Retention Behavior of Highly Hydrophobic, Structurally Related Corticosteroids with SDS Based Microemulsion

Abstract

The present work describes the retention behavior of structurally related corticosteroids separated with SDS based microemulsion. The analytes include cortisone acetate (CA), hydrocortisone (H), hydrocortisone acetate (HA), prednisolone (P), prednisolone acetate (PA), and prednisone (Ps), which are not easily separated by the conventional MEKC. Surfactants with linear and planar structures have been compared for the separation of these analytes. By fine tuning the parameters, the results revealed that complete baseline separation could be achieved with a microemulsion consisted of 0.8% (w/w) n-octane, 3.6% (w/w) SDS, 6.6% (w/w) 1-butanol, and 89% (w/w) phosphate buffer (40 mM, pH 8.0), under an applied voltage of +7 kV and a column dimension of 53 cm (effective length, 30 cm) × 75 μm id. Although the composition of the microemulsion has a high recognition property for these structurally related compounds, the high affinity of these analytes with the pseudostationary phase made the analysis time very long. The effects of the addition of acetonitrile, methanol, and isopropanol have also been studied. Acetonitrile was found to have the greatest impact on this separation.

Keywords:

• Corticosteroids
• EKC
• Microemulsion
• SDS

ACKNOWLEDGMENT

We thank the National Science Council of Taiwan for financial support.

Manuscript 6381

Notes

^a Fused-silica capillary: 73 cm (30 cm) × 75 μm id; microemulsion: 0.8% (w/w) n-octane, 6.6% (w/w) 1-butanol, 88.8–89.3% (w/w) phosphate buffer (40 mM, pH 8.0); sample injection: hydrostatic (9 cm, 5 s); sample concentration: 66.7 μg mL⁻¹; applied voltage: +10 kV; detection: 254 nm.

^b Capacity factor, k = (t_r/t_o − 1)/(1 + t_r/t_mc), where t_r, t_o (with methanol as marker) and t_mc (with dodecylbenzene as tracer) are the retention times of the solute, the void volume due to EOF, and the microemulsion, respectively.

^c Theoretical plate, N = 5.54 (t_r/w_½) ² , w_½ is the peak width at half height.

^d Resolution, R_s = 2 (t_r2 − t_r1)/(w₂ + w₁).

^a Conditions were as in Table 1, except the fused-silica capillary: 53 cm (30 cm) × 75 μm id; microemulsion: 0.8% (w/w) n-octane, 3.6% (w/w) SDS, 6.6% (w/w) 1-butanol, 74–85% (w/w) phosphate buffer (40 mM, pH 8.0) and the applied voltage: +7 kV.

^a Conditions were as in Table 1, except the fused-silica capillary: 53 cm (30 cm) × 75 μm id; microemulsion: 0.8% (w/w) n-octane, 3.6% (w/w) SDS, 6.6% (w/w) 1-butanol, 64–89% (w/w) phosphate buffer (40 mM, pH 8.0) and applied voltage: +7 kV.