Chromatography Forum

Hello all,

I know that the longer the column, the higher the back pressure will be for the same particle size.

If I keep the particle size and column length no change, what is the pressure change with different i.d. size?

Thanks in advance.

It is because there was a relative large extra column volume when I used 2.1 mm id column in the system, leading to peak broadening.

I am thinking about using 3 mm or 4.6 mm id column with the same packing, hoping that the effect of extra column volume will be minimize. (But I am not sure if the pressure will go up with larger bore column)

If all else is held constant, back pressure will vary inversely with the square of the column diameter; i.e., if you cut the diameter in half, you will see four times the pressure. The linear velocity will be four times higher as well (so retention time will be 1/4 what it was on the larger column).

It is because there was a relative large extra column volume when I used 2.1 mm id column in the system, leading to peak broadening.

I would suspect either sample or volume overloading (or both) to be the cause for the peak broadening – in this case.

Best Regards

If you change the column i.d. you need to change the flow rate in proportion to the square of the i.d. then everything remains the same (retention, backpressure etc.) but your extra-column badspreading will be less.

Thanks!!!

Then I will try a <2um column with 3 or 4.6 id bore when the existing column (5um, 4.6 id x 150 mm) come to the end of its service life.

Yesterday I tried 2 columns (same brand and same packing):

Column 1: 4.6mm id x 250mm, 5um, C18
Column 2: 10mm id x 250mm, 5um, C18

Mobile phase: 75:25 MeOH:H2O
Column 1 flow rate: 0.5ml/min, pressure 68 bar

Hence I used the formula:

(Column1 id / Column2 id)^2 = Flow rate 1 / Flow rate 2

Hoping that to keep the linear velocity the same. The calculated flow rate for Column 2 = 2.363ml/min.

Then I got 2 chromatograms with similar RT on the peaks. But the pressure of Column 2 is 128 bar.

Base on Tom & Uwe's comment, back pressure should be the same if I keep the linear velocity the same. Are there something wrong in my calculation?

Probably nothing wrong... Could be that the tubing in the system is creating all the backpressure...

Disconnect the column and replace it with a union. Then run 2.363 mL/min. Expected backpressure without column: ~ 60 bar. If not, the frits in the fat column are clogged (or something similar is happening).

BTW: I assume that all these columns are normal columns, and that the prep column is not an axially compressed column...