Determination of ammonia nitrogen in solid and liquid high-complex matrices using one-step gas-diffusion microextraction and fluorimetric detection

This paper introduces a new method for a one-step determination of ammonia nitrogen (NH3) in high complex solid and liquid samples from the agricultural and livestock sectors. To this end, we developed a simultaneous extraction and fluorimetric labeling of NH3, using gas diffusion microextraction (GDME), followed by the fluorescence measurement under 96-well microplate format. The GDME ensured a selective diffusion of NH3 through a commercial hydrophobic membrane, and confined the acceptor solution, which included the fluorimetric labeling reagent o-phthalaldehyde (OPA). The OPA-NH3 labeling reaction was optimized resorting to a full factorial experimental design, which showed that the reducing agent (Na2SO3) concentration was critical to achieve the highest sensitivity. A similar optimization approach for GDME showed that time and temperature significantly influenced the sensitivity of the assay, and also that the modifications in these two factors could be used to adjust the sensitivity according to the concentrations present in the samples. In our final conditions, it was possible to quantify NH3 in the range between 0.38 and 6.27 mg L−1 using a 10 min extraction at 25 °C in different samples (e.g., corn and grass silages, feces, urine). The developed method showed a high repeatability and reproducibility (intraday and interday relative standard deviations values of 4.5% and 9.5%, respectively) and an adequate limit of detection (0.22 mg L−1). This new methodology also highlighted the simplicity and versatility of GDME for the determination of volatile components of high-complex matrices, which will certainly drive future developments in the analysis of environmental and biological samples.