Guidelines for transferring a C18 method to Discovery? HS F5: Generally, bases are retained longer on the HS F5 than on a C18. Increasing the organic content of a C18 separation 5 to 10 percent will generally provide similar retention on an HS F5. Results with other compounds are highly variable. However, it is generally true that solutes with log?Po/w values less than 2.5 will be retained longer on HS F5 compared to a C18. The degree of difference is highly solute dependent.