irregular packing - refers to the shape of a silica gel based packing. Irregular silicas are available in microparticulate sizes. The packings are made by grinding silica gel into small particles and then sizing them into narrow fractions using classification machinery. Spherical packings are now used more often than irregular packings in HPLC, but less-expensive irregular packings are still widely used in prep LC.

See also particle shape.

irreversible adsoprtion - when a compound that has a very strong affinity for the adsorbent is injected onto a column, it can be adsorbed so strongly that it cannot be eluted from the column. A chemical reaction between the sample and the surface of the adsorbent is an example of irreversible adsorption.

isocratic - use of a constant-composition mobile phase in liquid chromatography.