Characterization and optimization of the preparation procedure for solution P-31 NMR analysis of organic phosphorus in sediments

Purpose: The single-step sodium hydroxide-ethylenediaminetetraacetic acid (NaOH-EDTA) extraction is currently the most common preparation technique for the measurement of organic phosphorus (P) composition in sediments using solution ³¹P nuclear magnetic resonance (NMR). In this study, detailed investigations were conducted to evaluate the performance of this technique, with an objective of finding an optimal procedure for the measurement of sediment organic P. Materials and methods: A single-step extraction with NaOH-EDTA was investigated on two types of sediment, i. e., Fe/Al-rich sediment and calcareous sediment. The influence of different sediment preparation methods, NaOH-EDTA compositions, solid:solution ratios, extraction time, pre-concentration techniques, and NMR sample frozen storage time on P extraction and solution ³¹P NMR analysis were investigated. Results and discussion: Both air- and freeze-drying increased organic P extraction for the calcareous sediment. An extraction time of 16 h was sufficient for quantitative recovery of extracted organic P for both Fe/Al-rich and calcareous sediments. The use of a 1:8 solid:solution ratio achieved stronger NMR signals and greater P diversity than the use of a 1:20 ratio. Extraction of the two sediments with 0.25 NaOH-50 mM EDTA favored ³¹P NMR detection by reducing the relaxation times required and the risk of organic P degradation compared to the use of stronger NaOH-EDTA solutions. Rotary evaporation was a better technique for pre-concentration of the NaOH-EDTA extracts than freeze drying. The concentrated extracts could be preserved by freezing (-20 °C) for at least 2 months. Conclusions: An optimized procedure was developed based on these investigations, including freeze-drying of fresh sediments, extraction with 0.25 M NaOH-50 mM EDTA for 16 h using a solid:solution ratio of 1:8, pre-concentration of the extract to the level of ~10 times of its original concentration using rotary evaporation, and storage of the NMR sample at -20 °C until ³¹P NMR analysis.