Preparation and evaluation of carbon coated alumina as a high surface area packing material for high performance liquid chromatography

Abstract

The retention of polar compounds, the separation of structural isomers and thermal stability make carbonaceous materials very attractive stationary phases for liquid chromatography (LC). Carbon clad zirconia (C/ZrO(2)), one of the most interesting, exhibits unparalleled chemical and thermal stability, but its characteristically low surface area (20-30 m(2)/g) limits broader application as a second dimension separation in two-dimensional liquid chromatography (2DLC) where high retentivity and therefore high stationary phase surface area are required. In this work, we used a high surface area commercial HPLC alumina (153 m(2)/g) as a support material to develop a carbon phase by chemical vapor deposition (CVD) at elevated temperature using hexane vapor as the carbon source. The loading of carbon was varied by changing the CVD time and temperature, and the carbon coated alumina (C/Al(2)O(3)) was characterized both physically and chromatographically. The resulting carbon phases behaved as a reversed phase similar to C/ZrO(2). At all carbon loadings, C/Al(2)O(3) closely matched the unique chromatographic selectivity of carbon phases, and as expected the retentivity was increased over C/ZrO(2). Excess carbon - the amount equivalent to 5 monolayers--was required to fully cover the oxide support in C/Al(2)O(3), but this was less excess than needed with C/ZrO(2). Plate counts were 60,000-76,000/m for 5 μm particles. Spectroscopic studies (XPS and FT-IR) were also conducted; they showed that the two materials were chemically very similar.

Figure 1

Schematic of chemical vapor deposition (CVD) apparatus. 1 and 2 are gas controllers for each direction (200 cc/min.)

Figure 2

Schematic of the device used to measure the resistivity of various carbon materials including C/ZrO₂, C/Al₂O₃, and graphite. The circuit consists of a reference voltage (V_s,); a measured voltage (V_AB,); the reference resistance (R_s,); the sample resistance (R_x); A and B are connected to the potentiometer that has input impedance of > 100 GΩ.

Figure 3

Chromatogram for a homolog series of nitroalkanes. LC conditions: 35/65 MeCN/water, T = 40 °C, F = 0.4 ml/min. 50 × 2.1 mm id. column, solutes: nitropropane, nitrobutane, nitropentane and nitrohexane (100 μg/ml), 1 μl injection.

Figure 4

Differential pore size distribution for pore volume and surface area for various carbon load computed by the BJH method from nitrogen adsorption (upper) and desorption (lower) data. (*) bare Al₂O₃; (◇) 6 %; (□) 24 %; (▲) 40 % C/Al₂O₃

Figure 5

Differential pore size distribution for pore volume (upper) and surface area (lower) for various carbon load on Al₂O₃ using the models: A, model 1 of smooth coating of uniform thickness; B, model 2 of smooth coating thicker in larger pores (i.e. not uniform thickness, but instead with uniform volume fraction of carbon in pores; carbon thickness/pore diameter constant for all values of pore diameter). See Fig. 4 for symbols.

Figure 6

Plot of log (resistance, ohms) for various carbon materials. See Fig. 2 and Eq. 1 for the calculation of the resistance. Bars indicate a standard deviation obtained from triplicate measurement (no error bar if not measured at least three times).

Figure 7

XPS C 1s spectra for (a) 24 % C/Al₂O₃ and (b) C/ZrO₂

Figure 8

Plot of logk′ vs. number of methylene groups for (a) nitroalkane homologs (nitropropane, nitrobutane, nitropentane and nitrohexane); (b) alkylbenzene homologs (benzene, toluene, ethylbenzene, propylbenzene and butylbenzene). LC conditions: F = 0.4 ml/min., T = 40 °C and (a) 35/65 MeCN/water; (b) 50/50 MeCN/water. (○) C/ZrO₂ (33 × 2.1 mm id. column); (◇) 6 %; (◽) 14 %; (□) 24 %; (▲) 40 % C/Al₂O₃ (50 × 2.1 mm id. column)

Figure 9

Plot of log (k′/k′_benezene) vs. benzene substituted compounds. LC conditions: F = 0.4 ml/min., T = 40 °C, 50/50 MeCN/water. (○) C/ZrO₂ (33 × 2.1 mm id. column); (◇) 6 %; (◽) 14 %; (□) 24 % C/Al₂O₃ (50 × 2.1 mm id. column); (*) ODS (50 × 2.1 mm id. column)

Figure 10

Ratio of the retention time of basic drugs on 24 % C/Al₂O₃ and C/ZrO₂. LC conditions: A, 20 mM perchloric acid; B, MeCN; 10 – 80 % MeCN, in 0–2.5 min. F = 1 ml/min. T = 40 °C, 33 × 2.1 mm id. column for both. PMA, p-Methoxyamphetamine; MDA, 3,4-Methylenedioxyamphetamine; PMMA, p-Methoxy-methamphetamine; MDMA, Methylenedioxy-N-methylamphetamine; MDEA, 3,4-Methylenedioxy-N-ethylamphetamine; MBDB, 3,4-Methylenedioxy-alpha-ethyl-N-methylphenethylamine

Figure 11

Chromatograms of mixture of 4 indolic metabolites on (a) C/ZrO₂ and (b) 24% C/Al₂O₃. LC conditions: A, 20 mM perchloric acid in water; B, MeCN; 8 – 35 % B in 0 –3.5 min for (a); 15– 60 % B in 0– 3.5 min for (b); F = 1 ml/min, T = 80 °C, 220 nm, 25 μl injection, 33 × 2.1 mm id. column for both. The analyte diluents (B/A) are 20/80 (black solid line), 40/60 (red dashed line), 80/20 (blue dotted line)