Determination of paramagnetic Mn(II) concentrations from bulk magnetic susceptibility shifts in NMR spectra

Abstract

The recently discovered negative health effects of Gd(III) complexes based magnetic resonance imaging (MRI) contrast agents have stirred interest into the development of alternatives based on Mn(II) complexes
due to its biocompatibility, and hence less health concerns. One important aspect of investigation on contrast agents in general, and Mn(II) complexes in particular, is an accurate determination of their concentrations in solution. Therefore, in this study a known approach of determining the concentration of paramagnetic species in solution, via the Bulk Magnetic Susceptibility (BMS) shift, is applied for Mn(II) and the feasibility of the method is evaluated. A proof of principle using Gd-DOTP concentrations in the range 0.7 – 6.6 mM Gd(III) to verify the applied BMS method was successfully performed. Subsequently, BMS measurements were performed with MnDOTA as well as free Mn(II) and the obtained concentrations were verified with ICP-OES. A good agreement between the two methods was found within the concentration range of 0.4 - 15.1 mM Mn(II) for both MnDOTA and free Mn(II). Although, BMS measurements showed slightly higher concentrations than those found by ICP-OES for both Mn-DOTA and free Mn(II). Additionally, the deviations between ICP-OES and BMS were greater for free Mn(II) compared to Mn-DOTA. Experiments for determining the accuracy of the BMS method showed a 2.5% upward systematic error. The random error decreased from 20% for 0.27 mM Mn(II) to almost 0% for 4.0 mM Mn(II) and higher concentrations. In conclusion, the BMS method proved to be accurate for determining Mn(II) concentrations and has
promising prospects for future research into Mn(II) complexes. However, deviations between ICP-OES and BMS measurements need further investigations. Key words: Bulk Magnetic Susceptibility shift, contrast agent, Mn(II) complexes.