Will a C18 Cover all Your Needs in HPLC?

The short answer is, of course, no. Yet, a C18 might be the most versatile phase and a first pick when developing analysis or purification methods for HPLC. However, when working with biomolecules and polar compounds, one may need to play with other parameters than hydrophobicity, like total charge at various pH as well as being able to run in highly aqueous conditions. In those situations, the boundaries of a traditional silica-based C18 might show some limitations: jeopardising the chemical stability of the phase at high pH or having issues to obtain any substantial sample retention in a water-rich mobile phase. Kromasil offers C18 alternatives to cover extended conditions: Kromasil EternityXT C18 and Kromasil Classic C18(w).

These two C18 alternatives are highly typical when it comes to hydrophobicity and IEX capacity at lower pH; as can be seen in the Tanaka test diagram. The other tested properties show differences that can give alternatives to reach separation goals.

In an example separating alkaloids, much better selectivity can be achieved at high pH when selecting Kromasil EternityXT C18, but still be confident that the stationary phase will not be impaired by running under those harsh conditions over a long time. Also, under reversed phase conditions, slightly different selectivity can be obtained with a wettable C18(w) compared to a classic C18. In the second example, an expected similarly strong hydrophobic retention is obtained for toluene, while caffeine, that has a notoriously weak retention on a classic C18, is substantially retained on the wettable C18(w). This is due to the interactions of polar compounds with the polar end-capping of the phase, enabling the ability to operate in fully aqueous conditions without risk for phase dewetting.

For more condition details:

Separation of alkaloids

Separation of polar compounds