Chromatography Forum

If I have a compound with a retention time of 4.5min, and I double the length of the column from 20cm to 40cm, does this mean that the retention time of the compound will be 9min?

If your linear velocity is the same, nearly YES.

with equation:

tr = L/ux

where,
tr - retention time (s)
L - length of the column (cm)
ux - velicity (cm/s)

If you double the length of the column, the retention time also doubles (if the velocity as Gerhard Kratz mentioneted is the same).

So does that mean that the resolution between two peaks will not change if the column length is increased?

In theory - YES

Actually, if you are using isocratic elution, then doubling the column length should double the resolution. However, the additional time on column permits more diffusion to occur. Consequently peaks will be wider and so the increase in resolution will be something less than 100%. The peak maxima should be 2x farther apart, though.

In theory - YES

Yes, in theory the resolution will increase with sqrt(2)=1.4 (Since double column lenght, double number of plates)

If I have a compound with a retention time of 4.5min, and I double the length of the column from 20cm to 40cm, does this mean that the retention time of the compound will be 9min?

Only if you run isocratic.

Peter

In theory - YES

Yes, in theory the resolution will increase with sqrt(2)=1.4 (Since double column lenght, double number of plates)

So if the resolution is between peaks is 1, and I double the column length, the resolution will improve a bit to 1.4?

Yes (if isocratic)

If increasing the resolution of two peaks is your main objective, I would try smaller particle size before using a longer column. A smaller particle size can sharpen the peaks, giving better resolution. Sometimes a shorter column with smaller particle size would be warranted. The column manufacturer can advise you on the column length and smaller particle size for your application once they know the original column and chromatography conditions.