Validation and Uncertainty Evaluation of an Atomic Absorption Spectrophotometric Method for Iron Determination in Fertiliser Samples

Abstract

Routine analysis of fertiliser samples is conducted to quantify their nutrient content; however, the measured nutrient concentrations may vary depending on the analytical methodology used. The principal objective of this study was to develop and validate a method for determining Iron (Fe) in fertiliser samples using atomic absorption spectrophotometry (AAS). The method was validated at the Soil and Water Testing Laboratory (SWTL), Dera Ghazi Khan, accredited to ISO/IEC 17025:2017, for repeatability, reproducibility, limit of detection (LOD), limit of quantification (LOQ), linearity, recovery, and bias, while the measurement uncertainty was estimated following the Eurachem Guide. The method produced a repeatability relative standard deviation (RSD) of 0.24%, and reproducibility was confirmed by a calculated t-value of 0.021, well below the tabulated t-value of 2.101 at the 95% confidence level. The LOD and LOQ were 0.014% and 0.05% Fe, respectively. A five-point calibration curve constructed over the range of 0-8 ppm Fe showed excellent linearity (R² = 0.99). The recovery of Fe from a certified quality-control sample (2% Fe) was 99.85%, well within the acceptance range of 95-105%. Laboratory performance was further confirmed through participation in the Magruder Fertiliser Sample Check Program (USA), with Z-scores of 1.34 and 0.00 recorded for two independent proficiency-testing rounds, both within the acceptable range of -2.0 to +2.0. The expanded measurement uncertainty, calculated at k = 2 (95% confidence level), was ±0.0004% Fe per unit of product. All validation parameters met the acceptance criteria, demonstrating that the Soil and Water Testing Laboratory, Dera Ghazi Khan, is competent to perform routine Iron analysis in fertiliser samples by the proposed AAS method.