A precise method for the analysis of δ¹⁸O of dissolved inorganic phosphate in seawater

Abstract

A method for preparation and analysis of the oxygen isotope composition (δ 18 O) of dissolved inorganic phosphate (DIP) has been developed and preliminary results for water samples from various locations are reported. Phosphate is extracted from seawater samples by coprecipitation with magnesium hydroxide. Phosphate is further purified through a series of precipitations and resin separation and is ultimately converted to silver phosphate. Silver phosphate samples are pyrolitically decomposed to carbon monoxide and analyzed for δ 18 O. Silver phosphate samples weighing 0.7 mg (3.5 µmol oxygen) can be analyzed routinely with an average standard deviation of about 0.3‰. There is no isotope fractionation during extraction and blanks are negligible within analytical error. Reproducibility was determined for both laboratory standards and natural samples by multiple analyses. A comparison between filtered and unfiltered natural seawater samples was also conducted and no appreciable difference was observed for the samples tested. The δ18 O values of DIP in seawater determined using this method range from 18.6‰ to 22.3‰, suggesting small but detectable natural variability in seawater. For the San Francisco Bay estuary DIP δ 18 O is more variable, ranging from 11.4‰ near the San Joaquin River to 20.1‰ near the Golden Gate Bridge, and was well correlated with salinity, phosphate concentration, and δ 18 O of water. AcknowledgmentsH.S.W. wishes to thank Henry P. Schwarcz and his laboratory at McMaster University, Hamilton, Ontario, Canada, where most of the exploratory Ce precipitation chemistry was performed.

Oxygen Isotopes for Unraveling Phosphorus Transformations in the Soil-Plant System: A Review

“…Other procedures (McLaughlin et al, 2006c;Weiner et al, 2011;Zohar et al, 2010a) use cerium phosphate to separate and purify phosphate from the extracts. This approach was first designed to process marine water samples, and cerium phosphate was chosen because cerium does not form any salt with Cl (McLaughlin et al, 2004(McLaughlin et al, , 2006c. This purification method, however, is not always applicable to soil extracts that have high concentrations of organic compounds and oxides (Tamburini et al, 2010).…”

Oxygen isotopes of phosphate and soil phosphorus cycling across a 6500 year chronosequence under lowland temperate rainforest

“…MgCl 2 was added to the remaining solution at a ratio of 2 g L −1 . The pH was raised to 10.5 to coprecipitate phosphate with magnesium hydroxide (MagIC) (Karl and Tien, 1992;McLaughlin et al, 2004). The concentration data is provided in Supplementary Table 1.…”

“…Sample pH was brought up to 5.5 and cerium nitrate added to precipitate cerium phosphate. The remainder of the process follows the method described in McLaughlin et al (2004) and Zohar et al (2010a). Sample splits were processed with 18 O enriched reagents as described in McLaughlin et al (2006) to determine if oxygen exchange occurs during processing and no measurable differences between the spiked and un-spiked samples were detected.…”

A method for analyzing the δ18O of resin‐extractable soil inorganic phosphate

“…4 filter paper (Whatman, Maidstone, UK), and the resin was washed with 20 mL DDW (double deionized water). The phosphate was then extracted and purified by the precipitation of cerium phosphate similar to the procedure described by McLaughlin et al [12] In detail: samples were buffered by adding 3 mL of 1 M potassium acetate (Sigma-Aldrich, St. Louis, MO, USA). When needed, adjustments to pH 5-6 were done using either 10 M KOH (Frutarom, Haifa, Israel) or 14.3 M HNO 3 (Merck).…”

“…This new method combines up-scaling of established methods for soil phosphate extraction [11] with a method for determining d 18 O P in seawater [12] together with some necessary modifications.…”