Gas chromatography is a process to separate gas molecules of different size due to their different adsorption at macroporous surfaces, e.g. at activated charcoal. The separation is managed by the use of starting gas or heating up.

Iodinated contrast agents (typically iodine-substituted benzene derivatives) are bound either in nonionic or ionic compounds. Ionic contrast agents consist of the negatively charged anion and the positively charged cation. Used components of the anion are for example diatrizoate, iodamide, iothalamate or metrizoate and of the cation the sodium or meglumine ion. The osmotic pressure depends on the number of particles in solution. Ionic contrast agents have a greater osmolarity; double that of nonionic contrast agents due to delivering more iodine atoms per molecule.
Ionic contrast agents were developed first and are still in use depending on the examination. Iodine based contrast media are water soluble and as harmless as possible to the body. However, ionic agents have more side effects compared to nonionic contrast agents due to their high osmolarity.

See also Ionic Dimer.

Ionization is the process of converting an atom or molecule with no electrical charge into an ion by adding or removing charged particles such as electrons or other ions, through high temperatures, electrical discharges, radiation etc.

See also Ion.

Nonionic monomers are used as x-ray contrast agents. To create a nonionic monomer, the tri-iodinated benzene ring is made water soluble by the addition of hydrophilic hydroxyl groups to organic side chains (CM ratio=3). Nonionic monomers have an intermediate osmolarity, intermediate viscosity and elevated hydrophilicity with three atoms of iodine per molecule. Lacking a carboxyl group, nonionic monomers do not ionize in solution. A nonionic monomer is potentially less chemotoxic than an ionic monomer.
Common nonionic monomers are iohexol (Omnipaque), iopamidol (Isovue®), ioversol (Optiray®), and iopromide (Ultravist®). Nonionic monomers are contrast agents with a wide range of indications due to their nonionic nature and lower osmolalities.

Osmolality is the number of moles of osmotically active particles (molecules) present in solution per kilogram of water.
High osmolality of contrast agents is related to some of the adverse reactions.

Osmolality of iodinated contrast media: