Titania microspheres were synthesized using hydrothermal methods to exploit a new liquid chromatography stationary phase. The prepared titania microspheres were approximately 7 μm in diameter, and the particle size distribution was relatively narrow and uniform. Furthermore, the average specific surface area was 276.0 m2·g ·1, the average pore volume was approximately 0.25 mL·g ·1, and the pore diameter was approximately 35.9 nm for sintering titania microspheres. These parameters indicate that the titania microspheres prepared for this study have excellent surface properties for chromatography. Additionally, columns filled with the titania microspheres were able to separate basic compounds, including benzene, nitrobenzene and o-nitroanisole. It could be proposed that the titania microspheres prepared for this study would be a promising stationary phase for liquid chromatography.
High-performance liquid chromatography (HPLC) has been widely accepted as one of the main analytical techniques for organic compounds [
Recently, an increasing number of metal oxide materials have been tested for HPLC column preparation [
This study was intended to test the hypothesis that titania microspheres synthesized by the hydrothermal method could display excellent surface parameters, an ideal mesopore structure, and the narrow size distribution needed for the HPLC stationary phase. Optimal experimental conditions for synthesizing titania microspheres were found. Furthermore, to test the chromatographic performance, the prepared titania microspheres were employed as the HPLC stationary phase to separate some basic compounds.
The titanium tetrachloride used as a precursor for the reaction was purchased from Sinopharm Chemical Reagent Company (Beijing, China); the ethanol and n-hexane used as the mobile phase were purchased from Tedia Company (USA). Ammonium sulfate, urea and other analytical grade reagents were obtained from Shanghai Chemical Reagent Company (Shanghai, China).
To synthesize uniform titania microspheres, 0.72 g of ammonium sulfate, 8.5 g of urea, 8.0 mL of millipore water and 8.0 mL of ethanol were added into a 50-mL conical beaker. The mixture was homogeneously stirred, and an aliquot of 0.6 mL of titanium tetrachloride was added drop by drop while continually stirring for 5 h. A transparent solution with a pH value of approximately 2.0 was then obtained and transferred into 50-mL polyfluortetraethylene cups that were heated at 393 K for 5 h in an oven (DGF- 30, Nanjing, China). Finally, the suspension was filtered, and the precipitate was washed twice with millipore water and ethanol, respectively. The prepared methods need high temperature over 500 K, thus the microspheres obtained from the precipitate were finally ground, dried at 353 K and carbonized at 773 K in a muffle furnace for subsequent characterization.
The morphology of the titania microspheres was observed using a scanning electron microscope (SEM, JSM-5610LV, Japan). The crystal shape of the titania microspheres was determined using an X-ray diffractometer (D/max-2500 VL/PC, Japan). A micromeric model ASAP-2010 surface analysis instrument (Micromeric Corporation, USA) was employed to generate nitrogen adsorption/desorption isotherms. A laser particle size analyzer, model Master Sizer2000 (Malvern Company, U.K) was used to determine the particle size distribution (PSD).
The chromatographic column (250 × 4.6 mm i.d.) was filled with the prepared titania microspheres as the stationary phase at a pressure of 450 psi using a high-pressure pump (STV-150, USA). During the filling procedure, a mixture of isopropanol and methanol were used as the propulsion solvent. The performance of the columns was tested by separating special basic substances with an HPLC/MS (1290 - 6460, Agilent Corporation, American).
Titania microspheres were synthesized using the hydrothermal method. The morphology was characterized, as shown in
6 - 10 µm. Further analysis of the particle size distribution showed that the average diameter of the titania microspheres was 7 µm.
Furthermore, the titania microspheres were characterized by X-ray diffraction (
It is critical for chromatographic columns to have enough free space available to provide a sufficient separation efficiency; this requires that the packing material have a large surface area to facilitate carrying large numbers of samples [
the sintered titania microspheres surfaces were more homogeneous, which is one of the physical requirements for the HPLC stationary phase.
As suggested by Thekkudan et al. [
Normal-phase separation of the aromatic compounds was performed on the packed titania column. It was found that there was 10 µl・mL−1 benzene, 0.4 µl・mL−1 nitrobenzene and 0.2 µl・mL−1 o-nitroanisole. These compounds were detected at 254 nm, and the mobile phase was a combination of n-hexane and ethanol with a gradient elution containing 10% ethanol at 7 min with a rate of 0.6 mL・min−1. The column temperature was controlled to be 35˚C, and the initial pressure of the column was approximately 280 bar. As shown in
The π electron of these molecules can react with the weakly acidic surface of titania microspheres [
Traditional silica columns would lead to a larger solubility in the basic mobile phases at higher temperatures. Simultaneously, tailed peaks appeared when alkaline substances were separated on a silica column. Thus, our work focused on exploring a new liquid chromatography stationary phase. The titania microspheres synthesized using hydrothermal treatments were approximately 7 µm in diameter and had a relatively narrow and uniform particle size distribution. The pore size of the titania microspheres was in the range of mesopores; the average specific surface area was 276.02 m2・g−1, and the average pore volume was 0.25 cm3・g−1. Furthermore, benzene, nitrobenzene and o-ni- troanisole all separated easily in the titania column. All of the parameters highlighted above indicate that titania microspheres prepared using a hydrothermal method would provide an excellent normal phase for liquid chromatography.
This work was financially supported by the Natural Science Foundation of Jiangsu Province (13KJB150023, 15KJB210003, BK20150976), and the Natural Science Foundation of China (41501239).
Zhang, J., Zhu, X.S. and Zhang, Y.P. (2016) Preparation and Characterization of Titania Microspheres and Their Application in a Liquid Chromatography Stationary Phase. Journal of Crystallization Process and Technology, 6, 21- 28. http://dx.doi.org/10.4236/jcpt.2016.63003