Determination of fatty alcohol ethoxylates with diphenic anhydride derivatization and liquid chromatography with spectrophotometric detection: a comparative study with other anhydrides.

A method for the determination of fatty alcohol ethoxylates (FAEs) using diphenic anhydride as derivatization reagent and RP-HPLC separation with UV-vis detection is presented and compared to derivatization with maleic, phthalic, and other cyclic anhydrides. With these anhydrides, the reaction rates increased when urea was added to the reaction medium, and the yields did not decrease when the samples contained moderate amounts of water. Gradient elution on a C8 column was performed with water/acetonitrile in the presence of 0.1% acetic acid. The use of diphenic anhydride was advantageous for both the chromatographic separation and the detection. Specifically, sensitivity at 200 and 220 nm was significantly better for the FAE diphenates, resulting in lower limits of detection at both wavelengths for the diphenates than for the maleates and phthalates (up to 30 and 4.3 times lower at 220 nm, respectively). Response factors for the diphenates decreased less than those of the phthalates when the number of ethylene oxide units, m, increased, reaching a constant value of ca. 0.62 when m>3. Peak symmetries and efficiencies were also better than those found for the other anhydrides. The optimized procedure was applied to the characterization and determination of FAEs in the effluent of a wastewater treatment plant and in sea water.