Unbiased Measurement of Phosphate and Phosphorus Speciation in Surface Waters

Abstract

Trace-level phosphate analysis and phosphorus speciation in surface water remained challenging due to adsorption and phosphate uptake by microorganisms. In this study a two-dimensional ion chromatography separation coupled to electrospray ionization high-resolution mass spectrometry (2D-IC-ESI-MS) allowed isotope dilution quantitation of phosphate with simultaneous analysis of 11 phosphate-containing metabolites and two inorganic condensed phosphates. Samples were collected from Lake St. Clair, Lake Ontario, and Georgian Bay (ON, Canada). Comparative experiments showed lower phosphate results for samples not immediately spiked and for external calibration quantitation. Field spiking with ¹⁸O-labeled phosphate combined with isotope dilution quantitation allows measurement of the phosphate levels existent at the collection time instead of the phosphate concentrations remaining in the samples at the analysis time. This is a significant advantage against the traditional colorimetric and ion chromatographic (IC) analysis methods, which are unable to compensate for the adsorption loss occurring in standards and samples, especially when phosphate is present at levels below 20 μg L^-1 as P (61 μg L^-1 as PO₄^3-). Two phosphate-containing metabolites, adenosine 5'-monophosphate (AMP) and d-glucose 6-phosphate (Glucose-P), were detected in a subset of samples collected from Lake St. Clair, with no statistically significant correlation between them and the simultaneously measured phosphate. Directly bioavailable P (phosphate), indirectly bioavailable P (phosphatase-hydrolyzed P) and nonbioavailable P (nonhydrolizable P) fractions were quantified by measuring phosphate, phosphate after phosphatase addition and total phosphorus. The proposed 2D-IC-ESI-MS method developed for a QExactive MS instrument with field spiking of the internal standard provides accurate phosphate results and eliminates quantitation errors caused by phosphate adsorption. This setup allows simultaneous collection of targeted and nontargeted analysis data and thus the detection of trace polar organic phosphorus metabolites as well.