Expanding the separation capability of sequential injection chromatography: Determination of melamine in milk exploiting micellar medium and on-line sample preparation

Abstract

Silica, C8 and C18 monolithic column were the only option for separation on a sequential injection chromatography (SIC) system due its low-pressure operation. Therewith, the separation of molecules poorly retained by this stationary phase was a difficult task. This work presents the first use of micellar chromatography on a SIC system, with the applicability demonstrated by the determination of melamine in milk. The mobile phase was composed by aqueous sodium dodecyl sulfate (SDS) solution and propanol (92.5:7.5). The adsorption of SDS monomers on the C18 stationary phase modified its chromatographic properties, enabling satisfactory melamine separation from the milk components. The sample pretreatment procedure was on-line implemented by dilution of the sample with SDS using a multiposition valve. Calibration equations obtained from melamine solutions prepared in different types of milk and water presented similar sensitivity, indicating absence of matrix effects. A linear response was observed within 2.0 and 6.0mgL−1 of melamine with a detection limit estimated at 0.6mgL−1 and coefficients of variation at 2.9% (n=6). The procedure was suitable for fast determination of melamine below the limits established by FDA and WHO (2.5mgL−1), with minimized reagent consumption. Results for different milk samples agreed with those obtained by high performance liquid chromatography at the 95% confidence level.