Preconcentration and separation of neutral steroid analytes using a combination of sweeping micellar electrokinetic chromatography and a Au nanoparticle-coated solid phase extraction sorbent

This paper describes the preconcentration of three neutral steroids (testosterone, progesterone, and testosterone propionate) through a combination of off-line preconcentration using a gold nanoparticle (Au NP)-coated silica gel solid phase extraction (SPE) sorbent prior to on-line preconcentration using sweeping micellar electrokinetic chromatography (MEKC). In the initial phase of this study, the sweeping-MEKC parameters for the capillary electrophoresis (CE) separation of the steroid analytes were optimized. The sweeping-MEKC effect was greatest when the sample matrix contained ca. 18% acetonitrile and 100 mM sodium dihydrogen phosphate (pH 7.0). Next, under the optimized sweeping-MEKC operating conditions, a commercial C18-bonded silica gel and a Au NP-coated silica gel were tested for their use as SPE sorbents for the SPE–sweeping-MEKC preconcentration of steroid-spiked urine samples. Of these two sorbents, the Au NP-coated SPE sorbent displayed a superior clean-up efficiency toward the sample matrix. Size exclusion chromatography (SEC) was used to characterize the interactions between urinary proteins and the Au NPs. The results indicated that the removal of the interfering signals from the urinary proteins was probably due to their interactions with residual Au metal surfaces of the Au NP-coated SPE sorbent. When combining Au NP-coated silica gel SPE with sweeping-MEKC for the CE analysis of steroid-spiked urine samples, the limits of detection (at a signal-to-noise ratio of N = 3) for testosterone, progesterone, and testosterone propionate were ca. 1.59, 1.20, and 1.15 μg/L, respectively; in addition, the detection sensitivities (based on peak heights) of these steroids improved by ca. 700-, 1090-, and 1100-fold, respectively, relative to those of the analytes that had not been subjected to preconcentration.