Why very long capillary GC columns are not preferred?

Capillary Column
Capillary Column
Capillary GC Column

It is a well-known fact that the separation efficiency of a chromatographic column increases with an increase in its length. Doubling the column length increases resolution by around 40%. Normally capillary GC columns range in length from 15 m to about 60 m. Such column lengths are generally sufficient to resolve complex compounds having closely spaced or even overlapping peaks. However, the basic question that needs to be answered is if this be the case then can the length of a column be increased indefinitely to gain significant improvements in resolution. The answer to this question is negative as beyond a certain point there is no significant gain in resolution but on the other hand operational difficulties get multiplied.

The article discusses how increase in length influences its main usage considerations.

Cost

As column length increases its cost also increases.

Analysis Time

Although there is an improvement in resolution there is a corresponding increase in analysis time.

Back Pressure

As the column length increases so does the column back pressure.

Risk of Breakage

Risk of breakage also increases with increasing length during mounting and removal from mounting cage inside column oven.

Column Bleed

Column bleed also increases with increasing column length.

On going through the above it becomes clear that increase in column length beyond a point is not the ideal solution of improving separation between the peaks. It is a good practice to start with shorter columns of about 30 m length when the length of the column is not specified in a method .In case there is no significant improvement on its performance try using a narrow column, different temperature program or stationary phase. Next try longer column up to about 60 m. Only for very complex mixtures you should consider columns of about 100 m length. However, before going for such an option it is advisable to change other parameters such as thinner film coating, optimization of carrier gas flow rate and different temperature programs and see if separation between difficult peaks improves.

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