Determination of trace anions in G2-grade phosphoric acid using two-dimensional ion-exclusion-ion-exchange chromatography with valve-switching technology

This study established a method for determining trace anions in G2-grade phosphoric acid under ultra-clean conditions that used two-dimensional ion-exclusion-ion-exchange chromatography coupled with valve-switching technology. The optimal flow rate of ultrapure water through the ion-exclusion column in the first dimension was determined to be 0.5 mL/min through comparative experiments. The valve-switching window was optimized to minimize enriching PO₄^3- on the anion-enrichment column and reduce interference from the phosphoric-acid matrix. The use of external water mode for the suppressor led to lower baseline noise, and optimizing the elution program of the second dimension enabled target trace anions to be well separated, thereby avoiding the risk of a false positive for NO^3-. The anions of Cl^-, Br^-, NO^3-, and SO₄^2- exhibited good linearities within the 0.5-20 μg/kg, with correlation coefficients ≥0.999. The limits of detection (LODs) and quantification (LOQs) were 0.09-0.29 μg/kg and 0.29-0.97 μg/kg, respectively. The spiked recoveries of each anion at four spiked levels ranged from 91.7% to 103.6%, with RSDs of 0.1%-4.9%. The developed method demonstrated excellent stability through repeated injections, and low detection limits; it is highly precise and accurate, and meets the detection requirements for trace anions in G2-grade phosphoric acid. Additionally, retention-time comparisons with single standard substances led to the discovery of other phosphorus-related anions and anion clusters, such as hypophosphite (HPO₃^-) and hexametaphosphate (PO₃^-)₆, in G2-grade phosphoric acid, in addition to conventional anions. This finding is theoretically and practically significant for the future development of purification processes for higher-purity phosphoric acid.