Journal of Chromatography A, 1394 (2015) 95–102

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Separation and identification of oligomeric phenylethoxysiloxanols by liquid chromatography-electrospray ionization mass spectrometry Yuting Jiang, Qian-Hong Wan ∗ School of Pharmaceutical Science & Technology, Tianjin University, Tianjin 900032, China

a r t i c l e

i n f o

Article history: Received 11 January 2015 Received in revised form 9 March 2015 Accepted 17 March 2015 Available online 24 March 2015 Keywords: Silanol oligomers Silsesquioxane Hydrolytic condensation Hybrid materials Liquid chromatography-mass spectrometry

a b s t r a c t Liquid chromatography-electrospray ionization mass spectrometry has been applied to qualitative analysis of oligomeric phenylethoxysiloxanols, a class of organosilanols as active intermediates to polyhedral silsesquioxanes. The phenylethoxysiloxanol samples were prepared by controlled acid-catalyzed hydrolysis and condensation of phenyltriethoxysilane at various molar equivalents of water (r1 ) and characterized by standard spectroscopic techniques. Using a gradient binary water–methanol mobile phase, these reaction products were resolved on octadecylsiloxane silica stationary phase and subsequently identified by online electrospray ionization mass spectrometric detection. Results show that the reaction products are composed of a multitude of linear and monocyclic siloxanol oligomers with various numbers of silicon atoms and hydroxyl groups, depending upon the reaction conditions used. With the r1 value increasing from 0.5 to 2.0, the chain lengths of the oligomers increase slightly but the numbers of hydroxyl groups increase considerably, accompanying by structural evolution from chains to rings. Characterization of the retention behavior of these oligomers indicates that hydrophobic interactions of phenyl and ethoxy groups with the stationary phase are responsible for their retention in reversed-phase liquid chromatography. © 2015 Elsevier B.V. All rights reserved.

1. Introduction With the quest for innovative advanced materials, enormous efforts have been directed to development of organic–inorganic hybrid materials that promise to combine hardness and stability of inorganic glasses with flexibility and low-temperature processing of organic polymers [1,2]. An interesting class of these materials is derived from silsesquioxane, an organosilicon compound with the empirical chemical formula RSiO3/2 where R refers to alkyl/aryl or organofunctional groups. A wide variety of silsesquioxanes in ladder, cage and cube forms have been synthesized and used as building blocks for nanocomposites in optoelectronics, heterogeneous catalysts, nanoglobular drug carriers, and other promising applications [3–5]. Typically, silsesquioxane based hybrid materials are prepared by acid- or base-catalyzed hydrolysis and condensation of the organo-modified trifunctionalsilane, also known as sol-gel process. This process proceeds from partial hydrolysis of the organosilane to form reactive silanol monomers, to condensation of these monomers to result in oligomers (sol formation), and finally to polymerization by crosslinking of the oligomeric

∗ Corresponding author. Tel./fax: +86 22 2740 3650. E-mail address: [email protected] (Q.-H. Wan). http://dx.doi.org/10.1016/j.chroma.2015.03.043 0021-9673/© 2015 Elsevier B.V. All rights reserved.

species leading to a three-dimensional network (gel formation) [6,7]. Detailed knowledge about the oligomeric species present in solution is mandatory to understand the chemistry of the process and to tailor the properties of the product. The solid product can be characterized by a variety of techniques including electron microscopy, physisorption, porosimetry, infrared (IR), and 29 Si nuclear magnetic resonance (29 Si NMR) spectroscopy. However, studies of soluble oligomeric silanols in solution have proved to be challenging because only a few techniques are suited for such analyses such as gas chromatography (GC), 29 Si NMR spectroscopy, and electrospray ionization mass spectrometry (ESI-MS) [8–10]. For GC analysis, the silylation of the oligomeric silanols with alkytrichlorosilanes is required to facilitate their transfer to the gas phase. This process usually requires the use of acid- or base-catalyst and produces side reactions such as polymerization or depolymerization of the silicate species. Since the method requires a relatively long time range, and the modification of the reaction mixture, it is more useful for studying trends but not appropriate for the investigation of the oligomer distribution. Identification of the silicate species by 29 Si NMR is based on the characteristic chemical shifts of 29 Si atoms resulting from the distinctive shielding effects of the chemical bonds attached to the silicon atoms. Despite the long tradition of using this technique for characterization purposes, some disadvantages are inherent to

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Fig. 1. Controlled acid-catalyzed hydrolysis and condensation of phenyltriethoxysilane to produce phenylethoxysiloxane and phenylethoxysiloxanol oligomers where n represents, respectively, the number of repeat units in the linear species (A-series) and the sum of the silicon atoms x, y and z in the cyclic species (B- and C-series).

the 29 Si NMR experiments. First, 29 Si NMR experiments are time consuming owing to both the low natural abundance of 29 Si nuclei and its small and negative magnetogyric ratio. This technique is inappropriate for quantitative measurements in systems with fast evolution because of the use of long recycle delays. Second, spectral interpretation is difficult for complex systems owing to peak overlapping of the 29 Si signals. In contrast, ESI-MS is a fast, sensitive, and selective analytical technique that allows for rapid screening of silicate oligomers in solution. Detection and quantification of analytes are based on measurements of the mass-to-charge ratio (m/z) and intensity of the target ions produced in the ESI source. A distinct advantage of ESI is that little fragmentation occurs to thermally labile species, thus simplifying the assignment of the signals and interpretation of the spectra. This technique has been applied to identify the silicate species and to monitor the temporal evolution of oligomers in acid and basic media [9]. Like all other techniques, however, ESI-MS is not without limitations. First, single stage MS is unable to distinguish geometric isomers of silicate oligomers, which increases with the number of silicon atoms in the molecule [11]. Second, ion suppression resulting from the presence of species in the sample which compete for ionization or inhibit ionization in other ways introduces great uncertainty in quantification of analytes, leading to biased oligomer distribution [12]. Obviously, there is a demand for a viable alternative method to overcome limitations inherent in the analytical techniques currently available. High-performance liquid chromatography (HPLC) has been widely used in oligomer analysis with a potential capability to resolve isomers [13–15]. Initial studies by Dorn and Skelly Frame demonstrated the speciation and quantification of organosilicon compounds by HPLC with inductively coupled plasma-atomic

emission spectrophotometry (ICP-AES) [16]. As degradation products of poly(dimethylsiloxane) polymer, water soluble silanol oligomers ranging from monomer to trimer were baseline separated by reversed-phase HPLC with water-acetonitrile as mobile phase and detected by ICP-AES with high sensitivity. This approach was extended by using ICP-MS as a detector allowing for detection of silanol species at lower levels [17]. Most of the work reported to date, however, has been limited to the determination of very low molecular weight silanols in environmental and industrial samples. Moreover, HPLC-ICP is not suited for studies of oligomer distribution because it requires the use of reference standards for identification, which are unfortunately not available in most cases. Recently, we showed for the first time that HPLC coupled with ESIMS (LC-ESI-MS) can be used to separate and identify oligomeric siloxanes derived from acid-catalyzed hydrolytic condensation of vinyltrimethoxysilane and to monitor the structural evolution of the oligomers with the amount of water in the reaction mixture [18]. Results from this work revealed several advantages of this approach over the conventional ones. First, separation and identification of oligomers can be done simultaneously allowing for broad range of complex samples to be addressed. Second, resolution of isomeric compounds is achievable by the chromatographic separation, facilitating their identification by subsequent MS detection. Furthermore, ion suppression effects in LC-ESI-MS are reduced by the sample dilution occurred during the chromatographic process. Herein, we extended this approach to characterize oligomeric silanols as active intermediates to polyhedral silsesquioxanes and to study their retention behavior in gradient reversed-phase chromatography. We chose phenyltriethysilane (PhTES) derived silanols as model compounds because they are active intermediates

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Fig. 2. IR spectra of phenylethoxysiloxanols prepared at various molar equivalents of water to organosilane, r1 . (a) r1 = 0.5; (b) r1 = 1.0; (c) r1 = 1.5; (d) r1 = 2.0.

during the synthesis of phenylsilsequioxanes [19]. We started with the preparation of four series of hydrolytic condensation products at various molar ratios of water to PhTES and subsequent structural characterization by standard spectroscopic techniques. Then we proceeded with the gradient elution LC separation of oligomers and their online ESI-MS identification. Finally, we investigated the retention behavior of these oligomers under gradient elution conditions to pinpoint the key factors that control the solute retention and selectivity. 2. Experimental 2.1. Chemicals Methanol (HPLC grade) and hydrochloric acid (analytical grade, 37 wt% HCl) were purchased from Concord Chemical Reagents (Tianjin, China). Phenyltriethoxysilane (PhTES, 98%) was obtained from WD Silicone (Wuhan, Hubei, China). Deionized water was prepared by Milli-Q water purification systems (Millipore, Bedford, MA, USA) 2.2. Preparation of oligomeric phenylethoxysiloxanols The oligomeric phenylethoxysiloxanols were synthesized by acid-catalyzed hydrolytic condensation of PhTES at various molar equivalents of water. Typically, a mixture of PhTES (24.0 g, 0.1 mol) and ethanol (4.6 g, 0.1 mol) was added to a round-bottom flask equipped with a mechanical stirrer. When the mixture was mixed homogeneously, an aqueous hydrochloric acid solution in the molar ratio of water to PhTES, r1 , in the range of 0.50–2.0 and the molar ratio of HCl to PhTES, r2 , fixed at 0.002 was added dropwise over a period of 5 min with stirring. After stirring at 30 ◦ C for another 30 min, the solvents were removed by vacuum evaporation at 80 ◦ C to yield viscous liquid products.

Fig. 3. 29 Si NMR spectra of phenylethoxysiloxanols prepared at various molar equivalents of water to organosilane, r1 . The inset shows integrated peak fractions of species T0 , T1 , T2 , and T3 as a function of the molar equivalents of water to organosilane, r1 . (a) r1 = 0.5; (b) r1 = 1.0; (c) r1 = 1.5; (d) r1 = 2.0.

(Waldbronn, Germany) operated in the positive ion mode. The ionization parameters were set as follows: dry temperature, 350 ◦ C; nebulizer gas (nitrogen), 30 psi; dry gas flow, 8 L/min; HV capillary voltage, 3.85 kV. The capillary exit block and skimmer voltages were set at 142 and 40 V, respectively. The ESI-MS instrument was controlled by Agilent ChemStation and the signals were acquired and processed by Bruker Compass DataAnalysis 4.0. The mass spectra were recorded in the m/z range of 200-2200. The sample solutions were prepared in 1-mL PP vials by dissolving the liquid products in ethanol at 50 mg/mL. A 20-␮L aliquot of the sample solutions was introduced in flow-injection mode into the ionization source at 0.4 mL/min with flow splitting ratio at 1:2.

2.3. Structural characterization 2.4. Chromatographic separation The infrared (IR) spectra of the liquid products were acquired on a Tensor 27 FTIR spectrophotometer (Bruker Optics, Ettlingen, Germany) using KBr pellets. 29 Si NMR spectra were acquired on 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 (Palo Alto, CA, USA). The neat liquid samples were transferred into PE tubes for spectral measurements. Direct ESI mass spectra were acquired on an Agilent 6310 ion trap mass spectrometer equipped with an electrospray ionization interface

An Agilent 1200 Series HPLC system consisting of a degasser, a quaternary pump, a column oven, an autosampler, a diode array detector, and an ESI-ion trap mass spectrometer was used throughout. Separations of the reaction products were carried out on a BaseLine C18 column (250 × 4.6 mm I.D., 5 ␮m particle size, 100 A˚ pore size) (BaseLine Chromtech Research Center, Tianjin, China) at 30 ◦ C. A binary solvent system consisting of water (solvent A) and methanol (solvent B) was used in linear gradient mode from 80%

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Fig. 4. Direct ESI-mass spectra of phenylethoxysiloxanols prepared at various molar equivalents of water to organosilane, r1 . (a) r1 = 0.5; (b) r1 = 1.0; (c) r1 = 1.5; (d) r1 = 2.0.

to 100% B over 60 min and then held at 100% B for 10 min. The flow rate was 1 mL/min. The sample solutions, injection volumes and online ESI-MS conditions were the same as used in direct ESI-MS experiments. 3. Results and discussion 3.1. Structural characterization The controlled acid-catalyzed hydrolytic condensation of phenyltriethoxysilane (PhTES) proceeds as illustrated in Fig. 1, resulting in soluble products comprising various amounts of linear/branched (A-series), monocyclic (B-series) and bicyclic (Cseries) oligomeric phenylethoxysiloxanes depending upon the

reaction conditions employed. The hydrolysis of the terminal or pendant ethoxy groups yields phenylethoxysilxanol oligomers denoted as An (OH)N , Bn (OH)N , and Cn (OH)N , respectively. The n and N refer to the respective numbers of silicon atoms and hydroxyl groups in the molecule. In order to generate soluble, reproducible oligomers with controlled molecular weights and amounts of functional end-groups, PhTES was oligomerized in ethanol solutions of varying molar equivalents of water at 30 ◦ C in the presence of fixed amount of HCl catalyst. The molar ratio of water to PhTES (r1 ) in the reaction mixture was varied from 0.5 to 2.0 while r1 = 1.5 corresponds to stoichiometric conditions for the total reaction of hydrolysis and condensation. The viscous liquids obtained were characterized by IR, NMR and mass spectrometry.

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Fig. 5. HPLC separations of phenylethoxysiloxanols prepared at various molar equivalents of water to organosilane, r1 . Conditions: column, BaseLine C18 (250 mm x 4.6 mm I.D., 5 ␮m particles); column temperature, 30 ◦ C; mobile phase, water (solvent A) and methanol (solvent B) gradient elution from 80% to 100% B over 60 min and held at 100% B for 10 min; UV detection, 254 nm. (a) r1 = 0.5; (b) r1 = 1.0; (c) r1 = 1.5; (d) r1 = 2.0.

Fig. 2 shows the IR spectra of the four samples prepared at r1 in the range 0.5–2.0. The absorption bands at 3620 cm−1 , 3390 cm−1 , and 3062-2892 cm−1 were assigned to the hydroxyl, phenyl, and ethoxy groups, respectively. The absorption bands at 1077 cm−1 assigned to siloxane (Si O Si) linkages, indicate the formation of the silicon based oligomers.

The 29 Si NMR spectra of these samples show four groups of peaks (Fig. 3), which are labeled as T0 (−57 to −59 ppm), T1 (−61 to −66 ppm), T2 (−70 to −74 ppm), and T3 (−78 to −80 ppm) [20]. These peaks represent silicon atoms containing 0–3 silicon units through the siloxane (Si O Si) bonds, corresponding to monomeric, terminal, linear and branching/cyclic structure,

Fig. 6. Total ion chromatograms of phenylethoxysiloxanols prepared at various molar equivalents of water to organosilane, r1 . (a) r1 = 0.5; (b) r1 = 1.0; (c) r1 = 1.5; (d) r1 = 2.0. The chromatographic conditions used are the same as for Fig. 5. Peak identification is based on extracted ion chromatograms presented in Figs. S1–S4 in Supplementary data.

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the monomeric (T0 ), terminal (T1 ) and higher order (T2 ) fractions are 63%, 35% and 3% of the total peak areas, suggesting that the degree of oligomerization is quite limited and the low molecular weight species, especially the monomer and dimer dominate the reaction products. At r1 = 1.0, the T1 and T2 fractions account for 89.2%, indicating that linear species with little branching predominate in the reaction products. At r1 ≥ 1.5, the T3 fraction began to show up and increase steadily, suggesting the polycondensation of the mononers leading to formation of branching or cyclic species. Fig. 4 shows positive ion ESI mass spectra of phenylethoxysiloxanols prepared at various values of r1 . All molecules are found as singly positive charged species, obtained primarily by the addition of a proton to the molecule. They are assigned to the oligomers with their molecular weights calculated by the generic formulas presented in Fig. 1. For r1 = 0.5, linear species (A-series) appear to be predominant with n ranging from 2 to 6. The nominal mass separation between two major peaks is 166 Da, which is equal to the formula mass of the repeat unit [R(C2 H5 O)SiO], where R is phenyl group. The loss of 28 Da, the formula weight of [C2 H4 ], from the corresponding adjacent phenylethoxysilane oligomers denoted by An indicates the formation of phenyethoxysiloxanol oligomers denoted by An (OH)N . The molar mass distribution of the oligomers is centered on trimer as judged by the peak areas, contrary to the corresponding 29 Si NMR data in which monomer is the dominant species. A plausible explanation for the apparent inconsistency is that the monomer and dimer are volatile species so that a large portion of them are blown away by the nebulization gas giving rise to biased ion transfer in the ESI source. There is no much change in the pattern of the oligomer distribution when the r1 value increased to 1.0, except that intensities of siloxane species A2 and A3 are increased and become predominant. The decrease in the intensity of siloxanols relative to that of siloxanes indicates the consumption of the siloxanols as reactive species during condensation reaction. For r1 = 1.5, monocyclic (B-series) oligomers with n ranging from 4 to 6 are identified as minor species together with the predominant A-series. With further increase in the amount of water in the reaction mixture, i.e., r1 = 2.0, the number of ion peaks and intensities of cyclic species increases considerably at the expense of the linear species. Thus the center of the oligomer distribution is shifted towards larger n. 3.2. Separation and identification

Fig. 7. Correlation of retention time (tR /t0 ) with the number of silicon atoms (n) for (a) A-series, (b) B-series, and (c) C-series of phenylethoxysiloxanols prepared at r1 = 2.0. The chromatographic conditions used are the same as for Fig. 5.

respectively. The integrated peak areas were used to demonstrate the structural evolution in the hydrolysis and condensation of PhTES. As shown in the inset in Fig. 3, with increasing r1 or the amount of water in the reaction mixture, the T2 and T3 fractions increase at the expense of the T0 fraction whereas the T1 fraction reaches a peak at r1 = 1.0 and then declines continuously. At r1 = 0.5,

Separation of the sol–gel reaction products into individual siloxane and siloxanol oligomers is desirable for a number of purposes, including studies of chemical composition distribution and structure growth. Preparation of singly purified oligomers as reference standards often involves an isolation procedure such as preparative liquid chromatography. For this reason, studies on liquid chromatographic separation of siloxane and siloxanol oligomers and their retention behavior are of practical implications to hybrid materials research. Based on previous experiences with silicon compounds, we chose reversed-phase liquid chromatography with gradient elution for separation of these siloxanol molecules with varying polarities. A diode array detection was used for eluent monitoring in the initial LC method development. Using a binary gradient consisting of methanol and water, the reaction products were separated into a multitude of oligomer peaks on an octadecylsiloxane (C18 ) silica column, as shown in Fig. 5. It is interesting to note that the number of resolved peaks increases with the amount of water in the reaction mixture, from about 5 at r1 = 0.5 to over 10 main peaks at r1 = 2.0, consistent with the trend observed in the ESI-MS experiments. Complete elution of the oligomers from the column was reached as evidenced by the fact that prolonged elution with 100% B at the end of the separation produces no additional peaks.

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Table 1 Integrated peak areas of phenylsiloxanols prepared at various molar ratios of water to silane agent (r1 ) measured from extracted ion chromatograms shown in Figs. S1–S4. Oligomer

r1 = 0.5 A-series B-series C-series r1 = 1.0 A-series B-series C-series r1 = 1.5 A-series B-series C-series r1 = 2.0 A-series B-series C-series

Peak area (%) n=1

n=2

n=3

n=4

n=5

n=6

n=7

n=8

n=9

Subtotal

1.03 – –

9.20 – –

23.70 2.40 –

31.01 7.13 –

12.01 7.40 –

2.39 3.07 –

– 0.65 –

– – –

– – –

79.35 20.65 –

0.66 – –

6.01 – –

19.54 1.93 –

30.19 7.11 –

15.82 6.43 –

4.81 4.08 0.29

1.25 1.38 0.08

0.13 0.31 –

– – –

78.40 21.23 0.37

1.0 – –

3.53 – –

17.5 1.8 –

26.94 9.75 –

15.22 9.96 –

3.91 5.87 0.87

0.89 1.85 0.31

0.13 0.49 –

– – –

69.11 29.71 1.18

0.84 – –

1.57 – –

15.37 1.73 –

17.86 17.14 –

6.88 20.70 0.92

2.17 7.42 2.86

0.47 1.99 0.92

0.08 0.54 0.31

– – 0.14

45.25 49.60 5.15

Since there are no reference standards for the oligomers under study, the resolved peaks could not be assigned to the individual compounds according to their retention times. To assist with peak identification, an ESI-MS detection was used in place of the UV detection to monitor the oligomers eluted from the column. The resulting total-ion chromatograms (TICs) are shown in Fig. 6. The peak identification was facilitated by extracting individual ion peaks at the m/z values corresponding to each linear and monocyclic species (see Figs. S1–S4 for extracted ion chromatograms (EICs) of four condensation products in Supplementary data). Linear siloxanols with various numbers of silicon atoms and hydroxyl groups are predominant in all the cases studied, but the cyclic species show gradual increase with the amount of water in the reaction mixtures. Table 1 presents the integrated peak areas for major oligomeric species obtained from the EICs, which can be used to illustrate the trends of the structure evolution. The peak area fraction of cyclic oligomers increases from about 20% at r1 = 0.5 to over 50% at r1 = 2.0, indicating the concentrations of cyclic species increased with the amount of water as expected from non-random cyclization theory proposed by McMormick et al. [21]. Many jammed or overlapped peaks are observed in the chromatograms presented, reflecting the presence of large numbers of oligomers and related geometric isomers in the samples. Evidence for isomeric separation is presented by the EICs of oligomers (see Figs. S1–S4 in Supplementary data), in which split peaks are observed for a single species. Work is underway to identify these isomers by tandem mass spectrometry. 3.3. Retention behavior The retention behaviors of phenylethoxysiloxane and phenylethoxysiloxanol olgiomers in reversed-phase LC systems were examined and correlation of the retention times with the number of silicon atoms is shown in Fig. 7. Apart from major species marked in chromatograms in Fig. 6, unmarked minor species which are identifiable from EICs (see Figs. S1–S4 in Supplementary data) were also included in this correlation. The error bar is used to indicate the range of tR /t0 values associated with geometric isomers. There is a general trend that the retention increases with the number of silicon atoms in the oligomer molecule for any given series. For the groups of oligomers with the same number of silicon atoms, the retention decreases with the number of hydroxyl groups. These retention behaviors observed

can be rationalized in terms of hydrophobic or non-polar interactions between solute and the C18 stationary phase [13]. These oligomers contain both hydrophobic phenyl and ethoxy groups and hydrophilic hydroxyl group, therefore their retention increases with the number of hydrophobic moieties while decreases with the number of hydrophilic moiety.

4. Conclusions Separations of phenylethoxysiloxanol oligomers prepared in various molar ratios of water to phenyltriethoxysilane (r1 ) have been achieved on a C18 stationary phase using gradient elution with a binary mobile phase of methanol and water. The resolved oligomer species are identified by online ESI-MS with their respective molecular structures. This method can be used to monitor the structural evolution of oligomeric silanols with increasing r1 value in the reaction mixture. The acid-catalyzed hydrolysis and condensation of phenyltriethoxysilane are shown to proceed from partial hydrolysis of the organosilane to formation of linear and monocyclic silanol oligomers with increasing amount of water in the reaction mixture. The retention of the silanol oligomers increases with the number of phenyl and ethoxy groups while decreases with the number of hydroxyl group in the molecules, thus indicating the hydrophobic interactions between the solute and the stationary phase to be the principal retention mechanism. Although the sol–gel process has been extensively investigated, the intermediate products such as silanols have remained largely uncharacterized and authentic samples are not available. The difficulty of isolating such complex intermediates lies in the known labile nature of structurally similar products associated with the reactive silanol groups [19]. To our knowledge, this is the first time that separation of a variety of phenylethoxysiloxanols by liquid chromatography has been demonstrated and that their structures have been elucidated unambiguously by online MS detection. Due to the presence of large number of species in the samples, the current separation system is plagued by peak overlap or coelution of different molecules, making it impossible to distinguish the species without MS detection. To overcome this limitation, future work will be directed at developing comprehensive two-dimensional liquid chromatographic techniques so as to provide an even more powerful tool with greater peak capacity for basic and applied research on silicon based hybrid materials [22,23].

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Acknowledgements The authors thank Mr. Kongying Zhu and Dr. Xinghua Jin from Tianjin University for their technical assistance with the 29 Si NMR and MS measurements. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chroma. 2015.03.043. References [1] C. Sanchez, P. Belleville, M. Popall, L. Nicole, Applications of advanced hybrid organic–inorganic nanomaterials: from laboratory to market, Chem. Soc. Rev. 40 (2011) 696–753. [2] G.L. Drisko, C. Sanchez, Hybridization in materials science–evolution, current state, and future aspirations, Eur. J. Inorg. Chem. (2012) 5097–5105. [3] R.H. Baney, M. Itoh, A. Sakakibara, T. Suzuki, Silsesquioxanes, Chem. Rev 95 (1995) 1409–1430. [4] P.G. Harrison, Silicate cages: precursors to new materials, J. Organometallic Chem. 542 (1997) 141–183. [5] Y. Abe, T. Gunji, Oligo- and polysiloxanes, Prog. Polym. Sci. 29 (2004) 149–182. [6] C.J. Brinker, Hydrolysis and condensation of silicates: effects on structure, J. Non-Cryst. Solids 100 (1988) 31–50. [7] J. Pinkas, Chemistry of silicates and aluminosilicates, Ceramics-Silicaty 49 (2005) 287–298. [8] J. Planelles-Arogo, C. Vicent, B. Julian, E. Cordoncillo, P. Escribano, New insights on organosilane oligomerization mechanisms using ESI-MS and 29Si NMR, New J. Chem. 33 (2009) 1100–1108. [9] D.J. Belton, O. Deschaume, C. Perry, An overview of the fundamentals of the chemistry of silica with relevance to biosilicification and technological advances, FEBS J. 279 (2012) 1710–1720. [10] I.H. Lim, W. Schrader, F. Schuth, The formation of zeolites from solution–analysis by mass spectrometry, Micropor. Mesopor. Mater. 166 (2013) 20–36.

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Separation and identification of oligomeric phenylethoxysiloxanols by liquid chromatography-electrospray ionization mass spectrometry.

Liquid chromatography-electrospray ionization mass spectrometry has been applied to qualitative analysis of oligomeric phenylethoxysiloxanols, a class...
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