Enhancement in sample preconcentration by the on-line incorporation of cloud point extraction to flow injection analysis inside the chemiluminescence cell and the determination of total serum bilirubin

The on-line incorporation of cloud point extraction (CPE) to flow injection analysis (FIA) was previously based on the use of a cotton-packed column to entrap the analyte-containing surfactant aggregates after salt-induced CPE, and then the preconcentrated analyte was eluted into a separate detection cell for subsequent chemiluminescence (CL) detection (via the peroxyoxalate CL reaction). In the work, the on-line CPE/FIA technique was improved by the following: (1) sample preconcentration and CL detection were both carried out directly inside the collection column, thus avoiding the decrease in detection sensitivity due to sample dispersion and dilution, and (2) CL detection was performed through the reaction between nitrite and hydrogen peroxide, which is compatible with aqueous samples and should allow for chemical excitation to occur more efficiently inside the collection column. In addition to more effective sample preconcentration, the CL detection of the entrapped analytes directly inside the collection column, i.e., a unique heterogeneous microenvironment in which analyte-containing surfactant aggregates were embedded within the densely packed filtering material, may also contribute to the overall increase in CL intensity (e.g., a CL enhancement factor of ca. 1000). Under optimum experimental conditions, the calibration curve was found to be linear for the CL detection of bilirubin (5 to 120 μg L^-1), the limit of detection (S/N = 3) was 1.8 μg L^-1, and the R.S.D. was ca. 2.6% (n = 30) for 20 μg L^-1 bilirubin. Good agreements were obtained for the determination of total bilirubin in certified reference human serum samples between the present approach and an established clinical method.