There are three main types of capillary columns: 

• Wall-coated open tubular (WCOT):In WCOT columns, the inner capillary tube is coated or chemically bonded with a thin layer of stationary phase (typically 0.25 nm thick). 
• Support-coated open tubular (SCOT):In SCOT columns, the liquid stationary phase is infused or absorbed into a carrier material, which is then securely attached to the inner wall of the capillary.  
• Porous-layer open tubular (PLOT): PLOT columns utilize a porous material like polymer, molecular sieve or alumina.