C18 high-performance liquid chromatography (HPLC) columns are used in the fields of chemical analysis and environmental sciences. The columns are often used in industries like pharmaceuticals for analysis of individual components of chemical mixtures. The 18 in the name means that the molecules contain 18 carbon atoms which bond to the silica particles found within the column. C18 columns come in a variety of sizes. They can also come with or without an end cap, in different pore and particle sizes, have varying degrees of hydrophobicity and vary in their abilities when it comes to separating separate acidic or basic components. C18 columns get used in the hydrophobic stationary phase of reversed-phase chromatography, which represents a significant branch in the field of analytical chemistry. The goal of the process is to separate analytics using varying degrees of hydrophobicity. The silica contained within C18 helps it attach to an appropriate surface.