Determination of Nitrogen in Fertilizer by Combustion: Collaborative Study
Abstract Fourteen laboratories participated in a collaborative study to compare abilities of AOAC modified copper catalyst Kjeldahl method, 978.02, and the generic combustion method, 990.03, to analyze the nitrogen content of fertilizer materials. Combustion analyses are more time efficient, more accurate, and less hazardous than Kjeldahl analyses. There were 3 different types of instrumentation involved in the collaborative study: (1) Leco FP-428 Nitrogen Determinator; (2) Perkin-Elmer 2410 Series II Nitrogen Analyzer; (3) Carlo-Erba 1500 Series II Nitrogen Analyzer. Thirty samples of fertilizer containing 1–67% N included 2 ACS grade standard materials: ammonium nitrate, theory 35.00% N; and dicyandiamide, theory 66.64% N. A diammonium phosphate and urea mixture (3 + 1 ; 1.0 mm grind) and 2 ACS grade standard materials of ammonium nitrate and ammonium sulfate were supplied for repetitive combustion analyses. Overall method performance of the combustion method was at least as good as the modified Kjeldahl method. Repeatability standard deviation (Sr) values for the combustion method ranged from 0.09 to 0.34 vs the modified Kjeldahl method range of 0.06–0.49; reproducibility standard deviation (SR) values for the combustion method ranged from 0.13 to 1.07 vs the range of 0.09–3.57 for the modified Kjeldahl method. The grand mean was 20.78% for the combustion method, and 20.79% for the modified Kjeldahl method using various fertilizers. The average ranges of sr and SR for the methods were, respectively, 0.17 and 0.29 for the combustion method, and 0.19 and 0.54 for the modified Kjeldahl method. The method was adopted first action by AOAC INTERNATIONAL.