Evaluation of Spectrophotometric Flow Injection and Microwave-Induced Plasma Optical Emission Spectrometry Methods for the Quantification of Phosphorus in Fertilizers

Background The growing need to increase the efficiency of routine analyses in quality control laboratories of agricultural fertilizer manufacturers underscores the importance of automating official methods or developing validated alternatives to enhance analytical throughput. Phosphorus, a key nutrient in fertilizers, demands precise and rapid analytical techniques that comply with current regulatory requirements. Objective This study aimed to evaluate Flow Injection Analysis (FIA) methods with molecular absorption spectrophotometric detection—based on official protocols—and compare them to a more advanced technique, Microwave-Induced Plasma Optical Emission Spectrometry (MIP-OES), for the quantification of elemental phosphorus. Methods Three methodologies were assessed: molecular absorption spectrophotometry based on the formation of yellow vanadophosphomolybdate, the formation of molybdenum blue, and MIP-OES. These methods were applied to fertilizer samples extracted using water and neutral ammonium citrate (NAC), as prescribed by official procedures. Results FIA methods provided a sampling throughput of up to 120 analyses per hour, while MIP-OES enabled up to 240 analyses per hour. Both spectrophotometric methods had limits of quantification (LOQ) near 1.0 mg/L. However, the yellow vanadophosphomolybdate method showed better linearity, up to 40 mg/L, compared to 20 mg/L for the molybdenum blue method. MIP-OES exhibited the highest sensitivity (LOQ < 0.2 mg/L) and the broadest linear range. All methods showed acceptable accuracy and precision when tested with certified reference materials. Conclusions FIA systems are advantageous over conventional methods due to their simplicity and low cost, making them suitable for routine laboratory settings. MIP-OES, while requiring greater instrumental investment, offers higher throughput and eliminates the need for reagent preparation. Highlights Considering their respective strengths—FIA for accessibility and MIP-OES for performance—it is recommended that both methods are included as Official Methods of Analysis by the Brazilian Ministry of Agriculture (MAPA) and other international regulatory bodies.