High performance liquid chromatography (HPLC), a form of column chromatography, can be performed in one of two ways. Reverse-phase (RP) HPLC separates full-length oligo product from truncated products based on relative hydrophobicity. Ion-exchange (IE) HPLC separates full-length oligonucleotides from truncated species based on relative charge difference. IDT uses HPLC to purify unmodified oligos as well as oligos with complex modifications such as linkers, spacers, modified bases, and hydrophobic modifications. The type of HPLC selected is dependent on oligo sequence composition and characteristics.