This was a question posed by one of our customers:
We are currently using 15 m columns with 0.25 mm ID and 0.25 µm film. We want to get faster separations and would like to know the difference in resolution and capacity for the following column dimensions:
Fast GC uses small diameter columns (0.18 mm) to help reduce run times. The reason this occurs is due to increased efficiency. A smaller ID column has a much greater efficiency due to increased mass transfer (there is less gas to travel through before interacting with the other side of the column). Since smaller ID columns have greater efficiency, the total length of the column can be decreased to achieve the same efficiency as a longer column with a bigger diameter. So, a 30 m x 0.25 mm x 0.25 µm column and a 20 m x 0.18 m x 0.18 µm column will have similar efficiency though the 0.18 mm ID column has only 20 meters of length.
It is the shortened length of the column that results in reduced run times. A 30 m x 0.18mm x 0.18 µm column will actually have longer retention than a 30 m x 0.25 mm x 0.25 µm column because there are a greater number of interactions between the sample and the phase. So, simply reducing the internal diameter will not shorten the run times. If initially using a 15 m x 0.25 mm x 0.25 µm column, we suggest going down to a 10 m x 0.18 mm x 0.18 µm column to reduce run times.
Notice also the film thickness decreases with the diameter. This is to keep the phase ratio (ß) the same since this term has an effect on selectivity. Decreasing the film thickness will decrease sample capacity though. To compensate for this difference, a higher split ratio (most often used method) or a thicker film can be employed. A higher split ratio will reduce the amount of analyte onto the column, but that is somewhat offset because the analytes will elute from the column in a shorter amount of time resulting in higher peaks. If a thicker film is used, this will affect the phase ratio and can impact selectivity and possibly elution orders. This will also increase retention of compounds. This may offset some of the advantages of going to a shorter column. Another effect is that a thicker film will also cause lower efficiency because there will be increased diffusion within the phase (though this contribution is small, though not insignificant).