Phosphorus speciation in environmental organic matter using 31P-NMR. Recent advances in the characterization of atmospheric organic matter

Phosphorus (P) is a vital element for all living organisms and is one of the most essential nutrients in both aquatic and terrestrial ecosystems. As a fundamental component of DNA and RNA, it plays a structural role in cell membranes (e.g., as part of phospholipids) and is crucial for various biological processes, such as energy transfer through ATP. In the environment, phosphorus exists in both inorganic forms (e.g., orthophosphate or PO₄³⁻) and organic forms. In environmental studies, organic phosphorus (P) is not measured directly but is estimated by subtracting the concentration of inorganic phosphorus species from total phosphorus. This process involves converting all forms of phosphorus into orthophosphate, typically through UV or persulfate oxidation, followed by colorimetric analysis. However, inefficiencies in this transformation can introduce errors and biases, leading to underestimation or overestimation of the organic phosphorus content.

Solid-state ³¹P NMR is a powerful technique that identifies phosphorus chemical species, organic or inorganic, without any previous sample treatment. This technique was widely used in the 2000s in marine chemistry in samples comprised of concentrated dissolved organic matter, fast-sinking particles, marine planktons, and sediments, revealing important features of organic P compounds such as phosphonates. A notable characteristic of P in concentrated dissolved organic matter is that P-esters and phosphonates consistently exhibit a nearly constant ratio of 75:25, regardless of the depth or location where the sample was collected. ³¹P-NMR, however, has not yet been used for atmospheric samples, and is the primary focus of this study. Here, we analyzed total suspended atmospheric particles collected during dust events, as well as ash produced from the biomass burning of olive trees. We find that the functional groups associated with phosphorus included orthophosphate and monophosphate esters, which shared the same chemical shift (H₃PO₄ and RH₂-PO₄), phosphate diesters (R₁R₂HPO₄), and pyrophosphate (H₄P₂O₇). P in our samples consisted of phosphate diesters (72–88%), followed by orthophosphate (10–19%) and pyrophosphate (1–8%). Unlike marine samples, phosphonates were absent, suggesting the absence of compounds containing carbon-phosphorus (C–P) bonds. Phosphate diesters are primarily found in naturally occurring organic compounds, such as nucleotides and their derivatives (e.g., DNA, RNA, AMP, ADP, ATP), including phospholipids, and thus constitute the majority of atmospheric organic phosphorus. As these compounds have C-O-P bonds they are readily hydrolyzed in the marine environment by the alkaline phosphatase enzyme, providing an important source of P in aquatic/marine ecosystems. Overall, our results suggest that organic-P, as estimated by ³¹P NMR, can account for up to 80% of total P in dust and burning biomass samples. Thus, the organic-P fraction of external inputs is likely as significant for marine primary productivity as inorganic inorganic-P.