Cheap C18-modified Silica Monolith Particles as HPLC Stationary Phase of Good Separation Efficiency Ashraf Ali, Faiz Ali, and Won Jo Cheong * Department of Chemistry, Inha University, Incheon 402-751, South Korea. *E-mail: wjcheong@inha.ac.kr Received January 19, 2015, Accepted February 23, 2015, Published online May 26, 2015 Keywords: Silica monolith particles, C18 modification, Partially sub-2 μm, High separation efficiency Columns packed with silica-based bonded phases have been used in reversed-phase liquid chromatography because of their high porosity, large surface area, compatibility with dif- ferent solvents, mechanical stability, ease of surface modifica- tion, and economic feasibility. Bulk monolithic columns have attracted a considerable amount of interest in the last decade, and both monolithic and packed chromatographic columns have their own advantages and disadvantages. 1–5 The columns packed with particles have a high efficiency but they are accom- panied with a high column back pressure due to lower permea- bility, while the monolithic columns have a high permeability but they result in inferior separation efficiency for the analysis of small molecules in HPLC. 6–13 In our laboratory, we have been using the pseudo-monolithic silica particles with C-18 ligand 1,14,15 or polystyrene film 1,2,16–18 as packing materials. In the current study, porous partially sub-2 μm silica monolith particles have been prepared by a sol–gel process with a six times increased production scale compared with our previous work. 16,17 These particles have been chemically modified with a C18 reagent and end-capped with a mixture of trimethylchlor- osilane (TMCS) and 1,1,1,3,3,3 hexamethyldisilazane (HMDS) and packed in HPLC columns (1.0 mm × 300 mm). Five ana- lytes (phenol, acetophenone, 4-methyl-2-nitroaniline, benzene, and toluene) are separated with good separation efficiency and resolution by using 60/40 (v/v) ACN/ H 2 O containing 0.1% TFA as the mobile phase with a flow rate of 25 μL/min. In our another previous study, 15 C18-modified partially sub-1 μm silica monolith particles were prepared and packed in a column of 15 cm length to show excellent separation effi- ciency with the number of theoretical plates (N) of about 180,000/m. Nevertheless, the N value per column was only 27,000 since the maximum length of the packed column was only 15 cm due to the reduced particle size. In this study, C18-modified partially sub-2 μm silica monolith particles have been prepared in an enhanced production scale and suc- cessfully packed in columns of 30 cm length to show the enhanced N value per column (37,000) although the N value per meter has been reduced to 123,000/m. Characterization of Bare and C18-modified Silica Monolith Particles Figure 1 shows the microscopic view (a) and SEM images of C18-bound silica monolith particles (b, c, d) of this study. The surface of C18-bound silica monolith particles is rather rough as shown in Figure 1(d), in comparison to the polystyrene- bound silica monolith particles of our previous studies. 16,17 This is natural because the C18 moiety is attached in the form of single alkyl chain and the roughness of the silica surface is almost maintained. A promising result was observed in the particle size distri- bution of silica monolith particles prepared in an enhanced production scale of this study (Table 1 and Figure 2) com- pared with that of the production of a smaller scale. 17 Both the d(0.5) value (average particle size) and the dispersity of distribution of this study were reduced considerably in com- parison to the results of the previous study, 17 which will be beneficial in view of mass transfer kinetics and separation efficiency. The BET/BJH N 2 adsorption/desorption plots for bare silica and C18-bound silica particles are shown in Figure 3. The BET/BJH adsorption–desorption reports of the previous and current studies are comparatively summarized in Table 2. The pore size was decreased from 296 Å for bare silica to 216 Å for C18-bound silica monolith particles after modi- fication as shown in Figure 3 and Table 2. The decrease in pore size after C18 modification is relatively greater in the cur- rent study. Some differences were again observed in the prop- erties of bare silica monolith particles between the batch of this study prepared on a larger scale and the batch of the pre- vious study 17 prepared on a smaller scale. The pore size, total pore volume, and surface area of current study were all some- what reduced in comparison to those of the previous study (Table 2). Chromatographic Performances It seems that the minor changes of physical properties of the silica monolith particles induced by the scale-up did not cause any trouble in separation performance of the resultant stationary phase of this study since the chromatographic per- formance of the column packed with the stationary phase of this study has proven to be better than those of a 5 μm com- mercial C18 phase or the C18-modified silica monolith parti- cles of the previous study as shown in Figure 4, and the column theoretical plates of this study (average 37,700) were found rather in the level of the top class HPLC columns as shown in Table 3. Note DOI: 10.1002/bkcs.10320 A. Ali et al. BULLETIN OF THE KOREAN CHEMICAL SOCIETY Bull. Korean Chem. Soc. 2015, Vol. 36, 1733–1736 © 2015 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Wiley Online Library 1733