Journal of Chromatography A, 1374 (2014) 231–237
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Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma
Selective separation of fluorinated compounds from complex organic mixtures by pyrolysis-comprehensive two-dimensional gas chromatography coupled to high-resolution time-of-flight mass spectrometry Yoji Nakajima ∗ , Yuko Arinami, Kiyoshi Yamamoto Asahi Glass Co., Ltd, Japan
a r t i c l e
i n f o
Article history: Received 15 September 2014 Received in revised form 23 November 2014 Accepted 24 November 2014 Available online 26 November 2014 Keywords: Comprehensive two-dimensional gas chromatography Fluorinated compound Polymer Time-of-flight mass spectrometry Pyrolysis
a b s t r a c t The usefulness of comprehensive two-dimensional gas chromatography (GC × GC) was demonstrated for the selective separation of fluorinated compounds from organic mixtures, such as kerosene/perfluorokerosene mixtures, pyrolysis products derived from polyethylene/ethylenetetrafluoroethylene alternating copolymer mixture and poly[2-(perfluorohexyl)ethyl acrylate]. Perfluorocarbons were completely separated from hydrocarbons in the two-dimensional chromatogram. Fluorohydrocarbons in the pyrolysis products of polyethylene/ethylene-tetrafluoroethylene alternating copolymer mixture were selectively isolated from their hydrocarbon counterparts and regularly arranged according to their chain length and fluorine content in the two-dimensional chromatogram. A reliable structural analysis of the fluorohydrocarbons was achieved by combining effective GC × GC positional information with accurate mass spectral data obtained by high-resolution time-of-flight mass spectrometry (HRTOF-MS). 2-(Perfluorohexyl)ethyl acrylate monomer, dimer, and trimer as well as 2(perfluorohexyl)ethyl alcohol in poly[2-(perfluorohexyl)ethyl acrylate] pyrolysis products were detected in the bottommost part of the two-dimensional chromatogram with separation from hydrocarbons possessing terminal structure information about the polymer, such as ␣-methylstyrene. PyrolysisGC × GC/HRTOF-MS appeared particularly suitable for the characterization of fluorinated polymer microstructures, such as monomer sequences and terminal groups. © 2014 Elsevier B.V. All rights reserved.
1. Introduction In recent years, fluorinated compounds have rapidly emerged as attractive substances for many fields, such as coatings, lubricants, films, electric materials, optical materials, refrigerants, food containers, analytical reagents, medical devices, and pharmaceuticals [1–5]. A variety of these compounds have widely spread throughout the environment, including in wildlife and humans [6–9]. Their selective separation from highly complex organic mixtures, such as biological tissues, environmental samples, and polymer pyrolysis products, is crucial to understanding their pharmacokinetics, environmental fate, and structure–property relationship [6–9]. Although pyrolysis (Py)-gas chromatography (GC)/mass
∗ Corresponding author at: Asahi Glass Co., Ltd. Research Center, 1150 Hazawacho, Kanagawa-ku, Yokohama 221-8755, Japan. Tel.: +81 45 374 7221; fax: +81 45 374 8892. E-mail address: [email protected] (Y. Nakajima). http://dx.doi.org/10.1016/j.chroma.2014.11.062 0021-9673/© 2014 Elsevier B.V. All rights reserved.
spectrometry (MS) is often used to structurally characterize polymers, including fluorinated polymers [10–12], the pyrolysis of fluorinated polymers often generates highly complex organic mixtures that cannot be completely resolved by conventional onedimensional (1D) GC [11,13]. Over the past decades, comprehensive two-dimensional gas chromatography (GC × GC) has been developed and applied to separate highly complex organic mixtures, such as petroleum, geochemical, and environmental samples [14–18]. Its separation ability is several tens of times higher than 1D GC. In addition to an enhanced separation, it facilitates analyte identification because organic compounds are detected in an orderly fashion according to their volatility and polarity in the two-dimensional (2D) chromatogram. Furthermore, its coupling with high-resolution time-of-flight mass spectrometry (HRTOF-MS) has proven a very powerful analytical approach for complex organic mixtures [14,16,19]. Few reports have addressed GC × GC analyses of fluorinated compounds. Korytár et al. have reported that mono- and di-fluorinated brominated diphenyl ethers (BDEs) can be separated
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from their non-fluorinated counterparts in the 2D chromatogram [20]. A few synthetic polymer analyses have been attempted by pyrolysis-comprehensive two-dimensional gas chromatography (Py-GC × GC). Eckerle et al. have characterized the short-chain branch structure in polyethylene (PE) by Py-GC × GC [21]. However, this technique has not been utilized for the structural characterization of fluorinated polymers to date. This study demonstrates the usefulness of GC × GC/HRTOF-MS for the selective separation of fluorinated compounds from organic mixtures, such as kerosene/perfluorokerosene mixture, pyrolysis products originating from PE/ethylene (E)–tetrafluoroethylene (TFE) alternating copolymer mixture, and poly[2-(perfluorohexyl)ethyl acrylate]. 2. Material and methods 2.1. Materials Kerosene, perfluorokerosene, and PE (HI-ZEX MILLION® 145 M) were purchased from Wako Pure Chemical Industries (Osaka, Japan), Tokyo Kasei Kogyo (Tokyo, Japan), and Mitsui Chemical (Tokyo, Japan), respectively. The ethylene (E)tetrafluoroethylene (TFE) alternating copolymer (E/TFE molar ratio = 46/54) was polymerized from the corresponding monomers in a 3,3-dichloro-1,1,1,2,2-pentafluoropropane/1,3-dichloro1,1,2,2,3-pentafluoropropane mixture (AK-225® , Asahi Glass). Poly[2-(perfluorohexyl)ethyl acrylate] was polymerized from the corresponding monomer using N,N -azobis(isobutyronitrile) as an initiator and cumyl dithiobenzoate as a chain transfer agent in AK-225® . 2.2. Methods The Py-GC × GC/HRTOF-MS system comprised a pyrolyzer (PY2020iD, Frontier Lab Corporation, Koriyama, Fukushima, Japan) and a Zoex KT2006 GC × GC jet modulator (Zoex Corporation, Houston, TX, USA) installed on an Agilent 7890 gas chromatograph (Agilent Technologies, Palo Alto, CA, USA) connected to a JEOL AccuTOF HRTOF-MS instrument (JEOL Corporation, Tokyo, Japan). A pyrolysis sample cup containing about 0.1 mg of sample material was dropped into the pyrolyzer microfurnace. The pyrolysis temperature was set at 600 ◦ C for the polymer analysis and 260 ◦ C for the kerosene/perfluorokerosene mixture analysis. The pyrolyzer–GC interface and the GC inlet were kept at 270 and 260 ◦ C, respectively. The GC oven was held at 40 ◦ C for 5 min before heating to 260 ◦ C at a rate of 3 ◦ C min−1 . The separation was performed on the column set of a DB-1MS fused silica capillary column (30 m × 0.25 mm i.d., 0.25 m film thickness, Agilent Technologies) as the first column and a DB-17 fused silica capillary column (2 m × 0.10 mm i.d., 0.10 m film thickness, Agilent Technologies) as the second column. Helium used as a carrier gas for the GC apparatus was supplied at a rate of 1.50 mL min−1 . The modulation period was set to 4 s. The modulator hot gas was maintained at 200 ◦ C for 5 min before heating to 350 ◦ C at a rate of 3 ◦ C min−1 . The hot gas duration time was 250 ms. The HRTOF-MS was operated at a multi-channel plate voltage of 1900 V and a constant resolving power of approximately 5000 FWHM over a mass range of m/z 45–800 using electron impact ionization (electron-accelerating voltage: 70 V). The data were acquired at 25 Hz. Perfluorokerosene was introduced into the HRTOF-MS ion source during the initial seconds of an analytical run for the m/z calibration. A raw data analysis was conducted using the MassCenter software (JEOL). Contour plots (2D chromatogram) were analyzed using the GC Image software (Zoex). Non-fluorinated compounds were identified based on the National Institute of Standards and Technology (NIST) MS library. Fluorinated compounds that were not recorded in the library were
identified based on fragmentation analyses of accurate mass spectra obtained by HRTOF-MS while referring to the known pyrolysis products of ETFE and fluorinated acrylic polymers described in the previous reports [13,22]. 3. Results and discussion 3.1. Separation of kerosene/perfluorokerosene mixture Fig. 1 shows (A) 1D and (B) 2D total ion current (TIC) chromatograms of the kerosene/perfluorokerosene (50/50 wt%) mixture. Abscissa and ordinate axes represent the retention times in the first non-polar column and in the second polar column, respectively. Petroleum derivatives, such as kerosene, have been extensively studied by GC × GC [17,23,24]. Kerosene components were spread over the upper part of the 2D chromatogram and consisted of highly ordered structures based on their physicochemical properties. These components were eluted according to their boiling point in the first dimension and in terms of their polarity, which was categorized into four chemical groups, i.e., alkanes, naphthenes, mono-aromatics, and di-aromatics, in the second dimension. Alkanes generally appear at the shortest retention times in the second dimension for natural organic compounds, such as hydrocarbons and biologically derived compounds. Therefore, the region below the alkanes in the second dimension is often called “the dead band” and does not show any peaks. Interestingly, fully fluorinated kerosene, or perfluorokerosene, was eluted in the dead band and was completely separated from its hydrocarbon counterparts (kerosene). Highly fluorinated organic compounds usually exhibit unique partition, sorption, and chromatographic properties and often generate a so-called fluorous phase that does not mix with either water or hydrocarbon phases [25–29]. Goss and Bronner have suggested that highly fluorinated compounds show a weaker van der Waals force than their non-fluorinated counterparts, resulting in their unique partition property [29]. Therefore, the weaker van der Waals force may reduce interactions between the highly fluorinated compounds and the GC columns, significantly shortening their retention times as compared to their hydrocarbon counterparts. 3.2. Separation of pyrolysis products of fluorinated polymers The ETFE alternating copolymer (Fig. 2) is a representative fluorinated polymer exhibiting remarkable thermal stability, chemical durability, electric property, and mechanical strength, making it interesting for various industrial applications, such as chemical equipment materials, wire-coating insulations, and covering sheets for sports dorms [30]. Therefore, a detailed structural characterization is considerably important for ETFE product development and manufacturing. However, such a structural characterization has encountered technical difficulties because of the insolubility of ETFE in any solvent. Py-GC/MS has emerged as a powerful technique for the structural characterization of insoluble polymers, such as fluorinated polymers [11–13]. Here, PE, the hydrocarbon counterpart of ETFE, acted as an analytical reference in a mixture with ETFE. The Py-GC × GC/HRTOF-MS analysis of the PE/ETFE (50/50 wt%) mixture provides highly structured 2D TIC chromatogram (Fig. 3). Pyrolyzed PE mainly produced n-alkanes, n-alkenes, and n-alkadienes with different numbers of carbon atoms [11,21]. These products were eluted as clusters (Fig. 3(B)). In each cluster, retention times followed the order: n-alkane > n-alkene > n-alkadiene in the first dimension and nalkane < n-alkene < n-alkadiene in the second dimension. On the other hand, ETFE pyrolysis products were eluted in the dead band of the 2D chromatogram and separated from those of
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Fig. 1. (A) Reconstructed 1D and (B) 2D TIC chromatograms of the kerosene/perfluorokerosene mixture obtained by GC × GC/HRTOF-MS. The reconstructed 1D chromatogram was generated with GC image software using the 2D chromatogram. The label Cn indicates the number of carbon atoms (n) in n-alkanes.
PE. These products mainly consisted of unsaturated fluorohydrocarbons with one degree of unsaturation and several degrees of fluorination (fluorine content). Retention times (tR) in both dimensions and retention indices (I) in the first dimension GC are shown in Table 1 for the representative ETFE pyrolysis fluorohydrocarbon products (peaks a1–d5, Fig. 3). Each retention index I in GC was calculated using [31] as follows: Ix = 100n +
100 (tRx − tRn ) (tRn+1 − tRn )
(1)
where x denotes the target compound, n indicates the number of carbon atoms for n-alkanes Cn H2n+2 eluted right before x, and tR represents the retention time in the first dimension GC. Fluorohydrocarbons were regularly ordered by chain length (number of carbon atoms) and fluorine content (wt%) in the 2D
chromatogram (Fig. 3(B)). All fluorohydrocarbons were eluted earlier than their hydrocarbon counterparts in both dimensions. In the second dimension, fluorohydrocarbons with a relatively high fluorine content tended to be eluted earlier (Fig. 4). A band of peaks, which distributes over the right bottommost part of the 2D chromatogram (e.g., see Fig. 3), consists of column bleeds. Fluorinated polymers often generate hydrogen fluoride upon pyrolysis, causing additional column degradation and column bleeds. In conventional GC, these column bleeds often interfere with the target compound peaks. In contrast, these bleeds were distinct from the target compound peaks of GC × GC. Most pyrolysis products of fluorinated polymers are not recorded in the NIST MS library. Therefore, their structures were tentatively deduced on the basis of the nominal masses of molecular and fragment ions. In addition, HRTOF-MS can improve the
Fig. 2. Chemical structures of ETFE alternating copolymer and poly[2-(perfluorohexyl)ethyl acrylate].
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Fig. 3. 2D TIC chromatogram of the PE/ETFE mixture obtained by Py-GC × GC/HRTOF-MS (A) and its enlarged view (B). The label Cn indicates the number of carbon atoms (n) in n-alkanes. Numbers in red correspond to peak labels for representative ETFE pyrolysis products. Peak assignments are listed in Table 1. Numbers in parentheses indicate the fluorine content (wt%) of the compounds.
reliability of a structural analysis by providing accurate mass measurements of molecular and fragment ions, as shown in Fig. 5 and Table 2 for compound a3 (Fig. 3). Measured m/z values of molecular and fragment ions were in good agreement with theoretical m/z values within an accuracy of 4.2 mDa. The mass spectrum showed that the molecular ion (M+ ) corresponded to m/z 434.0673, consistent with the formula C13 H12 F14 . In addition to the molecular ions, several fragment ions were also observed in the higher m/z region. For m/z 338.0505 and 255.0389, the formulas were C10 H9 F11 and C8 H7 F8 , respectively, which
in turn corresponded to the losses of C3 H3 F3 (fluorohydrocarbon olefin) and C5 H5 F6 from compound a3. Prominent fragment ions in the lower m/z region contained m/z 51.0031, 77.0224, 127.0149, and 147.0216, which matched CHF2 , C3 H3 F2 , C4 H3 F4 , and C4 H4 F5 , respectively. This spectral evidence and a consideration of the main chain structure of ETFE, which consists mainly of alternating monomer units [30] suggest that compound a3 is 1,1,4,4,5,5,8,8,9,9,12,12,13,13-tetradecafluorotridec-1-ene (Fig. 5). Similarly, accurate mass spectra are expected to provide useful information for the identification of pyrolysis products of
Table 1 Peak labels, formulas, fluorine contents (wt%), second dimension tR (s), first dimension tR (min), and first dimension I for representative ETFE pyrolysis products in Fig.3. Peak label
Formula
Fluorine content (wt%)
2nd D tR (s)
1st D tR (min)
1st D I
a1 a2 a3 a4 a5 b1 b2 b3 b4 b5 c1 c2 c3 c4 c5 d1 d2 d3 d4 d5
C11 H12 F10 C12 H12 F12 C13 H12 F14 C14 H12 F16 C15 H12 F18 C15 H16 F14 C16 H16 F16 C17 H16 F18 C18 H16 F20 C19 H16 F22 C19 H20 F18 C20 H20 F20 C21 H20 F22 C22 H20 F24 C23 H20 F26 C23 H24 F22 C24 H24 F24 C25 H24 F26 C26 H24 F28 C27 H24 F30
56.9 59.4 61.3 62.8 64.0 57.6 59.4 60.9 62.1 63.1 58.0 59.4 60.6 61.6 62.5 58.2 59.4 60.4 61.3 62.1
0.66 0.54 0.46 0.42 0.40 0.54 0.46 0.44 0.42 0.38 0.54 0.46 0.42 0.42 0.42 0.50 0.46 0.42 0.42 0.42
26.09 26.69 31.02 31.49 35.56 39.76 40.16 43.76 44.16 47.56 51.09 51.42 54.49 54.82 57.76 60.82 61.02 63.69 63.96 66.49
1078 1090 1179 1189 1279 1379 1389 1480 1490 1580 1681 1690 1780 1790 1883 1984 1991 2084 2093 2185
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Fig. 4. Relationship between fluorine content (wt%) and second dimension tR for representative ETFE pyrolysis products. Table 2 Measured and calculated m/z values, mass errors (mDa), and formulas for major fragment ions of compound a3 (Fig. 5). Measured m/z
Calculated m/z
Mass error (mDa)
Formula
51.0031 77.0224 127.0149 147.0216 255.0389 338.0505 434.0673
51.0046 77.0203 127.0171 147.0233 255.0422 338.0529 434.0715
−1.5 2.1 −2.2 −1.7 −3.3 −2.4 −4.2
CHF2 C3 H3 F2 C4 H3 F4 C4 H4 F5 C8 H7 F8 C10 H9 F11 C13 H12 F14
fluorinated polymers. A combination of these accurate mass spectral analyses with GC × GC positions, which are regularly arranged according to the structural features of the pyrolysis products (e.g., chain length and fluorine content), also enables an highly reliable and efficient identification of these products. Fluorinated acrylate (FA) polymers, such as poly[2(perfluoroalkyl)ethyl acrylate], are widely used as protective, water–oil repellent coating agents in textile finishes, carpets, paper, and upholstery [22,32]. In particular, copolymers involving 2-(perfluoroalkyl)ethyl acrylate and non-fluorinated monomers,
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such as alkyl acrylate and vinyl chloride, find wide commercial applications as coating agents [22,32]. The microstructural analysis of 2-(perfluoroalkyl)ethyl acrylate homo- and copolymers is important for providing a better understanding of the structure–property relationships necessary for the development of new protective coating agents. Fig. 6 shows the 2D TIC chromatogram of pyrolysis products of poly[2-(perfluorohexyl)ethyl acrylate] (FA homopolymer, Fig. 2). The main chain pyrolysis products, which consisted of 2-(perfluorohexyl)ethyl alcohol (6:2 fluorotelomer alcohol (6:2 FTOH)) as well as FA monomer, dimer, and trimer, were detected in the bottommost part of the 2D chromatogram. In addition, the pyrolysis product containing the terminal structure information for the polymer, ␣-methylstyrene, appeared in the upper part of the second dimension, apart from the main chain pyrolysis products. Carbonyl and alcohol groups in the FA and 6:2 FTOH structure are considered as polar groups. Therefore, they are expected to strongly interact with the polar liquid phase of the second column by dipole–dipole and hydrogen bonding interactions, resulting in longer retention times. However, the FA-related products were eluted significantly earlier than n-alkanes (black closed circles, Fig. 6) in the second column. This suggests that the week van der Waals interactions caused by the fluorocarbon structure (C6 F13 ) impact the retention properties of FA-related products more significantly than the dipole–dipole and hydrogen bonding interactions originating from the polar carbonyl and alcohol groups. In general, FA is often copolymerized with non-fluorinated monomers to generate functional polymers [32]. GC × GC is expected to completely separate non-fluorinated pyrolysis products from the FA-related products. Consequently, PyGC × GC/HRTOF-MS seems to be particularly suitable for the characterization of fluorinated polymer microstructure, such as monomer sequences and terminal groups. In addition, GC × GC/HRTOF-MS may be effective in the selective detection of trace fluorotelomers in highly complex non-fluorinated compound mixtures, such as biological and environmental samples, during studies on the fluorotelomer properties in living species and the environment.
Fig. 5. Mass spectrum and proposed chemical structure of compound a3 shown in Fig. 3.
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Fig. 6. 2D TIC chromatogram of FA homopolymer obtained by Py-GC × GC/HRTOF-MS. For the sake of comparison, black closed circles represent the approximate retention times for n-alkanes obtained in a separate analytical run (Fig. 3).
4. Conclusion The usefulness of GC × GC for the selective separation of fluorinated compounds from hydrocarbons was demonstrated. Perfluorokerosene was completely separated from kerosene in the 2D chromatogram. Fluorohydrocarbons in the ETFE pyrolysis products were regularly arranged according to their fluorine content and chain length in the 2D chromatogram. A combination of the positional information provided by GC × GC with accurate mass spectra obtained by HRTOF-MS detection facilitated a reliable structural analysis of the pyrolysis products. Along with 6:2 FTOH, FA monomer, dimer, and trimer were observed in the bottommost part of the 2D chromatogram. These compounds were distinct from hydrocarbons containing the terminal structure information about the polymer. Consequently, this combination offers effective synergies for the microstructural analysis of fluorinated polymers. Furthermore, GC × GC/HRTOF-MS may also be useful for the selective separation and structural analysis of fluorinated compounds in biological and environmental samples, which is essential for a better understanding of properties and fates of the fluorinated compounds in life and the environment. Acknowledgments We thank A. Funaki, M. Sasaki, and T. Kakiuchi (Asahi Glass Co., Ltd.) for their help during sample preparation. We thank K. Shirai and T. Ishii (JEOL Co., Ltd.), and T. Ieda (GERSTEL K.K. Japan) for their kind technical support. We also thank the editor and two anonymous reviewers for their constructive comments, which helped us to improve the manuscript. References [1] M. Pagliaro, R. Ciriminna, New fluorinated functional materials, J. Mater. Chem. 15 (2005) 4981–4991.
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Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma
Selective separation of fluorinated compounds from complex organic mixtures by pyrolysis-comprehensive two-dimensional gas chromatography coupled to high-resolution time-of-flight mass spectrometry Yoji Nakajima ∗ , Yuko Arinami, Kiyoshi Yamamoto Asahi Glass Co., Ltd, Japan
a r t i c l e
i n f o
Article history: Received 15 September 2014 Received in revised form 23 November 2014 Accepted 24 November 2014 Available online 26 November 2014 Keywords: Comprehensive two-dimensional gas chromatography Fluorinated compound Polymer Time-of-flight mass spectrometry Pyrolysis
a b s t r a c t The usefulness of comprehensive two-dimensional gas chromatography (GC × GC) was demonstrated for the selective separation of fluorinated compounds from organic mixtures, such as kerosene/perfluorokerosene mixtures, pyrolysis products derived from polyethylene/ethylenetetrafluoroethylene alternating copolymer mixture and poly[2-(perfluorohexyl)ethyl acrylate]. Perfluorocarbons were completely separated from hydrocarbons in the two-dimensional chromatogram. Fluorohydrocarbons in the pyrolysis products of polyethylene/ethylene-tetrafluoroethylene alternating copolymer mixture were selectively isolated from their hydrocarbon counterparts and regularly arranged according to their chain length and fluorine content in the two-dimensional chromatogram. A reliable structural analysis of the fluorohydrocarbons was achieved by combining effective GC × GC positional information with accurate mass spectral data obtained by high-resolution time-of-flight mass spectrometry (HRTOF-MS). 2-(Perfluorohexyl)ethyl acrylate monomer, dimer, and trimer as well as 2(perfluorohexyl)ethyl alcohol in poly[2-(perfluorohexyl)ethyl acrylate] pyrolysis products were detected in the bottommost part of the two-dimensional chromatogram with separation from hydrocarbons possessing terminal structure information about the polymer, such as ␣-methylstyrene. PyrolysisGC × GC/HRTOF-MS appeared particularly suitable for the characterization of fluorinated polymer microstructures, such as monomer sequences and terminal groups. © 2014 Elsevier B.V. All rights reserved.
1. Introduction In recent years, fluorinated compounds have rapidly emerged as attractive substances for many fields, such as coatings, lubricants, films, electric materials, optical materials, refrigerants, food containers, analytical reagents, medical devices, and pharmaceuticals [1–5]. A variety of these compounds have widely spread throughout the environment, including in wildlife and humans [6–9]. Their selective separation from highly complex organic mixtures, such as biological tissues, environmental samples, and polymer pyrolysis products, is crucial to understanding their pharmacokinetics, environmental fate, and structure–property relationship [6–9]. Although pyrolysis (Py)-gas chromatography (GC)/mass
∗ Corresponding author at: Asahi Glass Co., Ltd. Research Center, 1150 Hazawacho, Kanagawa-ku, Yokohama 221-8755, Japan. Tel.: +81 45 374 7221; fax: +81 45 374 8892. E-mail address: [email protected] (Y. Nakajima). http://dx.doi.org/10.1016/j.chroma.2014.11.062 0021-9673/© 2014 Elsevier B.V. All rights reserved.
spectrometry (MS) is often used to structurally characterize polymers, including fluorinated polymers [10–12], the pyrolysis of fluorinated polymers often generates highly complex organic mixtures that cannot be completely resolved by conventional onedimensional (1D) GC [11,13]. Over the past decades, comprehensive two-dimensional gas chromatography (GC × GC) has been developed and applied to separate highly complex organic mixtures, such as petroleum, geochemical, and environmental samples [14–18]. Its separation ability is several tens of times higher than 1D GC. In addition to an enhanced separation, it facilitates analyte identification because organic compounds are detected in an orderly fashion according to their volatility and polarity in the two-dimensional (2D) chromatogram. Furthermore, its coupling with high-resolution time-of-flight mass spectrometry (HRTOF-MS) has proven a very powerful analytical approach for complex organic mixtures [14,16,19]. Few reports have addressed GC × GC analyses of fluorinated compounds. Korytár et al. have reported that mono- and di-fluorinated brominated diphenyl ethers (BDEs) can be separated
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from their non-fluorinated counterparts in the 2D chromatogram [20]. A few synthetic polymer analyses have been attempted by pyrolysis-comprehensive two-dimensional gas chromatography (Py-GC × GC). Eckerle et al. have characterized the short-chain branch structure in polyethylene (PE) by Py-GC × GC [21]. However, this technique has not been utilized for the structural characterization of fluorinated polymers to date. This study demonstrates the usefulness of GC × GC/HRTOF-MS for the selective separation of fluorinated compounds from organic mixtures, such as kerosene/perfluorokerosene mixture, pyrolysis products originating from PE/ethylene (E)–tetrafluoroethylene (TFE) alternating copolymer mixture, and poly[2-(perfluorohexyl)ethyl acrylate]. 2. Material and methods 2.1. Materials Kerosene, perfluorokerosene, and PE (HI-ZEX MILLION® 145 M) were purchased from Wako Pure Chemical Industries (Osaka, Japan), Tokyo Kasei Kogyo (Tokyo, Japan), and Mitsui Chemical (Tokyo, Japan), respectively. The ethylene (E)tetrafluoroethylene (TFE) alternating copolymer (E/TFE molar ratio = 46/54) was polymerized from the corresponding monomers in a 3,3-dichloro-1,1,1,2,2-pentafluoropropane/1,3-dichloro1,1,2,2,3-pentafluoropropane mixture (AK-225® , Asahi Glass). Poly[2-(perfluorohexyl)ethyl acrylate] was polymerized from the corresponding monomer using N,N -azobis(isobutyronitrile) as an initiator and cumyl dithiobenzoate as a chain transfer agent in AK-225® . 2.2. Methods The Py-GC × GC/HRTOF-MS system comprised a pyrolyzer (PY2020iD, Frontier Lab Corporation, Koriyama, Fukushima, Japan) and a Zoex KT2006 GC × GC jet modulator (Zoex Corporation, Houston, TX, USA) installed on an Agilent 7890 gas chromatograph (Agilent Technologies, Palo Alto, CA, USA) connected to a JEOL AccuTOF HRTOF-MS instrument (JEOL Corporation, Tokyo, Japan). A pyrolysis sample cup containing about 0.1 mg of sample material was dropped into the pyrolyzer microfurnace. The pyrolysis temperature was set at 600 ◦ C for the polymer analysis and 260 ◦ C for the kerosene/perfluorokerosene mixture analysis. The pyrolyzer–GC interface and the GC inlet were kept at 270 and 260 ◦ C, respectively. The GC oven was held at 40 ◦ C for 5 min before heating to 260 ◦ C at a rate of 3 ◦ C min−1 . The separation was performed on the column set of a DB-1MS fused silica capillary column (30 m × 0.25 mm i.d., 0.25 m film thickness, Agilent Technologies) as the first column and a DB-17 fused silica capillary column (2 m × 0.10 mm i.d., 0.10 m film thickness, Agilent Technologies) as the second column. Helium used as a carrier gas for the GC apparatus was supplied at a rate of 1.50 mL min−1 . The modulation period was set to 4 s. The modulator hot gas was maintained at 200 ◦ C for 5 min before heating to 350 ◦ C at a rate of 3 ◦ C min−1 . The hot gas duration time was 250 ms. The HRTOF-MS was operated at a multi-channel plate voltage of 1900 V and a constant resolving power of approximately 5000 FWHM over a mass range of m/z 45–800 using electron impact ionization (electron-accelerating voltage: 70 V). The data were acquired at 25 Hz. Perfluorokerosene was introduced into the HRTOF-MS ion source during the initial seconds of an analytical run for the m/z calibration. A raw data analysis was conducted using the MassCenter software (JEOL). Contour plots (2D chromatogram) were analyzed using the GC Image software (Zoex). Non-fluorinated compounds were identified based on the National Institute of Standards and Technology (NIST) MS library. Fluorinated compounds that were not recorded in the library were
identified based on fragmentation analyses of accurate mass spectra obtained by HRTOF-MS while referring to the known pyrolysis products of ETFE and fluorinated acrylic polymers described in the previous reports [13,22]. 3. Results and discussion 3.1. Separation of kerosene/perfluorokerosene mixture Fig. 1 shows (A) 1D and (B) 2D total ion current (TIC) chromatograms of the kerosene/perfluorokerosene (50/50 wt%) mixture. Abscissa and ordinate axes represent the retention times in the first non-polar column and in the second polar column, respectively. Petroleum derivatives, such as kerosene, have been extensively studied by GC × GC [17,23,24]. Kerosene components were spread over the upper part of the 2D chromatogram and consisted of highly ordered structures based on their physicochemical properties. These components were eluted according to their boiling point in the first dimension and in terms of their polarity, which was categorized into four chemical groups, i.e., alkanes, naphthenes, mono-aromatics, and di-aromatics, in the second dimension. Alkanes generally appear at the shortest retention times in the second dimension for natural organic compounds, such as hydrocarbons and biologically derived compounds. Therefore, the region below the alkanes in the second dimension is often called “the dead band” and does not show any peaks. Interestingly, fully fluorinated kerosene, or perfluorokerosene, was eluted in the dead band and was completely separated from its hydrocarbon counterparts (kerosene). Highly fluorinated organic compounds usually exhibit unique partition, sorption, and chromatographic properties and often generate a so-called fluorous phase that does not mix with either water or hydrocarbon phases [25–29]. Goss and Bronner have suggested that highly fluorinated compounds show a weaker van der Waals force than their non-fluorinated counterparts, resulting in their unique partition property [29]. Therefore, the weaker van der Waals force may reduce interactions between the highly fluorinated compounds and the GC columns, significantly shortening their retention times as compared to their hydrocarbon counterparts. 3.2. Separation of pyrolysis products of fluorinated polymers The ETFE alternating copolymer (Fig. 2) is a representative fluorinated polymer exhibiting remarkable thermal stability, chemical durability, electric property, and mechanical strength, making it interesting for various industrial applications, such as chemical equipment materials, wire-coating insulations, and covering sheets for sports dorms [30]. Therefore, a detailed structural characterization is considerably important for ETFE product development and manufacturing. However, such a structural characterization has encountered technical difficulties because of the insolubility of ETFE in any solvent. Py-GC/MS has emerged as a powerful technique for the structural characterization of insoluble polymers, such as fluorinated polymers [11–13]. Here, PE, the hydrocarbon counterpart of ETFE, acted as an analytical reference in a mixture with ETFE. The Py-GC × GC/HRTOF-MS analysis of the PE/ETFE (50/50 wt%) mixture provides highly structured 2D TIC chromatogram (Fig. 3). Pyrolyzed PE mainly produced n-alkanes, n-alkenes, and n-alkadienes with different numbers of carbon atoms [11,21]. These products were eluted as clusters (Fig. 3(B)). In each cluster, retention times followed the order: n-alkane > n-alkene > n-alkadiene in the first dimension and nalkane < n-alkene < n-alkadiene in the second dimension. On the other hand, ETFE pyrolysis products were eluted in the dead band of the 2D chromatogram and separated from those of
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Fig. 1. (A) Reconstructed 1D and (B) 2D TIC chromatograms of the kerosene/perfluorokerosene mixture obtained by GC × GC/HRTOF-MS. The reconstructed 1D chromatogram was generated with GC image software using the 2D chromatogram. The label Cn indicates the number of carbon atoms (n) in n-alkanes.
PE. These products mainly consisted of unsaturated fluorohydrocarbons with one degree of unsaturation and several degrees of fluorination (fluorine content). Retention times (tR) in both dimensions and retention indices (I) in the first dimension GC are shown in Table 1 for the representative ETFE pyrolysis fluorohydrocarbon products (peaks a1–d5, Fig. 3). Each retention index I in GC was calculated using [31] as follows: Ix = 100n +
100 (tRx − tRn ) (tRn+1 − tRn )
(1)
where x denotes the target compound, n indicates the number of carbon atoms for n-alkanes Cn H2n+2 eluted right before x, and tR represents the retention time in the first dimension GC. Fluorohydrocarbons were regularly ordered by chain length (number of carbon atoms) and fluorine content (wt%) in the 2D
chromatogram (Fig. 3(B)). All fluorohydrocarbons were eluted earlier than their hydrocarbon counterparts in both dimensions. In the second dimension, fluorohydrocarbons with a relatively high fluorine content tended to be eluted earlier (Fig. 4). A band of peaks, which distributes over the right bottommost part of the 2D chromatogram (e.g., see Fig. 3), consists of column bleeds. Fluorinated polymers often generate hydrogen fluoride upon pyrolysis, causing additional column degradation and column bleeds. In conventional GC, these column bleeds often interfere with the target compound peaks. In contrast, these bleeds were distinct from the target compound peaks of GC × GC. Most pyrolysis products of fluorinated polymers are not recorded in the NIST MS library. Therefore, their structures were tentatively deduced on the basis of the nominal masses of molecular and fragment ions. In addition, HRTOF-MS can improve the
Fig. 2. Chemical structures of ETFE alternating copolymer and poly[2-(perfluorohexyl)ethyl acrylate].
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Fig. 3. 2D TIC chromatogram of the PE/ETFE mixture obtained by Py-GC × GC/HRTOF-MS (A) and its enlarged view (B). The label Cn indicates the number of carbon atoms (n) in n-alkanes. Numbers in red correspond to peak labels for representative ETFE pyrolysis products. Peak assignments are listed in Table 1. Numbers in parentheses indicate the fluorine content (wt%) of the compounds.
reliability of a structural analysis by providing accurate mass measurements of molecular and fragment ions, as shown in Fig. 5 and Table 2 for compound a3 (Fig. 3). Measured m/z values of molecular and fragment ions were in good agreement with theoretical m/z values within an accuracy of 4.2 mDa. The mass spectrum showed that the molecular ion (M+ ) corresponded to m/z 434.0673, consistent with the formula C13 H12 F14 . In addition to the molecular ions, several fragment ions were also observed in the higher m/z region. For m/z 338.0505 and 255.0389, the formulas were C10 H9 F11 and C8 H7 F8 , respectively, which
in turn corresponded to the losses of C3 H3 F3 (fluorohydrocarbon olefin) and C5 H5 F6 from compound a3. Prominent fragment ions in the lower m/z region contained m/z 51.0031, 77.0224, 127.0149, and 147.0216, which matched CHF2 , C3 H3 F2 , C4 H3 F4 , and C4 H4 F5 , respectively. This spectral evidence and a consideration of the main chain structure of ETFE, which consists mainly of alternating monomer units [30] suggest that compound a3 is 1,1,4,4,5,5,8,8,9,9,12,12,13,13-tetradecafluorotridec-1-ene (Fig. 5). Similarly, accurate mass spectra are expected to provide useful information for the identification of pyrolysis products of
Table 1 Peak labels, formulas, fluorine contents (wt%), second dimension tR (s), first dimension tR (min), and first dimension I for representative ETFE pyrolysis products in Fig.3. Peak label
Formula
Fluorine content (wt%)
2nd D tR (s)
1st D tR (min)
1st D I
a1 a2 a3 a4 a5 b1 b2 b3 b4 b5 c1 c2 c3 c4 c5 d1 d2 d3 d4 d5
C11 H12 F10 C12 H12 F12 C13 H12 F14 C14 H12 F16 C15 H12 F18 C15 H16 F14 C16 H16 F16 C17 H16 F18 C18 H16 F20 C19 H16 F22 C19 H20 F18 C20 H20 F20 C21 H20 F22 C22 H20 F24 C23 H20 F26 C23 H24 F22 C24 H24 F24 C25 H24 F26 C26 H24 F28 C27 H24 F30
56.9 59.4 61.3 62.8 64.0 57.6 59.4 60.9 62.1 63.1 58.0 59.4 60.6 61.6 62.5 58.2 59.4 60.4 61.3 62.1
0.66 0.54 0.46 0.42 0.40 0.54 0.46 0.44 0.42 0.38 0.54 0.46 0.42 0.42 0.42 0.50 0.46 0.42 0.42 0.42
26.09 26.69 31.02 31.49 35.56 39.76 40.16 43.76 44.16 47.56 51.09 51.42 54.49 54.82 57.76 60.82 61.02 63.69 63.96 66.49
1078 1090 1179 1189 1279 1379 1389 1480 1490 1580 1681 1690 1780 1790 1883 1984 1991 2084 2093 2185
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Fig. 4. Relationship between fluorine content (wt%) and second dimension tR for representative ETFE pyrolysis products. Table 2 Measured and calculated m/z values, mass errors (mDa), and formulas for major fragment ions of compound a3 (Fig. 5). Measured m/z
Calculated m/z
Mass error (mDa)
Formula
51.0031 77.0224 127.0149 147.0216 255.0389 338.0505 434.0673
51.0046 77.0203 127.0171 147.0233 255.0422 338.0529 434.0715
−1.5 2.1 −2.2 −1.7 −3.3 −2.4 −4.2
CHF2 C3 H3 F2 C4 H3 F4 C4 H4 F5 C8 H7 F8 C10 H9 F11 C13 H12 F14
fluorinated polymers. A combination of these accurate mass spectral analyses with GC × GC positions, which are regularly arranged according to the structural features of the pyrolysis products (e.g., chain length and fluorine content), also enables an highly reliable and efficient identification of these products. Fluorinated acrylate (FA) polymers, such as poly[2(perfluoroalkyl)ethyl acrylate], are widely used as protective, water–oil repellent coating agents in textile finishes, carpets, paper, and upholstery [22,32]. In particular, copolymers involving 2-(perfluoroalkyl)ethyl acrylate and non-fluorinated monomers,
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such as alkyl acrylate and vinyl chloride, find wide commercial applications as coating agents [22,32]. The microstructural analysis of 2-(perfluoroalkyl)ethyl acrylate homo- and copolymers is important for providing a better understanding of the structure–property relationships necessary for the development of new protective coating agents. Fig. 6 shows the 2D TIC chromatogram of pyrolysis products of poly[2-(perfluorohexyl)ethyl acrylate] (FA homopolymer, Fig. 2). The main chain pyrolysis products, which consisted of 2-(perfluorohexyl)ethyl alcohol (6:2 fluorotelomer alcohol (6:2 FTOH)) as well as FA monomer, dimer, and trimer, were detected in the bottommost part of the 2D chromatogram. In addition, the pyrolysis product containing the terminal structure information for the polymer, ␣-methylstyrene, appeared in the upper part of the second dimension, apart from the main chain pyrolysis products. Carbonyl and alcohol groups in the FA and 6:2 FTOH structure are considered as polar groups. Therefore, they are expected to strongly interact with the polar liquid phase of the second column by dipole–dipole and hydrogen bonding interactions, resulting in longer retention times. However, the FA-related products were eluted significantly earlier than n-alkanes (black closed circles, Fig. 6) in the second column. This suggests that the week van der Waals interactions caused by the fluorocarbon structure (C6 F13 ) impact the retention properties of FA-related products more significantly than the dipole–dipole and hydrogen bonding interactions originating from the polar carbonyl and alcohol groups. In general, FA is often copolymerized with non-fluorinated monomers to generate functional polymers [32]. GC × GC is expected to completely separate non-fluorinated pyrolysis products from the FA-related products. Consequently, PyGC × GC/HRTOF-MS seems to be particularly suitable for the characterization of fluorinated polymer microstructure, such as monomer sequences and terminal groups. In addition, GC × GC/HRTOF-MS may be effective in the selective detection of trace fluorotelomers in highly complex non-fluorinated compound mixtures, such as biological and environmental samples, during studies on the fluorotelomer properties in living species and the environment.
Fig. 5. Mass spectrum and proposed chemical structure of compound a3 shown in Fig. 3.
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Fig. 6. 2D TIC chromatogram of FA homopolymer obtained by Py-GC × GC/HRTOF-MS. For the sake of comparison, black closed circles represent the approximate retention times for n-alkanes obtained in a separate analytical run (Fig. 3).
4. Conclusion The usefulness of GC × GC for the selective separation of fluorinated compounds from hydrocarbons was demonstrated. Perfluorokerosene was completely separated from kerosene in the 2D chromatogram. Fluorohydrocarbons in the ETFE pyrolysis products were regularly arranged according to their fluorine content and chain length in the 2D chromatogram. A combination of the positional information provided by GC × GC with accurate mass spectra obtained by HRTOF-MS detection facilitated a reliable structural analysis of the pyrolysis products. Along with 6:2 FTOH, FA monomer, dimer, and trimer were observed in the bottommost part of the 2D chromatogram. These compounds were distinct from hydrocarbons containing the terminal structure information about the polymer. Consequently, this combination offers effective synergies for the microstructural analysis of fluorinated polymers. Furthermore, GC × GC/HRTOF-MS may also be useful for the selective separation and structural analysis of fluorinated compounds in biological and environmental samples, which is essential for a better understanding of properties and fates of the fluorinated compounds in life and the environment. Acknowledgments We thank A. Funaki, M. Sasaki, and T. Kakiuchi (Asahi Glass Co., Ltd.) for their help during sample preparation. We thank K. Shirai and T. Ishii (JEOL Co., Ltd.), and T. Ieda (GERSTEL K.K. Japan) for their kind technical support. We also thank the editor and two anonymous reviewers for their constructive comments, which helped us to improve the manuscript. References [1] M. Pagliaro, R. Ciriminna, New fluorinated functional materials, J. Mater. Chem. 15 (2005) 4981–4991.
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