closed at one end and has a rim 4 forming a base which enables the tube to be free standing the tube contains a measured amount of washed and dried chromatographic grade alumina powder 5 and a measured amount of an iron saturating substance 6 bonded to the base of the interior surface of the tube 3 the polyethylene cap defines a hemispherical enclosure large enough to contain the alumina and constructed so that the internal diameter of the cap is identical to that of the base the cap is designed to fit into the base such that there is a continuous unbroken internal surface the tube preferably is 44 mm high not including the cap has an outside diameter of 15 mm at the top and 12 5 mm at the base and has a working capacity of 2 ml more preferably the tube is 40 mm high in a preferred embodiment the iron saturating substance is attached to the base of the tube 3 by adding 10 ul of a solution of fecl 3 at a concentration of 500 mg fe 3 l in 0 1 mole l citric acid containing 0 2 sodium azide and heating the tube in an oven at 70 u00b0 until dry approximately 3 hours 1 invert the tube and remove the cap so that the alumina is retained in the cap 2 add 1 ml of water to the tube and wait two minutes or longer to dissolve the dried iron saturating substance which is attached to the bottom of the tube 3 add 0 5 ml of specimen serum and stand three minutes or longer to allow the iron to bind to.