All C18’s Are Equal - But Some Are More Equal Than Others

Firstly apologies to George Orwell for taking his words in vain however never has the above observation been truer than in the field of HPLC Columns containing the Octadecyl silane (C18 a.k.a ODS or RP-18) hydrophobic grouping bonded chemically to the surface of an amorphous silica surface. These are the most widely used columns in reversed phase chromatography with over 70% of chromatographers choosing this type of column for their separation, often for no better reason than its usually the one supplied with an instrument, or it is the one most likely to be lying around and available from a colleague. Admittedly it is the most stable of all chemistries available to the chromatographer yet stability is not always the prime reason for selecting a column chemistry. So how does a chromatographer decide which C18 column to choose and, as is the norm these days, when a new C18 comes onto the market how different/close is it selectivity wise to other known C18`s? The situation is further complicated by the myriad of exotic endcapping (secondary capping) techniques employed by manufacturers these days. More than 600 Brands on the market make it a potential minefield for the scientist if some help is not at hand to give characterisation clues to indicate the likely success of achieving a desired separation when a degree of knowledge about the analytes is available and the original column specified in the method is either vaguely described or unavailable.