The main type of columns used in gas chromatography is a capillary column. Packed columns are created of fused silica or stainless steel. To increase separation of a gas sample columns must be created of a large length and are coiled to fit inside of the oven. Capillary columns are separated into two categories; wall coated open tubular and support coated open tubular. WCOT are capillary tubes with a thin layer of stationary phase, and in SCOT, the tube is lined with a thin film of support material. As the sample moves through the column the individual parts of the sample become contained with the material inside of the column and are then released. This action allows for a sample to become separated.