Chiral Separations 2: Optimization of Chiral Separations

The cyclodextrin columns are unique in their ability to resolve a variety of chiral compounds. In my last blog I discussed the individual column separations of various chiral compounds. Today, I’d like to show how to optimize that separation. Variation in linear velocity and temperature ramp rate can greatly affect the resolution of enantiomers. Depending on the type of chiral column, initial GC oven temperature can affect peak width. Column sample capacity varies with different compounds, and overloading results in broad tailing peaks and reduced enantiomeric separation.

Linear Velocity (Column Flow)

In our analysis, we worked with hydrogen. Hydrogen’s optimal linear velocity is around 40 cm/s. However, as shown in Figure 1, better resolutions were achieved with higher flows/linear velocity (60-80 cm/s).

Figure 1: Resolution factors as function of linear velocity. Rt-βDEXsa: 40°C (80°C for lactones) to 200°C @ 2°C/min. (hold 1 min.). Hydrogen carrier gas.

Temperature Program

The chiral resolution can improve with decrease in the temperature ramp speed. The best temp programs are 1-2 °C/min (Figure 2).

Figure 2: Resolution factors as function of temperature ramp. Rt-βDEXsa: 40°C to 200°C (hold 1 min.). Linear velocity 80 cm/s. Hydrogen carrier gas

For optimal chiral separation use:

• Faster linear velocities (60-80 cm/sec.) with hydrogen carrier gas.
• Slower temperature ramp rates (1-2°C/min.).
• Appropriate minimum operating temperature (40 or 60°C).
• On-column concentrations of 50ng or less.