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ISSN 1615-9306 · JSSCCJ 38 (9) 1441–1624 (2015) · Vol. 38 · No. 9 · May 2015 · D 10609
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1484 Xiuli Zhou Qian-Hong Wan School of Pharmaceutical Science and Technology, Tianjin University, China Received October 26, 2014 Revised January 31, 2015 Accepted February 3, 2015
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Research Article
Separation and identification of oligomeric ethyl silicates by liquid chromatography with electrospray ionization mass spectrometry Reversed-phase liquid chromatography coupled with electrospray ionization mass spectrometry was used to study the molecular structures of components and molar mass distributions in ethyl silicate-40, a versatile liquid precursor for silicon-based materials. Identity testing by standard spectroscopic techniques showed that a commercial sample of ethyl silicate-40 was composed of linear/branched ethoxysiloxane oligomers with the silicon atoms ranging from 2 to 12 together with minor monocyclic species. Analysis of the sample by liquid chromatography coupled with evaporative light scattering detection resulted in an elution profile consisting of a series of peak clusters. Peak identification showed that the linear/branched homologous series of oligomers were eluted in the order of increasing number of silicon atoms in the molecules and the time duration (width) of the resulting peak clusters increased in the same fashion corresponding to increasing number of geometric isomers. In addition, small amounts of monocyclic oligomers present in the sample were found to be less retained than each linear/branched counterpart. Finally, the molar mass distribution parameters for ethyl silicate-40 determined by the developed method were in good agreement with the literature values. Overall, this work demonstrates that reversed-phase liquid chromatography coupled with electrospray ionization mass spectrometry is an indispensable tool for the comprehensive characterization of complex mixtures of this type. Keywords: Ethoxysiloxane oligomers / Ethyl silicate / Evaporative light scattering / Liquid chromatography with mass spectrometry / Molar mass distribution DOI 10.1002/jssc.201401184
1 Introduction Alkyl silicates, especially ethyl silicate in either the monomer or polymer form, have found widespread applications as liquid precursors of silicon-based materials [1–4]. They can be hydrolyzed and condensed to produce silica colloids that act as a binding agent for the manufacture of ceramic materials that are resistant to the highly corrosive environments and have the mechanical strength, heat resistance, and high dielectric properties. They can also be vaporized and thermally decomposed upon the surfaces of semiconductor chips to form electrically insulating layers of silica for the fabrication of integrated circuits. There are numerous other chemical applications, for example, as components of parting paints in precision casting [5], processable precursors for silica films [6,7], fibers [8], microspheres [9,10], and monolithic columns Correspondence: Professor Qian-Hong Wan, School of Pharmaceutical Science and Technology, Tianjin University, 92 Weijin Road, Tianjin 900032, China E-mail: [email protected] Fax: +86-22-2740-3650
Abbreviations: EIC, extracted ion chromatogram; ELSD, evaporative light scattering detector; SEC, size-exclusion chromatography; TIC, total ion chromatogram; TEOS, tetraethoxysilane
C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
[11–13], as crosslinkers in polymer compositions [14], and as consolidants for the preservation of artistic and architectural stone work [15]. New applications are constantly being developed. Among the variety of ethyl silicate products commercially available, ethyl silicate-40, prepared by controlled hydrolytic condensation of tetraethoxysilane (TEOS), is of particular importance for many technological applications. Despite their broad range of applications, precise information concerning the molecular structures and their distribution in ethyl silicate products has been scarce, limiting our ability to understand its processing behavior, polymer formation, materials function, and structure-property relationships. For example, the storage stability of the ethyl silicate-40 products depends upon the average sizes of ethoxysiloxane oligomers and the distribution of such sizes. Size-exclusion chromatography (SEC) has been a method of choice for determining the molar mass distributions of the oligomers in ethyl silicate-40 [16]. However, the data reported in the literature are not consistent probably because of the lack of commercially available individual oligomers as reference standards for molar mass calibration [17]. Besides, SEC provides insufficient resolution for the quantitation of individual oligomers. Spectroscopic techniques such as IR and NMR spectroscopy reveal information on the average chemical composition and end groups of the oligomeric compounds, which is however insufficient for determination of the distribution of www.jss-journal.com
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individual oligomer species [18–20]. MS with soft ionization techniques such as ESI and MALDI is a powerful tool capable of the identification and quantification of linear, branched, and cyclic species in solution, enabling oligomer distribution data to be extracted [21, 22]. However, there are very few reports in the literature on the application of ESI-MS or MALDIMS to analysis of ethyl silicates. RP-HPLC has been used extensively for the separation and quantification of synthetic oligomers/polymers over the last three decades [23–26]. A distinct advantage of this technique is that individual oligomeric species are eluted in the order of increasing number of repeat units, thus enabling well-resolved separation of individual oligomers within reasonable analysis time to be achieved under gradient elution conditions. But to date, there are no published studies showing the feasibility of applying RP-HPLC to studies of molecular structures and molar mass distribution for ethyl silicate-40 and its related oligo- and poly-siloxanes. The purpose of this work was to demonstrate the use of gradient RP-HPLC coupled with ESI-MS (LC–ESI-MS) for the separation and identification of individual oligomers in ethyl silicate-40 so that their distributions could be measured for better characterization of this commercial product. We started with testing the identity of the ethyl silicate-40 sample by standard spectroscopic techniques including IR and NMR spectroscopy and direct-probe ESI-MS. We then proceeded to fine-tuning the conditions for HPLC separation and MS identification of oligomeric species. Since the oligomers contain no chromophores in their molecules, evaporative light scattering detection (ELSD) was also used in conjunction with ESI-MS for eluate detection [27]. Finally, we compared the relative performance of direct ESI-MS, LC–ESI-MS, and LC– ELSD for the determination of molar mass averages and distributions for ethyl silicate-40.
2 Materials and methods
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trap LC/MS system (Waldbronn, Germany) equipped with an ESI source operated in the positive mode. The ionization parameters were as follows: dry temperature 350⬚C, nebulizer gas 30 psi, dry gas 8 L/min, HV capillary voltage 3850 V, HV end plate offset –500 V. The ESI-MS instrument was controlled by the ChemStation from Agilent and the spectral data were acquired and processed using Bruker Compass DataAnalysis 4.0. Aliquots of 10 L of the methanol solution of ethyl silicate-40 at 140 mg/mL were introduced directly into the ionization source at 0.4 mL/min with flow splitting ratio at 1:2. The mass spectra were recorded in the m/z range of 200–2200.
2.3 Chromatographic separation An Agilent 1200 Series HPLC system consisting of a degasser, a quaternary pump, column oven, autosampler, and UV-visible wavelength detector was used throughout. Separations of the ethoxysiloxane oligomers were carried out on a BaseLine C18 column (250 × 4.6 mm id, 5 m particle size, 100 Å pore size) at 35⬚C. A binary solvent system consisting of water (solvent A) and methanol (solvent B) was used in linear gradient mode from 90–100% B over 60 min and then held at 100% B for 10 min. The flow rate was 1 mL/min. The eluent from the outlet of the UV detector was introduced either into an ESI-MS or Alltech 500 ELSD (Deerfield, IL, USA) equipped with an oil-free air compressor. The MS parameters in LC–ESI-MS experiments were the same as for direct ESI-MS experiments. For ELSD detection, the drift tube temperature was set at 100⬚C and the air flow adjusted to 3.6 L/min corresponding to an inlet pressure of 51 kPa. The output ELSD signals were acquired and processed using a N2000 Chromatography Data System (BaseLine Chromtech Research Center, Tianjin, China). Aliquots of 20 L of the methanol solution of ethyl silicate-40 at 200 mg/mL were injected for HPLC analysis.
2.1 Chemicals Methanol (HPLC grade) and ethyl silicate-40 (technical grade, 38–42 wt% SiO2 ) were purchased from Concord Chemical Reagents (Tianjin, China) and Quzhou Ruilijie Chemical (Zhejiang, China), respectively. Deionized water prepared by Millipore Milli-Q system (Billerica, MA, USA) was used throughout.
2.4 Data analysis The number- and weight-average molar masses, Mn , Mw , of ethyl silicate-40 and their corresponding degrees of polymerization, nn , nw , were calculated by the following formulas [26, 28, 29]: Mn =
ci /
(ci /Mi )
(1)
2.2 Structural characterization The IR spectra of ethyl silicate-40 were acquired on a Tensor 27 FTIR spectrophotometer (Bruker Optics, Ettlingen, Germany) using KBr pellets. 29 Si NMR spectra were acquired using a Varian InfinityPlus 300 MHz spectrometer equipped with a 7.5 mm double-resonance MAS probe operating at 59.56 MHz with a 90⬚ pulse width. The neat ethyl silicate-40 was transferred into a PE tube for spectral measurements. Direct ESI mass spectra were acquired using an Agilent 6310 ion C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Mw =
(c i Mi )/
ci
(2)
nn =Mn /Mo
(3)
nw =Mw /Mo
(4)
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Figure 1. Acid-catalyzed hydrolysis and condensation of tetrae thoxysilane with water to produce ethoxysiloxane oligomers where n and m represent the number of repeat units and rings, respectively.
where i is the increment index; ci refers to the weight concentration of oligomer molecules with molar mass Mi , and Mo is the formula weight of the repeating unit in the macromolecules.
3 Results and discussion 3.1 Structural characterization Typically, ethyl silicate-40 is synthesized by controlled acidcatalyzed hydrolytic polycondensation of quadrafunctional TEOS as illustrated in Fig. 1. The partially hydrolyzed products may contain various amounts of monomer, linear, branched, and cyclic ethoxysiloxane oligomers depending upon the reaction conditions employed. Tight control of the molar ratios of water to TEOS in the reaction mixture below 1.5 is the key to obtain soluble processable precursors for silica. For molar ratios above 1.5, condensation of silanol groups results in insoluble cross-linked networks or gelation of TEOS [19]. For identity verification, standard spectroscopic techniques including IR and 29 Si NMR spectroscopy and directprobe ESI-MS were applied to the technical-grade ethyl silicate-40 from a commercial source. The resulting spectra are shown in Fig. 2. The absorption bands at 2978– 2894 and 1083 cm−1 are assigned to ethoxy and siloxane groups, respectively (Fig. 2A). The absence of silanol group bands at 3690 cm−1 (free Si–OH) and in the range 3400– 3200 cm−1 (hydrogen-bonded Si–OH) and in the range 950– 810 cm−1 (free Si–OH) suggests that the ethyl silicate-40 does not contain any silanol groups [30]. The 29 Si NMR spectrum of the same sample shows five groups of peaks (Fig. 2B) corresponding to silicon atoms bearing: zero siloxane units Q0 (–81.32 ppm, TEOS), one siloxane unit Q1 (–89.29 ppm, terminal units), two siloxane units Q2 (–96.66 ppm, linear units), three siloxane units Q3 (–104.5 ppm, semidendritic units), and four siloxane units Q4 (–112.3 ppm, dendritic units) [15]. There are some splittings of chemical shift for Q4 and these may result from the molecular internal cyclization reaction and the interference of adjacent silicon atoms. Besides, there are no chemical shifts for silanol groups indicating that the process of polycondensation in the reaction is complete. The presence of the Q3 and Q4 peaks indicates the ethyl silicate40 contains branched oligomers and the degree of branching calculated from the integrated peak areas of the individual species (Qi ) was 0.39 [31]. C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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The ESI mass spectrum of ethyl silicate-40 is shown in Fig. 2C. Characteristic molecular ions corresponding to each oligomer species are observed as ammonium and sodium adducts represented by molecular formulas EtO[SiO(OEt)2 ]n (SiO2 )m Et + NH4 + /Na+ . Here n and m refer to the numbers of ethoxysiloxane repeating units and rings in the oligomer molecule, respectively. In single stage MS, the linear and branched species cannot be distinguished so that they are classified as a single group for clarity. For linear/branched homologous series where m = 0, the major molecular ions are spaced by 134 Da, the formula weight of the repeating unit SiO(OEt)2 . This series is denoted as A-series with the subscript number n equal to the number of repeating units. Except for dimer and trimer, each linear/branched oligomer was preceded by a second, smaller peak giving rise to a second series of monocyclic oligomers (where m = 1) of 74 Da lower than the first. This series is denoted as B-series with the subscript number referring to the total number of silicon atoms in the molecule (n + 1). The assignments of the molecular ions to their respective oligomers are summarized in Table 1. The ethoxysiloxane oligomers with silicon atoms ranging from 2 to 12 are observed in ethyl silicate-40, and their size distribution is left skewed, a feature typical for condensation polymerization.
3.2 Oligomeric separation and identification Initial selection of chromatographic conditions for separation of ethyl silicate-40 was carried out with ELSD. Using a binary gradient consisting of water and methanol, the sample was resolved on a C18 column into eight or nine clusters of peaks with retention times ranging from 10 to 55 min (Fig. 3). Each cluster consists of four to five peaks. The baseline drifted upward with the retention time, suggesting ELSD signals susceptible to the solvent changes occurred in the gradient system. Attempts to improve the separation in a single run were made by using different solvents such as acetonitrile and tetrahydrofuran under various gradient elution conditions but met with limited success (data not shown). The gradient elution HPLC conditions chosen with ELSD were adopted for LC–ESI-MS studies of ethyl silicate-40. Figure 4 shows the total-ion chromatogram (TIC) of ethyl silicate-40. Several peak clusters are recognizable even though the separation does not appear as sharp as with ELSD. It is also noted that the MS detection is more sensitive than ELSD especially for low mass oligomers with retention times less than 15 min. No baseline drift was observed with the MS detection. These contrasts may be attributable to the different detection mechanisms involved in two techniques. Figure 5A and B show the respective extracted ion chromatograms (EICs) for the molecular ions of the linear/branched (A-series) and monocyclic (B-series) oligomers. Both groups of the oligomers are eluted indeed in the order of increasing chain length. The B-series is less retained than the A-series of the same number of silicon atoms and therefore peak-overlaps between A- and B-series are common. It is noted that each www.jss-journal.com
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Figure 2. (A) IR, (B) 29 Si NMR, and (C) ESI-mass spectrum of ethyl silicate-40. The labels Qi in (B) refer to the silicon atoms bearing i number of siloxane (Si–O–Si) units from 0 to 4 and the % values represent the percentages of integrated peak areas for each species.
Figure 3. Separation of oligomeric ethyl sili cate-40 by LC–ELSD. Conditions: column, Baseline C18 (250 mm × 4.6 mm id, 5 m particles); column temperature, 35⬚C;mobile phase, water (solvent A), and methanol (solvent B) gradient elution from 90–100% B over 60 min and held at 100% B for 10 min; detection, ELSD (drift tube temperature, 100⬚C, nebulizing gas flow, 3.6 L/min, and the inlet pressure, 51 kPa). Peak identification: A-series, An ; B-series, B(n+1) , where n refers to the number of repeat units in the molecule.
oligomer is eluted as a peak envelope consisting of isomeric species, the time duration (width) of which is increased with the retention time considerably due to partial resolution of the geometric isomers during the chromatographic process. This is especially the case for high mass oligomers that have a large number of possible isomeric structures. Deconvolution or separate isomer identification of the peak envelope has not been possible without performing a MS/MS study of the isomeric species or comparing the retention times with those of purified single isomers. LC–ESI-MS analysis resulted in tentative molecular masses for the peaks present in Fig. 5 and these masses along with the oligomer formulas are given in Table 1. Characteristic spectra of the linear/branched and monocyclic oligomers are shown in Fig. 6. C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
The ELSD peaks were identified by comparison of their retention times with those of EIC peaks on the assumption that the difference in time delay between the ELSD and MS signals was negligible. Support for this assumption was provided by conducting offline ESI-MS analysis of the fractions collected from the column outlet at 1 min intervals before ELSD and comparing the mass spectra obtained with those of LC–ESI-MS. Our results show that the offline mass spectra for various fractions are generally consistent with online ones for the same time range and thus verify the validity of overlaying the EICs over ELSD trace for peak identification. Comparison of ELSD and MS profiles indicates that the low mass oligomers including dimers, trimers, tetramers, and pentamers, which are too volatile to be detected by ELSD can be easily picked up by ESI-MS. www.jss-journal.com
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Table 1. Identification of oligo(ethoxysiloxane)s in ethyl silicate-40 by direct ESI-MS and LC-ESI-MS in positive ion mode
Direct ESI-MS Oligomer
Molar mass, g/mol
Linear/branched series (An , n = 2–13) 342 EtO[SiO(OEt)2 ]2 Et EtO[SiO(OEt)2 ]3 Et 476 610 EtO[SiO(OEt)2 ]4 Et 744 EtO[SiO(OEt)2 ]5 Et 878 EtO[SiO(OEt)2 ]6 Et EtO[SiO(OEt)2 ]7 Et 1012 1146 EtO[SiO(OEt)2 ]8 Et 1280 EtO[SiO(OEt)2 ]9 Et EtO[SiO(OEt)2 ]10 Et 1414 1548 EtO[SiO(OEt)2 ]11 Et EtO[SiO(OEt)2 ]12 Et 1682 1816 EtO[SiO(OEt)2 ]13 Et Monocyclic series (Bn+1 , n = 3–12) EtO[SiO(OEt)2 ]3 (SiO2 )1 Et 536 670 EtO[SiO(OEt)2 ]4 (SiO2 )1 Et 804 EtO[SiO(OEt)2 ]5 (SiO2 )1 Et 938 EtO[SiO(OEt)2 ]6 (SiO2 )1 Et 1072 EtO[SiO(OEt)2 ]7 (SiO2 )1 Et EtO[SiO(OEt)2 ]8 (SiO2 )1 Et 1206 EtO[SiO(OEt)2 ]9 (SiO2 )1 Et EtO[SiO(OEt)2 ]10 (SiO2 )1 Et EtO[SiO(OEt)2 ]11 (SiO2 )1 Et EtO[SiO(OEt)2 ]12 (SiO2 )1 Et
1340 1474 1608 1742
LC-ESI-MS
NH4 + -adduct ion, m/z
Na+ -adduct ion, m/z
NH4 + -adduct ion, m/z
Na+ -adduct ion, m/z
361.8 495.5 628.2 761.8 895.5 1029.6 1164.4 1297.9 1432.2 1566.3 N.D.a) N.D.a)
366.0 500.0 633.8 767.5 901.3 1035.5 1169.7 1303.1 1437.0 1571.0 N.D.a) N.D.a)
361.5 495.0 627.5 762.0 896.0 1030.4 1165.4 1299.2 1433.9 1566.0 1700.0 1831.0
366.0 500.0 633.7 767.4 901.2 1035.0 1169.0 1304.6 1438.9 1571.0 1705.6 1836.9
555.4 688.8 822.0 955.1 1089.0 1224.1
559.8 693.6 827.3 961.2 1095.4 1229.8
555.0 688.2 822.1 956.2 1090.6 1225.7
559.8 693.6 827.1 961.3 1095.5 1230.8
1358.4 1492.8 1626.3 N.D.a)
1364.3 1497.0 1632.4 N.D.a)
1359.2 1492.0 1626.0 1760.4
1365.9 1497.0 1632.7 1765.0
a) N.D. refers to signals are not detected.
Figure 4. Total ion chromatogram of oligo meric ethyl silicate-40 by LC–ESI-MS. Conditions are specified in the text.
3.3 Molar mass averages and distributions Molar mass averages and distributions are important parameters for QC of oligomeric ethyl silicate-40 as they are known to impact the processable behavior and mechanical properties of the end products. Direct-probe ESI-MS, LC–ESI-MS, and LC–ELSD techniques were used to determine these distribution parameters. Abundances of the characteristic molecular ions in mass spectrum of ethyl silicate-40 in direct ESI-MS, peak areas of the oligomers in EICs in LC–ESI-MS, and peak C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
heights of the oligomers in chromatogram in LC–ELSD were taken, respectively, for parameter calculations. The peak heights were used in LC–ELSD because the oligomer peaks were not baseline resolved in this case. As shown in Table 2, the average molecular masses, Mn and Mw , obtained by direct ESI-MS and LC–ESI-MS are in good agreement with those reported in the literature but only about two-thirds of those given by LC–ELSD. This difference is most likely attributed to the fact that the oligomers of repeating units less than 5 are largely undetectable in LC–ELSD, rendering www.jss-journal.com
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Figure 5. Extracted ion chromatograms of (A) linear/branched and (B) monocyclic oligomers separated by LC–ESI-MS.
Table 2. Weight-, number-averaged molar masses, Mw , Mn , and corresponding degrees of polymerization, nw , nn of ethyl silicate-40 determined by direct ESI-MS, LC-ESI-MS, and LC-ELSD
Analytical method
Mw
Mn
Mw /Mn
nw
nn
Direct ESI-MS LC-ESI-MS LC-ELSD
778.7 865.0 1126
755.8 799.8 1070
1.03 1.07 1.09
5.81 6.40 8.40
5.64 6.00 7.99
the molar mass distributions skewed to high mass range. The MS-based approaches seem to produce more realistic parameters as a result of less biased ionization and detection of the oligomers and therefore are particularly suited for samples like ethyl silicate-40 for which no single oligomers are available for conventional SEC calibrations. C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 6. Mass spectra of (A) linear/branched and (B) monocyclic oligomers.
4 Conclusions For the first time, HPLC–ESI-MS has been successfully applied to separate and identify the oligomeric species in ethyl silicate-40. The sample is found to be primarily composed of linear/branched ethoxysiloxane oligomers with silicon atoms ranging from 2 to 13. The retention strength and peak envelope width of the oligomer are increased in the order of increasing number of silicon atoms in the molecule. In addition, there are cyclic homologous series present in small amounts that are eluted before the linear/branched www.jss-journal.com
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counterparts. Moreover, the peak areas of each oligomers can be obtained from LC–ESI-MS, thus providing materials chemists and engineers with realistic molar mass distribution parameters. Taken together, the results from this work demonstrate that the combination of the separation power of LC with the structural identification capability of MS can be used advantageously for comprehensive characterization of complex mixtures such as ethyl silicate-40 in terms of molecular structure determination, chemical composition distribution and molar mass distribution. The authors thank Mr. Kongying Zhu and Dr. Xinghua Jin from Tianjin University for their technical assistance with the 29 Si NMR and LC–ESI-MS measurements. The authors have declared no conflict of interest.
5 References [1] Iler, R. K., The Chemistry of Silica, Wiley, New York 1979. [2] Hampden-smith, M. J., Klemperer, W. G., Brinker C. J. (Eds.), Better Ceramics Through Chemistry V, Materials Research Society, Pittsburgh, PA, USA 1992.
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[13] Krenkova, J., Svec, F., J. Sep. Sci. 2009, 32, 706–718. [14] Hedrick, J. L., Hawker, C. J., Miller, R. D., Twieg, R., Srinivasan, S. A., Trollsas, M., Macromolecules 1997, 30, 7607–7610. [15] Griswold, J., Uricheck, S., J. Am. Inst. Conserv. 1998, 37, 89–110. [16] Khabarov, V. B., Pronin, A. Ya., Buryak, A. K., Ospennikova, O. G., Pikulina, L. V., Doklady Chem. 2009, 429, 311–314. [17] Striegel, A.M., Yau, W.W., Kirkland, J. J., Bly, D. D., Modern Size-Exclusion Liquid Chromatography: Practice of Gel Permeation and Gel Filtration Chromatography, 2nd ed., John Wiley & Sons, Hoboken 2009. [18] Takamura, N., Gungi, T., Hatano, H., Abe, Y., J. Polym. Sci. Part A: Polym. Chem. 1999, 37, 1017–1026. [19] Jaumann, M., Rebrov, E. A., Kazakova, V. V., Muzafarov, A. M., Goedel, W. A., Moller, M., Macromol. Chem. Phys. 2003, 204, 1014–1026. [20] Zhu, X., Jaumann, M., Peter, K., Moller, M., Melian, C., Adams-Buda, A., Demco, D. E., Blumich, B., Macromolecules 2006, 39, 1701–1708. [21] Williams, R. J. J., Erra-Balsells, R., Ishikawa, Y., Nonami, H., Mauri, A. N., Ricardi, C. C., Macromol. Chem. Phys. 2001, 202, 2425–2433. [22] Mandel, H., Hay, A. S., J. Polym. Sci. Part A: Polym. Chem. 1998, 36, 2429–2437.
[3] Jutzi, P., Schubert, U. (Eds.), Silicon Chemistry: From the Atom to Extended Systems, Wiley-VCH, Weinheim 2003.
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[5] Greenwood, P., Pigment Resin Technol. 2010, 39(6), 315– 321. [6] Abe, Y., Kagayama, K., Takamura, N., Gunji, T., Yoshihara, T., Takahashi, N., J. Non-Cryst. Solids. 2000, 261, 39–45. [7] Wang, S.-J., Fan, X.-D., Si, Q.-F., Kong, J., Liu, Y.-Y., Qiao, W.-Q., Zhang, G.-B., J. Appl. Polym. Sci. 2006, 102, 5818– 5824. [8] Abe, Y., Shimano, R., Gunji, T., J. Polym. Sci. Part A Polym. Chem. 2003, 41, 2250–2255. [9] Hohenesche, C. F., Ehwald, V., Unger, K. K., J. Chromatogr. A 2004, 1025, 177–187.
[25] Cumme, G. A., Blume, E., Bublitz, R., Hoppe, H., Horn, A., J. Chromatogr. A 1997, 791, 245–253. [26] Kong, X., Diao, X., Wan, Q.-H., J. Chromatogr. A 2014, 1364, 198–203. [27] Nair, L. M., Konkel, J., Thomas, M., Koberda, M., J. Pharm. Biomed. Anal. 2006, 41, 725–730. [28] Montaudo, G., Montaudo, M. S., Puglisi, C., Samperi, F., Rapid Commun. Mass Spectrom. 1995, 9, 453–460. [29] Jackson, C., Larsen, B., McEwen, C., Anal. Chem. 1996, 68, 1303–1308.
[11] Mallik R., Hage, D.S., J. Sep. Sci. 2006, 29, 1686–17048.
[30] Crompton, T. R., Analysis of organosilicon compounds, in: Patai, S., Rappoport, Z. (Eds.), Organic Silicon Compounds Volumes 1 & 2, Chapter 6, John Wiley & Sons, Chichester 1989.
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ISSN 1615-9306 · JSSCCJ 38 (9) 1441–1624 (2015) · Vol. 38 · No. 9 · May 2015 · D 10609
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1484 Xiuli Zhou Qian-Hong Wan School of Pharmaceutical Science and Technology, Tianjin University, China Received October 26, 2014 Revised January 31, 2015 Accepted February 3, 2015
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Research Article
Separation and identification of oligomeric ethyl silicates by liquid chromatography with electrospray ionization mass spectrometry Reversed-phase liquid chromatography coupled with electrospray ionization mass spectrometry was used to study the molecular structures of components and molar mass distributions in ethyl silicate-40, a versatile liquid precursor for silicon-based materials. Identity testing by standard spectroscopic techniques showed that a commercial sample of ethyl silicate-40 was composed of linear/branched ethoxysiloxane oligomers with the silicon atoms ranging from 2 to 12 together with minor monocyclic species. Analysis of the sample by liquid chromatography coupled with evaporative light scattering detection resulted in an elution profile consisting of a series of peak clusters. Peak identification showed that the linear/branched homologous series of oligomers were eluted in the order of increasing number of silicon atoms in the molecules and the time duration (width) of the resulting peak clusters increased in the same fashion corresponding to increasing number of geometric isomers. In addition, small amounts of monocyclic oligomers present in the sample were found to be less retained than each linear/branched counterpart. Finally, the molar mass distribution parameters for ethyl silicate-40 determined by the developed method were in good agreement with the literature values. Overall, this work demonstrates that reversed-phase liquid chromatography coupled with electrospray ionization mass spectrometry is an indispensable tool for the comprehensive characterization of complex mixtures of this type. Keywords: Ethoxysiloxane oligomers / Ethyl silicate / Evaporative light scattering / Liquid chromatography with mass spectrometry / Molar mass distribution DOI 10.1002/jssc.201401184
1 Introduction Alkyl silicates, especially ethyl silicate in either the monomer or polymer form, have found widespread applications as liquid precursors of silicon-based materials [1–4]. They can be hydrolyzed and condensed to produce silica colloids that act as a binding agent for the manufacture of ceramic materials that are resistant to the highly corrosive environments and have the mechanical strength, heat resistance, and high dielectric properties. They can also be vaporized and thermally decomposed upon the surfaces of semiconductor chips to form electrically insulating layers of silica for the fabrication of integrated circuits. There are numerous other chemical applications, for example, as components of parting paints in precision casting [5], processable precursors for silica films [6,7], fibers [8], microspheres [9,10], and monolithic columns Correspondence: Professor Qian-Hong Wan, School of Pharmaceutical Science and Technology, Tianjin University, 92 Weijin Road, Tianjin 900032, China E-mail: [email protected] Fax: +86-22-2740-3650
Abbreviations: EIC, extracted ion chromatogram; ELSD, evaporative light scattering detector; SEC, size-exclusion chromatography; TIC, total ion chromatogram; TEOS, tetraethoxysilane
C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
[11–13], as crosslinkers in polymer compositions [14], and as consolidants for the preservation of artistic and architectural stone work [15]. New applications are constantly being developed. Among the variety of ethyl silicate products commercially available, ethyl silicate-40, prepared by controlled hydrolytic condensation of tetraethoxysilane (TEOS), is of particular importance for many technological applications. Despite their broad range of applications, precise information concerning the molecular structures and their distribution in ethyl silicate products has been scarce, limiting our ability to understand its processing behavior, polymer formation, materials function, and structure-property relationships. For example, the storage stability of the ethyl silicate-40 products depends upon the average sizes of ethoxysiloxane oligomers and the distribution of such sizes. Size-exclusion chromatography (SEC) has been a method of choice for determining the molar mass distributions of the oligomers in ethyl silicate-40 [16]. However, the data reported in the literature are not consistent probably because of the lack of commercially available individual oligomers as reference standards for molar mass calibration [17]. Besides, SEC provides insufficient resolution for the quantitation of individual oligomers. Spectroscopic techniques such as IR and NMR spectroscopy reveal information on the average chemical composition and end groups of the oligomeric compounds, which is however insufficient for determination of the distribution of www.jss-journal.com
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individual oligomer species [18–20]. MS with soft ionization techniques such as ESI and MALDI is a powerful tool capable of the identification and quantification of linear, branched, and cyclic species in solution, enabling oligomer distribution data to be extracted [21, 22]. However, there are very few reports in the literature on the application of ESI-MS or MALDIMS to analysis of ethyl silicates. RP-HPLC has been used extensively for the separation and quantification of synthetic oligomers/polymers over the last three decades [23–26]. A distinct advantage of this technique is that individual oligomeric species are eluted in the order of increasing number of repeat units, thus enabling well-resolved separation of individual oligomers within reasonable analysis time to be achieved under gradient elution conditions. But to date, there are no published studies showing the feasibility of applying RP-HPLC to studies of molecular structures and molar mass distribution for ethyl silicate-40 and its related oligo- and poly-siloxanes. The purpose of this work was to demonstrate the use of gradient RP-HPLC coupled with ESI-MS (LC–ESI-MS) for the separation and identification of individual oligomers in ethyl silicate-40 so that their distributions could be measured for better characterization of this commercial product. We started with testing the identity of the ethyl silicate-40 sample by standard spectroscopic techniques including IR and NMR spectroscopy and direct-probe ESI-MS. We then proceeded to fine-tuning the conditions for HPLC separation and MS identification of oligomeric species. Since the oligomers contain no chromophores in their molecules, evaporative light scattering detection (ELSD) was also used in conjunction with ESI-MS for eluate detection [27]. Finally, we compared the relative performance of direct ESI-MS, LC–ESI-MS, and LC– ELSD for the determination of molar mass averages and distributions for ethyl silicate-40.
2 Materials and methods
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trap LC/MS system (Waldbronn, Germany) equipped with an ESI source operated in the positive mode. The ionization parameters were as follows: dry temperature 350⬚C, nebulizer gas 30 psi, dry gas 8 L/min, HV capillary voltage 3850 V, HV end plate offset –500 V. The ESI-MS instrument was controlled by the ChemStation from Agilent and the spectral data were acquired and processed using Bruker Compass DataAnalysis 4.0. Aliquots of 10 L of the methanol solution of ethyl silicate-40 at 140 mg/mL were introduced directly into the ionization source at 0.4 mL/min with flow splitting ratio at 1:2. The mass spectra were recorded in the m/z range of 200–2200.
2.3 Chromatographic separation An Agilent 1200 Series HPLC system consisting of a degasser, a quaternary pump, column oven, autosampler, and UV-visible wavelength detector was used throughout. Separations of the ethoxysiloxane oligomers were carried out on a BaseLine C18 column (250 × 4.6 mm id, 5 m particle size, 100 Å pore size) at 35⬚C. A binary solvent system consisting of water (solvent A) and methanol (solvent B) was used in linear gradient mode from 90–100% B over 60 min and then held at 100% B for 10 min. The flow rate was 1 mL/min. The eluent from the outlet of the UV detector was introduced either into an ESI-MS or Alltech 500 ELSD (Deerfield, IL, USA) equipped with an oil-free air compressor. The MS parameters in LC–ESI-MS experiments were the same as for direct ESI-MS experiments. For ELSD detection, the drift tube temperature was set at 100⬚C and the air flow adjusted to 3.6 L/min corresponding to an inlet pressure of 51 kPa. The output ELSD signals were acquired and processed using a N2000 Chromatography Data System (BaseLine Chromtech Research Center, Tianjin, China). Aliquots of 20 L of the methanol solution of ethyl silicate-40 at 200 mg/mL were injected for HPLC analysis.
2.1 Chemicals Methanol (HPLC grade) and ethyl silicate-40 (technical grade, 38–42 wt% SiO2 ) were purchased from Concord Chemical Reagents (Tianjin, China) and Quzhou Ruilijie Chemical (Zhejiang, China), respectively. Deionized water prepared by Millipore Milli-Q system (Billerica, MA, USA) was used throughout.
2.4 Data analysis The number- and weight-average molar masses, Mn , Mw , of ethyl silicate-40 and their corresponding degrees of polymerization, nn , nw , were calculated by the following formulas [26, 28, 29]: Mn =
ci /
(ci /Mi )
(1)
2.2 Structural characterization The IR spectra of ethyl silicate-40 were acquired on a Tensor 27 FTIR spectrophotometer (Bruker Optics, Ettlingen, Germany) using KBr pellets. 29 Si NMR spectra were acquired using a Varian InfinityPlus 300 MHz spectrometer equipped with a 7.5 mm double-resonance MAS probe operating at 59.56 MHz with a 90⬚ pulse width. The neat ethyl silicate-40 was transferred into a PE tube for spectral measurements. Direct ESI mass spectra were acquired using an Agilent 6310 ion C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Mw =
(c i Mi )/
ci
(2)
nn =Mn /Mo
(3)
nw =Mw /Mo
(4)
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Figure 1. Acid-catalyzed hydrolysis and condensation of tetrae thoxysilane with water to produce ethoxysiloxane oligomers where n and m represent the number of repeat units and rings, respectively.
where i is the increment index; ci refers to the weight concentration of oligomer molecules with molar mass Mi , and Mo is the formula weight of the repeating unit in the macromolecules.
3 Results and discussion 3.1 Structural characterization Typically, ethyl silicate-40 is synthesized by controlled acidcatalyzed hydrolytic polycondensation of quadrafunctional TEOS as illustrated in Fig. 1. The partially hydrolyzed products may contain various amounts of monomer, linear, branched, and cyclic ethoxysiloxane oligomers depending upon the reaction conditions employed. Tight control of the molar ratios of water to TEOS in the reaction mixture below 1.5 is the key to obtain soluble processable precursors for silica. For molar ratios above 1.5, condensation of silanol groups results in insoluble cross-linked networks or gelation of TEOS [19]. For identity verification, standard spectroscopic techniques including IR and 29 Si NMR spectroscopy and directprobe ESI-MS were applied to the technical-grade ethyl silicate-40 from a commercial source. The resulting spectra are shown in Fig. 2. The absorption bands at 2978– 2894 and 1083 cm−1 are assigned to ethoxy and siloxane groups, respectively (Fig. 2A). The absence of silanol group bands at 3690 cm−1 (free Si–OH) and in the range 3400– 3200 cm−1 (hydrogen-bonded Si–OH) and in the range 950– 810 cm−1 (free Si–OH) suggests that the ethyl silicate-40 does not contain any silanol groups [30]. The 29 Si NMR spectrum of the same sample shows five groups of peaks (Fig. 2B) corresponding to silicon atoms bearing: zero siloxane units Q0 (–81.32 ppm, TEOS), one siloxane unit Q1 (–89.29 ppm, terminal units), two siloxane units Q2 (–96.66 ppm, linear units), three siloxane units Q3 (–104.5 ppm, semidendritic units), and four siloxane units Q4 (–112.3 ppm, dendritic units) [15]. There are some splittings of chemical shift for Q4 and these may result from the molecular internal cyclization reaction and the interference of adjacent silicon atoms. Besides, there are no chemical shifts for silanol groups indicating that the process of polycondensation in the reaction is complete. The presence of the Q3 and Q4 peaks indicates the ethyl silicate40 contains branched oligomers and the degree of branching calculated from the integrated peak areas of the individual species (Qi ) was 0.39 [31]. C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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The ESI mass spectrum of ethyl silicate-40 is shown in Fig. 2C. Characteristic molecular ions corresponding to each oligomer species are observed as ammonium and sodium adducts represented by molecular formulas EtO[SiO(OEt)2 ]n (SiO2 )m Et + NH4 + /Na+ . Here n and m refer to the numbers of ethoxysiloxane repeating units and rings in the oligomer molecule, respectively. In single stage MS, the linear and branched species cannot be distinguished so that they are classified as a single group for clarity. For linear/branched homologous series where m = 0, the major molecular ions are spaced by 134 Da, the formula weight of the repeating unit SiO(OEt)2 . This series is denoted as A-series with the subscript number n equal to the number of repeating units. Except for dimer and trimer, each linear/branched oligomer was preceded by a second, smaller peak giving rise to a second series of monocyclic oligomers (where m = 1) of 74 Da lower than the first. This series is denoted as B-series with the subscript number referring to the total number of silicon atoms in the molecule (n + 1). The assignments of the molecular ions to their respective oligomers are summarized in Table 1. The ethoxysiloxane oligomers with silicon atoms ranging from 2 to 12 are observed in ethyl silicate-40, and their size distribution is left skewed, a feature typical for condensation polymerization.
3.2 Oligomeric separation and identification Initial selection of chromatographic conditions for separation of ethyl silicate-40 was carried out with ELSD. Using a binary gradient consisting of water and methanol, the sample was resolved on a C18 column into eight or nine clusters of peaks with retention times ranging from 10 to 55 min (Fig. 3). Each cluster consists of four to five peaks. The baseline drifted upward with the retention time, suggesting ELSD signals susceptible to the solvent changes occurred in the gradient system. Attempts to improve the separation in a single run were made by using different solvents such as acetonitrile and tetrahydrofuran under various gradient elution conditions but met with limited success (data not shown). The gradient elution HPLC conditions chosen with ELSD were adopted for LC–ESI-MS studies of ethyl silicate-40. Figure 4 shows the total-ion chromatogram (TIC) of ethyl silicate-40. Several peak clusters are recognizable even though the separation does not appear as sharp as with ELSD. It is also noted that the MS detection is more sensitive than ELSD especially for low mass oligomers with retention times less than 15 min. No baseline drift was observed with the MS detection. These contrasts may be attributable to the different detection mechanisms involved in two techniques. Figure 5A and B show the respective extracted ion chromatograms (EICs) for the molecular ions of the linear/branched (A-series) and monocyclic (B-series) oligomers. Both groups of the oligomers are eluted indeed in the order of increasing chain length. The B-series is less retained than the A-series of the same number of silicon atoms and therefore peak-overlaps between A- and B-series are common. It is noted that each www.jss-journal.com
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Figure 2. (A) IR, (B) 29 Si NMR, and (C) ESI-mass spectrum of ethyl silicate-40. The labels Qi in (B) refer to the silicon atoms bearing i number of siloxane (Si–O–Si) units from 0 to 4 and the % values represent the percentages of integrated peak areas for each species.
Figure 3. Separation of oligomeric ethyl sili cate-40 by LC–ELSD. Conditions: column, Baseline C18 (250 mm × 4.6 mm id, 5 m particles); column temperature, 35⬚C;mobile phase, water (solvent A), and methanol (solvent B) gradient elution from 90–100% B over 60 min and held at 100% B for 10 min; detection, ELSD (drift tube temperature, 100⬚C, nebulizing gas flow, 3.6 L/min, and the inlet pressure, 51 kPa). Peak identification: A-series, An ; B-series, B(n+1) , where n refers to the number of repeat units in the molecule.
oligomer is eluted as a peak envelope consisting of isomeric species, the time duration (width) of which is increased with the retention time considerably due to partial resolution of the geometric isomers during the chromatographic process. This is especially the case for high mass oligomers that have a large number of possible isomeric structures. Deconvolution or separate isomer identification of the peak envelope has not been possible without performing a MS/MS study of the isomeric species or comparing the retention times with those of purified single isomers. LC–ESI-MS analysis resulted in tentative molecular masses for the peaks present in Fig. 5 and these masses along with the oligomer formulas are given in Table 1. Characteristic spectra of the linear/branched and monocyclic oligomers are shown in Fig. 6. C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
The ELSD peaks were identified by comparison of their retention times with those of EIC peaks on the assumption that the difference in time delay between the ELSD and MS signals was negligible. Support for this assumption was provided by conducting offline ESI-MS analysis of the fractions collected from the column outlet at 1 min intervals before ELSD and comparing the mass spectra obtained with those of LC–ESI-MS. Our results show that the offline mass spectra for various fractions are generally consistent with online ones for the same time range and thus verify the validity of overlaying the EICs over ELSD trace for peak identification. Comparison of ELSD and MS profiles indicates that the low mass oligomers including dimers, trimers, tetramers, and pentamers, which are too volatile to be detected by ELSD can be easily picked up by ESI-MS. www.jss-journal.com
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Table 1. Identification of oligo(ethoxysiloxane)s in ethyl silicate-40 by direct ESI-MS and LC-ESI-MS in positive ion mode
Direct ESI-MS Oligomer
Molar mass, g/mol
Linear/branched series (An , n = 2–13) 342 EtO[SiO(OEt)2 ]2 Et EtO[SiO(OEt)2 ]3 Et 476 610 EtO[SiO(OEt)2 ]4 Et 744 EtO[SiO(OEt)2 ]5 Et 878 EtO[SiO(OEt)2 ]6 Et EtO[SiO(OEt)2 ]7 Et 1012 1146 EtO[SiO(OEt)2 ]8 Et 1280 EtO[SiO(OEt)2 ]9 Et EtO[SiO(OEt)2 ]10 Et 1414 1548 EtO[SiO(OEt)2 ]11 Et EtO[SiO(OEt)2 ]12 Et 1682 1816 EtO[SiO(OEt)2 ]13 Et Monocyclic series (Bn+1 , n = 3–12) EtO[SiO(OEt)2 ]3 (SiO2 )1 Et 536 670 EtO[SiO(OEt)2 ]4 (SiO2 )1 Et 804 EtO[SiO(OEt)2 ]5 (SiO2 )1 Et 938 EtO[SiO(OEt)2 ]6 (SiO2 )1 Et 1072 EtO[SiO(OEt)2 ]7 (SiO2 )1 Et EtO[SiO(OEt)2 ]8 (SiO2 )1 Et 1206 EtO[SiO(OEt)2 ]9 (SiO2 )1 Et EtO[SiO(OEt)2 ]10 (SiO2 )1 Et EtO[SiO(OEt)2 ]11 (SiO2 )1 Et EtO[SiO(OEt)2 ]12 (SiO2 )1 Et
1340 1474 1608 1742
LC-ESI-MS
NH4 + -adduct ion, m/z
Na+ -adduct ion, m/z
NH4 + -adduct ion, m/z
Na+ -adduct ion, m/z
361.8 495.5 628.2 761.8 895.5 1029.6 1164.4 1297.9 1432.2 1566.3 N.D.a) N.D.a)
366.0 500.0 633.8 767.5 901.3 1035.5 1169.7 1303.1 1437.0 1571.0 N.D.a) N.D.a)
361.5 495.0 627.5 762.0 896.0 1030.4 1165.4 1299.2 1433.9 1566.0 1700.0 1831.0
366.0 500.0 633.7 767.4 901.2 1035.0 1169.0 1304.6 1438.9 1571.0 1705.6 1836.9
555.4 688.8 822.0 955.1 1089.0 1224.1
559.8 693.6 827.3 961.2 1095.4 1229.8
555.0 688.2 822.1 956.2 1090.6 1225.7
559.8 693.6 827.1 961.3 1095.5 1230.8
1358.4 1492.8 1626.3 N.D.a)
1364.3 1497.0 1632.4 N.D.a)
1359.2 1492.0 1626.0 1760.4
1365.9 1497.0 1632.7 1765.0
a) N.D. refers to signals are not detected.
Figure 4. Total ion chromatogram of oligo meric ethyl silicate-40 by LC–ESI-MS. Conditions are specified in the text.
3.3 Molar mass averages and distributions Molar mass averages and distributions are important parameters for QC of oligomeric ethyl silicate-40 as they are known to impact the processable behavior and mechanical properties of the end products. Direct-probe ESI-MS, LC–ESI-MS, and LC–ELSD techniques were used to determine these distribution parameters. Abundances of the characteristic molecular ions in mass spectrum of ethyl silicate-40 in direct ESI-MS, peak areas of the oligomers in EICs in LC–ESI-MS, and peak C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
heights of the oligomers in chromatogram in LC–ELSD were taken, respectively, for parameter calculations. The peak heights were used in LC–ELSD because the oligomer peaks were not baseline resolved in this case. As shown in Table 2, the average molecular masses, Mn and Mw , obtained by direct ESI-MS and LC–ESI-MS are in good agreement with those reported in the literature but only about two-thirds of those given by LC–ELSD. This difference is most likely attributed to the fact that the oligomers of repeating units less than 5 are largely undetectable in LC–ELSD, rendering www.jss-journal.com
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Figure 5. Extracted ion chromatograms of (A) linear/branched and (B) monocyclic oligomers separated by LC–ESI-MS.
Table 2. Weight-, number-averaged molar masses, Mw , Mn , and corresponding degrees of polymerization, nw , nn of ethyl silicate-40 determined by direct ESI-MS, LC-ESI-MS, and LC-ELSD
Analytical method
Mw
Mn
Mw /Mn
nw
nn
Direct ESI-MS LC-ESI-MS LC-ELSD
778.7 865.0 1126
755.8 799.8 1070
1.03 1.07 1.09
5.81 6.40 8.40
5.64 6.00 7.99
the molar mass distributions skewed to high mass range. The MS-based approaches seem to produce more realistic parameters as a result of less biased ionization and detection of the oligomers and therefore are particularly suited for samples like ethyl silicate-40 for which no single oligomers are available for conventional SEC calibrations. C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 6. Mass spectra of (A) linear/branched and (B) monocyclic oligomers.
4 Conclusions For the first time, HPLC–ESI-MS has been successfully applied to separate and identify the oligomeric species in ethyl silicate-40. The sample is found to be primarily composed of linear/branched ethoxysiloxane oligomers with silicon atoms ranging from 2 to 13. The retention strength and peak envelope width of the oligomer are increased in the order of increasing number of silicon atoms in the molecule. In addition, there are cyclic homologous series present in small amounts that are eluted before the linear/branched www.jss-journal.com
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counterparts. Moreover, the peak areas of each oligomers can be obtained from LC–ESI-MS, thus providing materials chemists and engineers with realistic molar mass distribution parameters. Taken together, the results from this work demonstrate that the combination of the separation power of LC with the structural identification capability of MS can be used advantageously for comprehensive characterization of complex mixtures such as ethyl silicate-40 in terms of molecular structure determination, chemical composition distribution and molar mass distribution. The authors thank Mr. Kongying Zhu and Dr. Xinghua Jin from Tianjin University for their technical assistance with the 29 Si NMR and LC–ESI-MS measurements. The authors have declared no conflict of interest.
5 References [1] Iler, R. K., The Chemistry of Silica, Wiley, New York 1979. [2] Hampden-smith, M. J., Klemperer, W. G., Brinker C. J. (Eds.), Better Ceramics Through Chemistry V, Materials Research Society, Pittsburgh, PA, USA 1992.
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