A new application of the diacetyl monoxime method to the automated determination of dissolved urea in seawater

A new application of the diacetyl monoxime method for the determination of dissolved urea in seawater is presented, based on the Alpkem autoanalyzer (OI, Analytical). This system has several advantages over automated methods previously described in the literature. The urea cartridge can be easily built using nut/ferrule connections, and the external thermostatic bath is not required. The system is set up to operate at low urea concentrations (0–3 µM urea), but it is linear up to 100 µM urea. The sample volume needed for analysis is 0.4 ml, and the rate of analysis is 25 samples h^–1. The precision of analysis (CV) of seawater samples is 2.1% at 0.50 µM urea, and the detection limit is 0.02 µM urea. The interference on the colorimetric measurement due to formation of Brij-35 aggregate in strongly acidic media was eliminated by substituting this wetting agent with hexadecyl–trimethyl–ammonium bromide; thus, this method is suitable for other autoanalyzer technologies that use small-diameter transmission tubes and small-sized spectrophotometric flowcells.Communicated by R. Cattaneo-Vietti, Genova