Changing from Reverse Phase (RP) to Normal Phase (NP) Mode (or vice versa) in your HPLC System

Two closely related HPLC questions which have the same answer are commonly found in my email folder each week. Both questions deal with concerns about switching mobile phase solutions used in HPLC. Here are the two questions with an explanation regarding what information is needed to answer them:

1. Can I switch-over an HPLC system which has reverse phase (RP) columns and RP solvents installed to one with Normal Phase (NP) columns and solvents (e.g. such as a C18 column with water and acetonitrile switched over to a silica column with Heptane and IPA )?
2. What is the best way to switch or change-over from a reverse phase (RP) mobile phase made up of water with buffer and acetonitrile to one that is all organic for normal phase (NP)?

To answer these questions we first must review the specified materials used in the HPLC system that are in contact with the mobile phase solutions (the 'wet' parts). 

If you have a system rated for use with most RP solutions, then you will want to verify that the same system is also rated for use with the NP liquids you are considering too (Refer to the manufacturer's product manual or specification sheet). Some HPLC systems may require no changes at all, while others may not be compatible for use. Some systems have seals, tubing and/or valve components that may NOT be chemically compatible with the proposed solvents. Use with incompatible solvents (or additives) may result in damage or destruction of the system. The instrument manufacturer will often provide the needed information inside the specific instrument's Operator / User Manual (*always contact the manufacturer if you have any questions regarding the safe use of the system). In some cases, the vacuum degasser, pump piston seals, tubing, injector seals and/or other parts may need to be replaced with chemically compatible parts before use.  The newer seals or parts may also have different operational limits (e.g. pressure) or specifications than the ones they replace. For example, the maximum pressure ratings with the different parts may be different (some RP to NP conversions result in much lower max pressure ratings and reduced part lifetimes). Many of the newer vacuum degasser units may have no chemical compatibility with the proposed HPLC solvents or additives. Proceed with caution. Check and verify compatibility first.

Anytime you switch from one liquid type to another, you must insure that the new solution is fully soluble (miscible) with the old solution being displaced from the system. The solutions used must be miscible and must not result in precipitation of any contents or contamination of the flow path (and/or plugging of lines) may result. 

Basic guidelines: 

• If any buffers or additives have been used, begin by flushing the system with the same solution, but without the buffer or additives dissolved. We want to remove those buffers and salts first. Flush the entire flow path. Flushing out these salts will greatly reduce the chances of system contamination or plugging. In the case of aqueous buffer solutions, initially flushing with ultra pure water will remove them. 
• Next, if the new solvent is not fully miscible with the old solvent (e.g. Water to Hexane...), then flush the system with an intermediate solvent that is fully soluble with both liquids (in the Water to Hexane example, IPA would be an excellent choice). In fact, for many aqueous to normal phase conversions, IPA provides the miscibility needed to change back and forth from a highly polar aqueous mode to a non-polar organic mode. *Consult a table of Solvent Physical Properties for guidance
  

Summary: 

Verify instrument chemical compatibility and the possible need to replace any seals or parts.

With the proper precautions and checks, many research grade analytical HPLC system can be routinely switched between RP and NP modes.

To switch from NP to RP mode (or RP to NP), flush the system of any salts, buffers or additives, verify liquid miscibility of the solvents, then use an intermediate solvent if needed to change over. 
A table of commonly used HPLC Solvent Properties will help you determine which liquids can be used as intermediate solvents for this purpose.