Ask the Editor: Advantages of LC Separations at High Temperatures

A reader recently asked:

What are some of the advantages of LC separations at high temperatures?

Liquid chromatography (LC) separations performed at elevated temperatures offer the following advantages:

• Decreased mobile phase viscosity and improved solute mass transfer and diffusivity.
• Separations can be performed at higher flow rates as a result of the reduced mobile phase viscosity and lower system back pressure.
• Decreased mobile phase surface tension and dielectric constant cause reduced analyte retention, thus enabling faster analyses.
• Possible use of longer columns or multiple sequential columns enables increased theoretical plate number.

Limitations on the use of high temperatures for LC separations include the thermal stability of the stationary phase and analytes, although the analytes are exposed to the higher temperatures for a relatively short time.

LCGC's "LC Troubleshooting" columnist John Dolan identified other possible limitations: "High temperature operation requires an oven that can support the desired temperature and a mechanism to preheat the mobile phase to the column temperature. Most commercial systems cannot heat above 60-70 C, and performance at these temperatures varies widely with the product. Temperature mismatch of the mobile phase and column causes a thermal gradient along the column, resulting in broadened or split peaks."

Questions?LCGC technical editor Steve Brown will answer your technical questions. Each month, one question will be selected to appear in this space, so we welcome your submissions. Please send all questions to the attention of "Ask the Editor" at lcgcedit@lcgcmag.com. We look forward to hearing from you.