Using Phase Ratio to Reduce Run Time

How Does Each Parameter of Your GC Column Affect Chromatography?

Length will have a big effect on the resolution of critical pairs, the longer the column the more theoretical plates you have available for interactions of your analyte with the column and the better the separation.

Columns with high numbers of theoretical plates will still require the interactions of the analyte with the stationary phase to be fast and efficient to give the greatest separation possible within the column length. The internal diameter of your column can ensure you achieve this. Narrow columns can provide more of these fast, efficient interactions and offer good separation and sharp peak shapes. You should, however, be mindful when reducing column ID that you will limit the loading capacity of your column and will have to work at a reduced flow rate.

The third parameter of a GC column that affects chromatography efficiency, is the film thickness. A thinner film will give a faster rate of mass transfer. When a thin film is used, the transition of an analyte into and out of the stationary phase is easier leading to shorter retention times. These parameters all work in harmony and can be used to reduce run time by working with the phase ratio of the column to optimize your method.

Phase ratio is calculated from the column ID and film thickness using this equation:

ß = d / 4df (where d = column diameter in mm and df = film thickness in µm)

Once you have the phase ratio of the column used in your separation, you can improve separation by increasing phase ratio. A higher ß value represents a higher efficiency separation which in turn allows you to use a shorter column to achieve the same separation.

As an example, the initial analysis has a phase ratio of (β) = 44.2 using a 30 m x 0.53 mm x 3.00 µm column. If the film thickness remains the same but the column ID is reduced (in theory to reduce the analysis time), this would result in a reduction in phase ratio and lead to a loss of efficiency. To maintain efficiency and reduce run time the film thickness also must be reduced which will then give the same efficiency but a faster analysis time.

In summary:

• A smaller column ID will increase the frequency of interactions of the analyte with the stationary phase, which leads to an increase in efficiency.
• A thinner film will increase the rate of mass-transfer, which also leads to an increase in efficiency through reducing diffusion within the liquid film.
• Once you increase efficiency you can use a shorter column length to reduce run time whilst maintaining the original efficiency of your separation.

Through the optimization of your analysis using phase ratio you can reduce your GC analysis time, saving you time and producing a more efficient laboratory.