Accurate determination of carbonate-associated phosphate (CAP) by ICP-OES following cation-exchange resin separation

An improved and robust method for determining carbonate-associated phosphate (CAP) is presented. • AG50W-X12 effectively purifies P from carbonate samples with high Mg Ca matrices. • The use of the P 177.495 nm emission line enables a low detection limit of <3 ppb. • CAP values for carbonate reference materials (GSR-13, GSR-23 and Jdo-1) agree well with published data and exhibit improved precision. • CAP values of five carbonate standards (GSR-26, GSR-27, GSR-28, GSR-29, and BCS-CRM 393) are reported for the first time. Carbonate-associated phosphate (CAP) serves as a direct proxy for reconstructing past oceanic phosphorus (P) concentrations and provides valuable insights into the co-evolution of marine P cycling, primary productivity, and atmospheric oxygen levels throughout Earth's history. However, current CAP analytical methods face challenges, including poor reproducibility, high detection limits, and a scarcity of geological reference material datasets for inter-laboratory comparison. In this study, we developed a method to purify P from carbonate samples with high Ca Mg matrices using AG50W-X12 cation-exchange resin, followed by quantitative determination by ICP-OES using the strongest P emission line at 177.495 nm, achieving a detection limit for CAP values of 0.002 mmol/mol. The methodological robustness with respect to variations in acid concentration, dissolution time, and the fraction of carbonate dissolved was evaluated through multiple independent experiments. The method was then applied to determine CAP values of eight carbonate geological reference materials (mmol/mol): CAP GSR-13  = 0.018 ± 0.002 (2σ, N  = 3, RSD = 4.2%); CAP GSR-23  = 0.061 ± 0.008 (2σ, N  = 8, RSD = 6.9%); CAP GSR-26  = 0.093 ± 0.014 (2σ, N  = 3, RSD = 7.3%); CAP GSR-27  = 0.025 ± 0.006 (2σ, N = 3, RSD = 12.3%); CAP GSR-28  = 0.047 ± 0.008 (2σ, N = 3, RSD = 8.9%); CAP GSR-29  = 0.051 ± 0.010 (2σ, N = 3, RSD = 9.6%); CAP BCS-CRM 393  = 0.027 ± 0.005 (2σ, N = 3, RSD = 9.4%); CAP JDo-1  = 0.147 ± 0.008 (2σ, N = 3, RSD = 2.6%). Compared with previously published CAP values for three standards: CAP GSR-13  = 0.022 ± 0.004 (2σ, RSD = 9.1%), CAP GSR-23  = 0.075 ± 0.015 (2σ, RSD = 9.7%) or 0.070 ± 0.014 (2σ, RSD = 10.0%), and CAP JDo-1  = 0.135 ± 0.012 (2σ, RSD = 4.6%), our results exhibit comparable accuracy but improved analytical precision. Long-term reproducibility of the GSR-23 standards was better than 0.008 mmol/mol (2σ) over a three-month measurement period. The CAP values for GSR-26, GSR-27, GSR-28, GSR-29, and BCS-CRM 393 are reported here for the first time. This high-precision analytical method enables the identification of subtle variations in P concentrations during carbonate formation and significantly facilitates future investigations into the evolution of marine phosphorus cycling. Download: Download high-res image (133KB) Download: Download full-size image