At a certain column length, a sample injected into center of a packed bed spreads by radial diffusion but never reaches column wall, where wall effects can cause band broadening. Phenomenon observed by John Knox, who showed that a sample peak collected in the exact center of the column exit displayed a higher efficiency than a sample peak collected near the wall. The infinite diameter effect depends on column length, internal diameter, particle size, and mobile-phase properties. Very seldom applied in HPLC.