Ferrous determination in ground and brackish water by high-resolution electrospray ionization mass spectrometry

Abstract

Generally, methods using nebulizers such as in Electrospray Ionization Mass Spectrometry (ESI-MS) or Induced Coupled Plasma Mass Spectrometry(ICP-MS) for the determination of Fe(II) in seawater are difficult because of the high concentration of salt. In earlier studies, we determined Fe(II) in river water with 1,10-phenanthroline (phen) and measured the complexed species by ESI-MS, but a method to reduce the interference of salts in seawater is lacking. After filtration of water samples with a 0.45 µm membrane on site, the sample was immediately added into a previously prepared solution containing sodium perchlorate (NaClO4) and phen in nitrobenzene (NB). After shaking for 20 min, Fe(II) was extracted into nitrobenzene (NB) with the perchlorate ions. The NB was introduced to an ESI-MS, using a syringe. The Fe(II)-phen complex and perchlorate ions in NB were more stable than in the aqueous solution.Approximately 99.6 % of Fe(II) was extracted into the organic phase. In our previously presented data, Fe(II) was complexed with 1,10-phenanthroline in aqueous solution, and measured by HR-ESI-MS, a standard addition method was used, and a calibration curve was obtained. In this method, the samples were added to 0.025, 0.05, 0.1 and 0.2 Fe(II)/µmol/L and a calibration curve (R2 = 0.993) of the peak intensity was determined by HR-ESI-MS from the total peak areas of [Fe(phen)₂-NB]2+m/z 269.5 and [Fe(phen)3]2+m/z 298.2. In the present study, the concentration of Fe(II) in brackish water containing 1 % salt was determined to be 0.184 ± 0.05 µM, and the total iron concentration was determined by ICP-MS and found to be 0.70 µM. Fe(II)/Fetotal was 26 %, the groundwater sample contained Fe(II) 0.146 ± 0.002 µM, Fetotal was 0.23 ± 0.02 µM, and Fe(II)/Fetotal was 63 %. Groundwater is less sensitive to oxygen and therefore has a higher proportion of Fe(II) to total iron. In brackish water, on the other hand, the proportion of Fe(II) can be higher or lower because it is affected by oxygen supply from the surface and reduction from the bottom sediment. Therefore, the Fe(II) concentration was relatively high, but in brackish water, Fe(II) was low, but Fetotal was high.