Compounded injections of sodium or potassium phosphates are sterile solutions that contain a mixture of monobasic and dibasic phosphates in water for injection [1]. These solutions serve as a phosphate source to either prevent or correct hypophosphatemia (i.e., an abnormally low phosphate level in the blood) in patients with restricted or no oral intake. After dilution, these can be administered intravenously as electrolyte replenishers. They are also useful as additives for preparing specific intravenous fluid formulas.

An ion chromatography (IC) assay with suppressed conductivity detection is the standardized way to accurately quantify phosphate in phosphates compounded injections [2–3]. The Metrosep A Supp 17 column was established as a suitable alternative column within a standardized validation procedure [4] in cooperation with the U.S. Pharmacopoeia (USP). This column guarantees reliable separation, and the Metrohm suppressor module (MSM) ensures low background noise and robust long-term performance.

For the equivalence investigation of the Metrosep A Supp 17 - 150/4.0 column, compounded injections were prepared from the respective mono- and dibasic sodium or potassium salts of phosphate. Anhydrous salts from different manufacturers were used.

Sample stock solutions for the sodium phosphates compounded injection were prepared from 24 g of monobasic sodium phosphate and 14.2 g of dibasic sodium phosphate, both dissolved in 100 mL sterile water for injection. For the potassium phosphates compounded injection, 22.4 g of monobasic potassium phosphate and 23.6 g of dibasic sodium phosphate were dissolved in 100 mL sterile water for injection.

The sample stock solutions were further manually diluted in ultrapure water (1250-fold) to a nominal concentration of 0.23 mg/mL phosphate. All samples were prepared as individual duplicates.

A single point calibration with 0.230 mg/mL of phosphate, prepared from dibasic potassium phosphate in water, was used.