Development and Validation of an Ion Chromatography Method for Quantification of Ammonium Ions in STEALTH® Liposomes

Ammonium sulfate is one of the subsidiary components in the stealth liposome structure. The ratio of ammonium ion bound to liposome sphere to ammonium ions outside the liposome plays an important role in drug delivery formulation; accordingly, in order to quantify the ammonium ion in the liposome structure, a rapid and sensitive method was validated using a conductivity detector. Through this method, the amount of ammonium enclosed in the liposomal spheres is determined by subtracting the amount of extra-liposomal ammonium content from the total amount of ammonium present in the liposome structure. Destruction of the liposome structure with the aid of 1% w/w Triton-X-100 solution allows for the analysis of the total ammonium ion present in the liposome structure. In the present research, ultrafiltration made it possible to isolate and analyze the extra-liposomal ammonium ions. This measurement was performed using an ion exchange chromatography column, isocratic elution flow and a linearity of 0.9998. Based on this signal-to-noise method, LOD was determined as 0.0003 mM ammonium ion, with a signal-to-noise ratio of 3:1 and RSD of 1.4%. With a signal-to-noise ratio of 10:1, LOQ was determined as 0.001 mM ammonium ion with RSD 1.2%. In the determination of the total ammonium ion, the individual percentage recovery ranged from 100.00 to 100.93% and for the external ammonium ion analysis, the individual percentage recovery varied from 95.87 to 98.12% for all three levels and RSDs were 0.27, 0.71 and 0.71% for these concentrations.