The particulate material packed inside the column; also called the stationary phase. This usually consists of silica- or polymer-based particles, often chemically bonded with a chemical function al group. For analytical work, 3µm or 5µm spherical particles are used; for semi-preparative, 10µm or larger spherical or irregular particles are favored. The most common packings are as follows:

• Octadecyl (ODS, C18) the most popular phase, used for reversed-phase HPLC
• Octyl (C8) more polar than C18, also a popular reversed-phase column
• TMS (methyl, C1) weakly retentive in reversed-phase
• Silica (Si) unbonded silica used in normal phase mode, popular for preparative LC.
• Phenyl, Cyano, Amino specialty phases used for specific applications.