Iodinated media are derivatives of triiodobenzoic acid and capable of forming solutions that are uniquely capable of banding any biological particle, according to its buoyant density, often under isoosmotic conditions.

 In 1968 Bøyum (1) published his famous density barrier method for the isolation of mononuclear cells from human blood that used sodium metrizoate (Figure1) and Ficoll®. 

This technique received universal approval and Nycomed was the first company to introduce a readymade medium in 1973 based on his original formula. The modern version is called Lymphoprep™ and more recently metrizoate has been replaced by diatrizoate (Figure 2).

Both metrizoate and diatrizoate are ionic compounds which are able to interact with other charged groups on biological particles and also influence the distribution of ions across membranes, thus the development of nonionic iodinated compounds was the next important step. 

Metrizamide was introduced in 1973 (2,3) and iohexol in 1982 (4). Both of these non-ionic gradient media can form solutions of high density (>1 .30 g/ml at 60% w/v); in metrizamide the carboxyl group present in metrizoic acid is linked to glucosamine, while in iohexol the carboxyl group is linked to the amine group of 3-amino1,2-propanediol.