Alternate Selectivity

C18 columns have become the industry standard and are selected for HPLC methods most of the time. Dispersive interaction forces are sufficient to allow C18 columns to retain and separate many organic compounds; however, results are not optimum in all cases. Experience has shown that selectivity, retention and efficiency can all suffer with certain samples. Poor or marginal results can occur up to 50% of the time depending on sample type. Modern polar phases are highly complementary to alkyl phases and may produce much better results for these compounds. C18 columns are becoming much alike due to evolution of silica particles and development of improved bonding chemistry. Changing from one brand of C18 to another may not produce a different, better result. When resolution is not acceptable for any reason, an alternate stationary phase with more polarity than C18 or C8 may be the best answer. Polar stationary phases such as Ascentis Phenyl and RP-Amide can play a unique and important role in changing retention and selectivity for certain classes of compounds.

• Poor selectivity
• Peaks overlap or elute in wrong order for best quantitation
• Poor retention
• No retention; sample elutes at column void volume
• Unstable separation; phase not wetted by high aqueous mobile phase causing loss of efficiency and retention
• Poor peak shape (low efficiency)
• Too much retention and selectivity (requires gradient)

Watch our instructional web seminar on The Use of Alternate Selectivity in Reversed-Phase HPLC for a detailed description of the various types of bonded phases. 

Additional Topics:

Developing HPLC Methods on Ascentis

Harnessing the Power of Chromatographic Selectivity