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  1. Elute 2 g of ion exchange resin with 8 ml of 8 M HNO3 overnight. Separate the resin by filtration.
  2. Take 1 ml of the solution into a centrifuge tube and add 10 mg Fe-carrier (FeCl3) and dilute with water to total volume of 10 ml. Take a 100 µL sample for AAS/ICP-MS/ICP-OES measurement.
  3. Add drop by drop concentrated ammonia until Fe(OH)3 precipitates (pH 8-9 ). Centrifuge the sample for a couple of minutes and discharge the supernatant.
  4. Dissolve the precipitate in 8 M HCl and transfer the solution into 100 ml extraction funnel. Extract the sample two times with 25 ml di-isopropyl ether. Discharge the aqueous phase.
  5. Combine the ether fractions and back-extract the iron into aqueous phase by extracting it twice with 15 ml of distilled water. Transfer the water solution into a centrifuge tube and discharge the ether fraction.
  6. Add ammonia to precipitate the iron (pH 8-9). Use a warm water bath to facilitate the precipitation. Centrifuge and discharge the supernatant. Wash the precipitate with small amount of water, centrifuge and discharge the rinse water.
  7. Dissolve the precipitate in 5 ml 0.2 M HCl in a warm water bath.
  8. Pour the solution into an ion exchange column (Dowex 50 w x 4; 50/100, pretreated with 20 ml of 0.2 M HCl). Wash the column with 10-20 ml of 0.2 M HCl.
  9. Elute the iron from the column with 40 ml of 0.5 % oxalic acid into a centrifuge tube.
  10. Add a drop of 30 % hydrogen peroxide. Precipitate the iron with concentrated ammonia on a warm water bath. Centrifuge and discharge the solution.
  11. Dissolve the residue into a small amount of strong HF. Transfer the sample into a liquid scintillation vial (weighed before addition). Add 1 mL of distilled water, weigh the sample.
  12. Take a 100 µL sample for the iron measurement with AAS/ICP-MS/ICP-OES.
  13. Measure the activity of 55Fe from the rest of the sample by liquid scintillation counting.