Temperature-promoted large-volume solute enrichment in column-switching miniaturized liquid chromatography: Determination of an antioxidant

Abstract

A two-valve sub-ambient temperature-promoted reversed-phase packed-capillary liquid-chromatography column-switching system has been tailored for sensitive determination of hydrophobic compounds. Such compounds are not easily dissolved in solvent mixtures of non-eluting properties that traditionally are used for solute enrichment in reversed-phase liquid chromatography. Enrichment-column solute focusing of large sample volumes was promoted by use of sub-ambient temperatures only, allowing the use of sample solvents that were stronger or equal to the mobile phase solvent strength. Subsequent column switching and enrichment-column temperature increment provided efficient low-dispersion back-flushed enrichment-column solute desorption onto the analytical column, where the solute was subjected to temperature-programmed gradient action. The antioxidant, Irganox 1076 (octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) extracted from low density polyethylene with 100% acetonitrile served as a hydrophobic model compound. The mobile phase consisted of acetonitrile containing 10 mM triethylamine and formic acid, and the 0.25 mm id enrichment-column and analytical column in lengths of 27 and 250 mm, respectively, were packed with 3.5 µm Kromasil C₁₈ particles. Sample volumes of up to 500 µL were successfully focused on the enrichment column at 5 °C using loading flow rates of up to 40 µL min⁻¹ prior to temperature programming to 90 °C. The concentration limit of detection of Irganox 1076 was 6 ng mL⁻¹when using an injection volume of 500 µL. The within-assay precision was in the range 3.5–6.8% (n = 6) while the between-day precision was 7.5% (n = 3) relative standard deviation. The method was linear within the investigated mass range 3–100 ng (R² = 0.9993).