetate) in the product. Figure 7b shows in-source DP-MS analysis of the same product prior to polymer fractionation. As expected, all the nonpolymeric materials are vaporized prior to polymer pyrolysis.

Using the spectrum subtraction strategy discussed earlier, we identified the product to be a copolymer of vinyl acetate/n-butyl acrylate. This identification was later confirmed by NMR. The n-butyl acrylic monomer was neglected in the first round Py-GC/ MS analysis because of its low abundance. CONCLUSIONS In-source DP-MS is shown to be an effective means for quick identification of polymer composition in a complex sample matrix. The variations in heating rate, sample loading, ion source temperature, and polymer molecular weight have only minor impacts on the relative abundances of the pyrolysis products. Under the same experimental conditions, the technique yields highly reproducible polymer mass spectra which are suitable for library searching purposes. Using the technique and a commercial software, a polymer mass spectral library containing more than 150 polymers, copolymers, and terpolymers was developed. For qualitative compositional analysis, in-source DP-MS has the advantages of short analysis time, high confidence level, and low

matrix interference. Our study suggests that a robust standard polymer library, which is independent of laboratory and mass spectrometer type, can be developed. However, more rigorous interlaboratory tests are needed to achieve this goal. ACKNOWLEDGMENT The authors thank their colleagues at W.R. Grace, Dr. G. R. Hatfield, Dr. P. A. Dreifuss, Dr. Md. A. Mabud, and Mr. J. J Litzau for discussing and reviewing this article and S. Murray, K. Sigismonti, and P. Ernst for preparing most of the figures. The authors also thank Professors J. H. Futrell and B. Munson for supporting the comparative analysis at the University of Delaware. Finally, we thank W.R. Grace and its Washington Research Center for the permission to publish this work. Received for review October 18, 1995. Accepted January 9, 1996.X AC951046R X

Abstract published in Advance ACS Abstracts, February 15, 1996.


1027

Rapid Polymer Identification by In-Source Direct Pyrolysis Mass

Recommend Documents