Diamond Technology

 

Pellicular Particles with Spherical Carbon Cores and Porous Nanodiamond Shell for RP HPLC

Landon A. Wiest, David S. Jensen, Chuan-Hsi Hung, Rebecca E. Olsen, Robert C. Davis, Michael A. Vail, Andrew Dadson, Matthew R. Linford

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Abstract

We report the development of chromatographic phases created by coating spherical 3 µm carbon particles with poly(allylamine) (PAAm) and nanodiamond for high-performance liquid chromatography (HPLC). These hybrid particles are prepared via layer-by-layer (LbL) deposition of PAAm and nanodiamond, resulting in core-shell (pellicular) particles. The starting material (core carbon particles) is first immersed in a solution of PAAm, which leads to the self-limiting adsorption of this polymer. After washing, the particles are immersed in a slurry of nanodiamond, which leads to its selflimiting deposition. Alternating, self-limiting, depositions of PAAm and nanodiamond are continued until the desired thickness of a porous PAAm/nanodiamond shell is formed around the carbon core. Finally, the core-shell particles are simultaneously functionalized and cross-linked with a mixture of 1,2- epoxyoctadecane and 1,2,7,8-diepoxyoctane to create a mechanically stable C18 phase. Core-shell particles are characterized by scanning electron microscopy (SEM), and their surface area, pore diameter, and volume are determined using the Brunauer-Emmett-Teller (BET) method. Particle size distribution (PSD) measurements are also obtained. Columns packed with these ca. 4 µm particles showed efficiencies of 56 000 N/m for n-butylbenzene. Van Deemter studies were performed, although the C term from this analysis and the particle size distribution pointed to particle agglomeration. The particles show considerable stability at high pH (11.3 and even 13) over extended periods of time. At pH 11.3, a ca. 5% loss in k was observed after 1 600 column volumes of mobile phase passed through the column. This experiment was followed by one at pH 13.0 in which a ca. 1% loss in k was observed over a 1 000 column volume period, suggesting considerable stability at high pH for this phase. When a narrower particle size distribution was obtained, the Van Deemter curve showed the expected lowering of its C term. The Supporting Information of this work contains a MATLAB program, which conveniently plots Van Deemter curves, including the individual contributions of the A, B, and C terms, the residuals to the fit, the values of A, B, and C, the rmse and R2 values for the fit, and the optimal values of u and H.

Introduction

Silica is the workhorse of modern liquid chromatography. Accordingly, its surface has been extensively studied and modified, which has led to a broad array of available functionalities for the chromatographer. However, despite its flexibility, silica lacks stability at both high and low pHs. Indeed, for many silica-based chromatographic materials, the useful window of pH stability lies between ca. 3 and 8, although if better silane ligands and/or modified silica, such as silica-polymer hybrids,5 are used this range can be extended. Nevertheless, there remains some question regarding the long-term stability of these materials, especially at elevated pH, where a loss of column efficiency, increase in back pressure, and ultimately bed collapse may occur.

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