The retention index approach in liquid chromatography: An historical review and recent advances

Historically, retention parameters were reliably used as identification criterion in chromatographic ana- lytical systems. However, pure analytical standards are necessary to determine the retention behaviour of a given compound. In recent decades, mass spectrometer (MS) became the detector of choice to obtain structural information on unknown peaks, thanks to the elucidation of fragments, often arisen from the loss of specific functional groups. The cost and the level of experience of the operators is surely higher compared to the use of retention data. Therefore, the aim of the present review is to describe the effort s in the introduction of the Linear Retention Index (LRI) in routine, interlaboratory applicable identification procedures. The requirements and the main challenges will be discussed, even compared to gas chro- matography methods, in which LRI is stably used for identification purposes, usually in combination with MS spectral libraries. The higher number of LC-amenable molecules and the wide range of LC mobile phase compositions make the building of universal LRI database a very challenging task. The limitations encountered in the past decades are reported, together with new proposals in order to overcome such issues.