Phosphate, an important nutrient, is the most frequent form of phosphorus present in surface waters. It has been predominantly measured by colorimetry following exactly or with minor modifications the method published by Murphy and Riley in 1962. While a relatively easy and fast analysis, measurement at low concentrations are known to be biased due to easy absorption of the phosphate ions to glassware from both samples and calibration standards. The reliable analysis of phosphate at trace levels remains challenging using existent methods. This is the first report of using an optimized isotope dilution method for unbiased analysis of phosphate in surface water samples. Pre-concentration, matrix elimination and separation were achieved using two-dimensional ion chromatography (IC) coupled to a QExactive mass spectrometer (MS). The sulfate ions, normally present in surface water at much higher concentrations than phosphate (20 mg/L vs. 20 µg/L) were removed using first IC column and the high resolution MS can differentiate between the remaining HSO4- ions (m/z=96.95956) and H2PO4- (m/z=96.96907). The internal standard was spiked in the field to compensate for the phosphate loss during storage and also to eliminate the instrumental variability during analysis. Other anionic P species were simultaneously monitored, including 4 inorganic condensed P species, 3 organic condensed phosphates (AMP, ADP and ATP) and 9 sugar phosphate compounds. The method was tested by analyzing over 1000 samples collected from the Georgian Bay and Lake St. Clair (ON, Canada) during late spring, summer and fall seasons. The estimated detection limit for phosphate is 0.1 µg/L as P.

Analysis of anionic phosphorus species and isotope dilution measurement of phosphate in surface water samples

256th National Meeting (2018)

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