RESUMO
There is increasing regulatory interest in the non-nutritive metals content of fertilizer materials, but at present there is no consensus analytical method for acid digestion and instrument detection of those elements in fertilizer matrixes. This lack of method standardization has resulted in unacceptable variability of results between fertilizer laboratories performing metals analysis. A method has been developed using microwave digestion with nitric acid at 200 degrees C, followed by inductively coupled plasma-optical emission spectrometry instrument detection, for the elements arsenic, cadmium, cobalt, chromium, molybdenum, nickel, lead, and selenium. The method has been collaboratively studied, and statistical results are here reported. Fourteen collaborators were sent 62 sample materials in a blind duplicate design. Materials represented a broad cross section of fertilizer types, including phosphate ore, manufactured phosphate products, N-P-K blends, organic fertilizers, and micro-nutrient materials. As much as possible within the limit of the number of samples, materials were selected from different regions of the United States and the world. Limit of detection (LOD) was determined using synthetic fertilizers consisting of reagent grade chemicals with near zero levels of the non-nutritive elements, analyzed blindly. Samples with high iron content caused the most variability between laboratories. Most samples reasonably above LOD gave HorRat values within the range 0.5 to 2.0, indicating acceptable method performance according to AOAC guidelines for analyses in the mg/kg range. The method is recommended for AOAC Official First Action status.
