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Add 9.5 mL of scintillation cocktail Optiphase ’Hisafe’3 (LKB, Wallac). Shake well. Measure after ½ hour with a liquid scintillation counter.

To be able to calculate55Fe radioactivity, counting efficiency need to be determined (efficiency calibration). Counting efficiency is determined from a 55Fe standard series using Fluorescein (C20H12O5) as a quenching agent.

A quenching standard series
Add reagents into plastic scintillation vials as follows:

	55Fe (X Bq/ml)	MQ-H2O	HF	Fluorescein	Scintillation cocktail
1	0.5 ml	2 ml	1 drop	0 ml	9.5 ml
2	0.5 ml	1.9 ml	1 drop	0.1 ml	9.5 ml
3	0.5 ml	1.8 ml	1 drop	0.2 ml	9.5 ml
4	0.5 ml	1.6 ml	1 drop	0.4 ml	9.5 ml
5	0.5 ml	1.0 ml	1 drop	1.0 ml	9.5 ml

Shake well. After ½ hour measure the samples in a liquid scintillation counter.

Draw a quenching standard quenching curve. Determine the counting efficiency for 55Fe from the curve.