The chromatographic separation of components is based upon competition between the column, mobile phase components, species, and the sample matrix. With the variety of matrices now being analyzed, there are a variety of chromatographic options available. This column will discuss three column types and the relative advantages and disadvantages of each.
Ion-Exhange Versus Reversed-Phase Ion-Pairing ChromatographyTwo common types of separation schemes used in chromatography are ion-exchange and reversed-phase ion-pairing chromatography. These schemes differ primarily in the column stationary phase, which affects the way components compete for active sites on the column.
Reversed-phase ion-pairing chromatography involves the use of reversed-phase columns that are characterized by nonpolar stationary phases composed of carbon chains, typically 18 carbons (C18), although other columns exist with different length carbon chains (for example, four and eight carbons). Because most inorganic species exist in charged states in solution, it appears that reversed-phase columns would not be able to perform the separation.
Both ion-exchange and reversed-phase ion-pairing chromatography each have their advantages and disadvantages. One of the main advantages of ion exchange is that there is only one interaction involved in the separation: the analytical species interacting with the stationary phase. With reversed-phase ion pairing, two interactions are involved with the separation: the ion-pairing reagent interacting with stationary phase of the column and the species interacting with the ion-pairing reagent. As a result, ion-exchange chromatography may have more matrix tolerance.
The downside of ion-exchange chromatography is that these columns typically are much more expensive than reversed-phase columns. Also, because reversed-phase columns have been in use for many years, they are reliable and established, meaning that results will be reproducible from one column to the next. This may not always be true with ion-exchange columns because new columns are constantly being developed with new ionic groups bound to the stationary phase. Because of this constant evolution, some ion-exchange columns may not always be well established, which could lead to irreproducibility from column to column.
The decision to use either ion-exhange or reversed-phase ion-pairing chromatography depends upon the application: the elements and species of interest, the sample matrix, and the levels of the different species that need to be measured.