Chromatographic methods for the quantification of free and chelated gadolinium species in MRI contrast agent formulations

Speciation measurements of gadolinium in liposomal MRI contrast agents (CAs) are complicated by the presence of emulsifiers, surfactants, and therapeutic agents in the formulations. The present paper describes two robust, hyphenated chromatography methods for the separation and quantification of gadolinium in nanoemulsion-based CA formulations. Three potential species of gadolinium, free gadolinium ion, gadolinium chelated by diethylenetriamine pentaacetic acid, and gadolinium chelated by 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine-N-diethylenetriaminepentaacetic acid, were present in the CA formulations. The species were separated by reversed-phase chromatography (reversed phase high-performance liquid chromatography, RP-HPLC) or by high-pressure size-exclusion chromatography (HPSEC). For RP-HPLC, fluorescence detection and post-column online isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS) were used to measure the amount of gadolinium in each species. Online ID-ICP-MS and species-specific isotope dilution (SID)-ICP-MS were used in combination with the HPSEC column. The results indicated that some inter-species conversions and degradation had occurred within the samples and that SID-ICP-MS should be used to provide the most reliable measurements of total and speciated gadolinium. However, fluorescence and online ID-ICP-MS might usefully be applied as qualitative, rapid screening procedures for the presence of free gadolinium ions.