AOAC RI ERP EBOOK FOR FERTILIZERS

These fertilizers are liquid and were used without any further modifications, unless specified in the method, (i.e., dilution). The above twelve commercially available U-F liquid fertilizers were tested by the authors’ lab (Lab 1) and by an independent lab (Lab 2) using AOAC Official Method 2003.14 for the determination of their unreacted (free) urea by HPLC. Each sample was analyzed for its free urea content in duplicate. The inter laboratory results were compared to each other for the consistency of this method for the determination of free urea contents in these fertilizers by comparing each value obtained from the means of duplicate analyses. Analytical results, means and standard deviations are shown in Table 2. The same materials were also analyzed by a commercial lab using the AOAC Official Method 953.01 (Urease Method) for the determination of their free urea contents. Each sample was analyzed in duplicate and their means were used for comparisons with the means of those results obtained by Lab 1, and Lab 2 using the AOAC Official method 2003.14. The commercial lab used powdered form of urease made from Jack bean meal. In addition, different sources and forms of urease enzyme were tested to understand what effects (if any) of the urease enzyme has on the determination of free urea contents of this class of fertilizers.

Analytical methods AOAC Official Method 2003.14 2

A. Principle

A precisely weighed portion of homogenous urea or homogenous water-soluble urea-containing liquid UF fertilizer sample was diluted to volume with solvent of the same composition used as the liquid chromatographic mobile phase. Ureawas determined via high performance liquid chromatography (HPLC) employing ultraviolet wavelength detection. The area beneath the absorption peak due to urea in the sample was compared with the area beneath the absorption peak for urea in an external standard solution prepared with pure urea in the mobile phase solvent.

B. Apparatus

(a) Liquid Chromatograph. – Requires a high performance liquid chromatograph capable of isocratic delivery of mobile phase at 2 ml/min at 204 bars (3000 psig) and having a UV absorption detector capable of stable operation at 195 nm (Acetonitrile and water absorption cutoff). Instrument operating conditions are listed in Table 1: