[Pranav Jha]

Why is the column used in GC coiled? Many answers deal with the coiled shape enabling to fit a longer tube into the column oven but then why does the column have to long on the first place? I got confused when i tried to understand the logic behind the straight column of HPLC and the coiled column of GC as i couldn't figure out why the column was not coiled in HPLC as well.

[MOTOBALL]

In general, the longer the column used for a chromatographic separation, the greater is the separating power (i.e., the number of compounds that can be separated, and the resolution between them).  Therefore use a very long GC column, and coil it so that it will fit into the oven.  GC is performed with helium gas as the "mobile phase", and there is very little resistance to the He flow (i.e., low back-pressure is genertated), because the column is wall-coated, open tubular (essentially hollow).

HPLC is performed with a liquid mobile phase and a column that is packed with very small, spherical particles which generates high to very high back-pressures with even a regular, analytical column (e.g. 250 mm long and 5 um particles).  Ultra-fast chromatography on columns packed with < 3 um particles can generate extremely high back-pressures and requires specialized equipment.

The essential to remember is GAS = low viscosity = low back-pressure.
                                   LIQUID = high viscosity = high back-pressure.

Therefore, you can only use short HPLC columns, and no need to coil them.  I suspect that obtaining good packing of a coiled, rather than a linear column, might also be a problem.

If you post this question in www.sepsci.org you will get an expert answer from an expert chromatographer, such as Tom Jupille.

[Pranav Jha]

i couldn't understand the term "back pressure"

[TheUnfocusedOne]

Keep in mind that the GC column doesn't have to be very long.  I've heard of people running a 5m column, while other will run 100m.  It depends on the amount of interaction you want your analyte to have with the stationary phase.  If you have a column made to seperate a ceratin class of compounds, it might not take much interaction to seperate.  A shorter column will increase throughput, since it takes less time for the sample to travel from the injector to the detector.  Also keep in mind that too long of a column will cause peak tailing, which will degrade your results since the amount of tailing varies and may shift your retention time, peak area or hieght.  Its a trade off really, you need to find a balance of results vs throughput.

Coiling is a space saving technique.  GC samples travel faster at lower pressures than an LC (as MOTOBALL said), so to maintain a high amount of interaction with the stationary phase, also know as the plate (the seperative capability of your chromatography) you need to make it longer.