New method for development of carbon clad silica phases for liquid chromatography: Part I. Preparation of carbon phases

Abstract

Owing to its combination of unique selectivity and mechanical strength, commercial carbon clad zirconia (C/ZrO₂) has been widely used for many applications, including fast two-dimensional liquid chromatography (2DLC). However, the low surface area available (only 20-30 m²/g for commercial porous ZrO₂) limits its retentivity. We have recently addressed this limitation by developing a carbon phase coated on the high surface area of HPLC grade alumina (C/Al₂O₃). This material provides higher retentivity and comparable selectivity, but its use is still limited by how few HPLC quality types of alumina particles (e.g., particle size, surface area, and pore size) are available. In this work, we have developed useful carbon phases on silica particles, which are available in various particle sizes, pore sizes and forms of HPLC grade. To make the carbon phase on silica, we first treat the silica surface with a monolayer or less of metal cations that bind to deprotonated silanols to provide catalytic sites for carbon deposition. After Al (III) treatment, a carbon phase is formed on the silica surface by chemical vapor deposition at 700 °C using hexane as the carbon source. The amount of Al (III) on the surface was varied to assess its effect on carbon deposition, and the carbon loading was varied at different Al (III) levels to assess its effect on the chromatographic properties of the various carbon adsorbents. We observed that use of a concentration of Al (III) corresponding to a full monolayer leads to the most uniform carbon coating. A carbon coating sufficient to cover all the Al (III) sites, required about 4-5 monolayers in this work, provided the best chromatographic performance. The resulting carbon phases behave as reversed phases with reasonable efficiency (50,000-79,000 plates/m) for non-aromatic test species.

Figure 1

Differential pore size distributions for pore volume and surface area for various carbon loads computed by the BJH method from nitrogen adsorption (upper) and desorption (lower) data. (*) SiO₂; (◇) half monolayer (4 μmol/m²) Al/SiO₂; (Δ) one monolayer (8 μmol/m²) Al/SiO₂.

Figure 2

Plot of carbon loading (% C, w/w) vs. CVD time for alumina (◇), 8 μmolAl/SiO₂ (Δ) and 2 μmolAl/SiO₂ (○). The arrow is to help compare induction times between 8 μmolAl/SiO₂ and 2 μmolAl/SiO₂. CVD temperature is 700 °C for all data.

Figure 3

Chromatogram for homolog series of nitroalkanes (nitropropane, nitrobutane, nitropentane and nitrohexane). LC conditions: 20/80 MeCN/water, T = 40 °C, F = 0.4 ml/min. 33 × 2.1 mm id. column for both phases.

Figure 4

Plot of log k vs. number of methylene groups for nitroalkane homologs (nitropropane, nitrobutane, nitropentane and nitrohexane). (a) 2 μmolAl/SiO₂: (▲) 32 %; (□) 8 % C; (b) 8 μmolAl/SiO₂: (Δ) 25 %; (□) 21 %; (◇)14 % C; (○) C/ZrO₂.; (*), 24 % C/Al₂O₃. LC conditions: F = 0.4 ml/min., T = 40 °C, 35/65 MeCN/water; all columns are 33 × 2.1 mm id. Error bars are not bigger than the markers in the plot.

Figure 5

Plot of k vs. % C (w/w) for nitrobenzene (▼), p-xylene (◆), ethylbenzene (■) and toluene (●). A: 2μmolAl/SiO₂; B: 8 μmolAl/SiO₂, Extrapolation in B is based on linear regression of all data points (R² for nitrobenzene, p-xylene, ethylbenzene and toluene are 0.990, 0.999, 0.999 and 0.999, respectively). LC conditions: F = 0.4 ml/min., T = 40 °C, 50/50 MeCN/water; all columns are 33 × 2.1 mm id. Error bars are not bigger than the markers in the plot.

Figure 6

a.Differential pore volume and area distributions for various carbon loads on 2 μmolAl/SiO₂ computed by the BJH method from nitrogen adsorption (upper) and desorption (lower). (*) bare SiO₂; (◇) 8 % C; (○) 32 % C. b.Differential pore volume and area distributions for various carbon loads on 8 μmolAl/SiO₂; adsorption (upper) and desorption (lower); (*) bare 8 μmolAl/SiO₂; (□) 21 % C; (Δ) 25 % C.

Figure 6

a.Differential pore volume and area distributions for various carbon loads on 2 μmolAl/SiO₂ computed by the BJH method from nitrogen adsorption (upper) and desorption (lower). (*) bare SiO₂; (◇) 8 % C; (○) 32 % C. b.Differential pore volume and area distributions for various carbon loads on 8 μmolAl/SiO₂; adsorption (upper) and desorption (lower); (*) bare 8 μmolAl/SiO₂; (□) 21 % C; (Δ) 25 % C.