Visualization of Urea Treatments Using Micro-X-Ray Fluorescence Spectrometry

Urea has many uses in agriculture. It is a common source of nitrogen fertilizer but is also used as an inert ingredient in solid formulations. Urea is listed on the U.S. EPA's list of minimal concern. When used as an inert ingredient, urea can be used as a dissolution aid. Its worldwide availability, low cost, and safe-handling profile are some of the drivers of its use. Therefore, a wide variety of chemicals are coated on urea for various reasons. Having a method for detecting the deposition of various chemicals on urea is useful for understanding how uniformly a chemical has deposited on urea. Dyes and pigments have been used in fertilizer and seed treatments as an indication that a treatment has been applied. However, this method assumes that the treatment has the same mobility as the dye through the urea granule. Alternatively, chromatography techniques such as high-performance liquid chromatography and gas chromatography can indicate how much treatment has been applied, but these techniques do not provide information regarding the uniformity of treatment from granule to granule or the penetration of the treatment into the granule. An analytical technique called micro-X-ray fluorescence (MXRF) was evaluated for visualizing urea treatments. MXRF is a nondestructive form of elemental analysis capable of collecting elemental maps over a region of interest. Because urea is a simple organic molecule that contains only carbon, hydrogen, oxygen, and nitrogen (elements not typically detected by X-ray fluorescence spectrometry [XRF]), treatment that contains elements that are detectable by XRF can be differentiated from urea and visualized. This technique was used to analyze urea samples treated with zinc and N-(n-butyl)thiophosphoric triamide (NBPT). The distribution of zinc and NBPT (sulfur and phosphorus) across the granule and within the granule could be observed with MXRF. Whereas a red dye of a commercial NBPT treatment fully penetrated a urea granule, MXRF indicated that NBPT remained on its surface.