back pressure and optimal flow rates estimation
Posted: Sat May 24, 2008 10:01 pm
I want to buy a C18 XBridge column but I don't exactly know which dimensions will be the best choice for my HPLC system.
I work with an Alliance 2690 and a PDA 996. The maximum back pressure allowed is 5000 psi. I classically use 250x3mm, 5µm C18 columns with CH3CN / water 5:95 at 0.6 ml/min. Back pressure obtained is 2700 PSI.
It is usually said that the resolution can be enhanced by using smaller particle and smaller internal diameter. This can be true if system void volume is low and if the back pressure induced can be supported by the system. For exemple, when working with a 125x2.1mm, 5 µm column at 0.25 ml/min, resolution obtained is poor probably because the peaks spreads in the flow cell (it's a standard flow cell, I think it's a 1cm length). Peak widths are slightly thinner with 0.005 inch tubing instead of 0.009 between column and detector but it's still not efficient. So, I think my system is not adapted to run at this low flow rate. I know that optimal flow rate decrease with internal diameter and increase when particle size decrease but I don't exactly know how to estimate it.
Is there a rule or an equation to calculate optimum flow rate depending on the column?
May I estimate the back pressure induced depending on the flow rate and the column dimensions? Are there equations available to calculate it?
What following XBridge columns will operate at optimum flow rate with a back pressure < 5000 psi (with 95% water)?
250x3mm, 5µm
250x4.6mm, 3.5µm
150x4.6mm, 3.5µm
150x3mm, 3.5µm
150x2.1mm, 3.5µm
100x2.1mm, 3.5µm
75x4.6mm, 2.5 µm
50x3mm, 2.5 µm
In your opinion, which of these columns will give the best resolution with my HPLC considering its void volume (650µl), large flow cell and pressure limitation?
At which flow rate?
Thanks in advance for your comments.
I work with an Alliance 2690 and a PDA 996. The maximum back pressure allowed is 5000 psi. I classically use 250x3mm, 5µm C18 columns with CH3CN / water 5:95 at 0.6 ml/min. Back pressure obtained is 2700 PSI.
It is usually said that the resolution can be enhanced by using smaller particle and smaller internal diameter. This can be true if system void volume is low and if the back pressure induced can be supported by the system. For exemple, when working with a 125x2.1mm, 5 µm column at 0.25 ml/min, resolution obtained is poor probably because the peaks spreads in the flow cell (it's a standard flow cell, I think it's a 1cm length). Peak widths are slightly thinner with 0.005 inch tubing instead of 0.009 between column and detector but it's still not efficient. So, I think my system is not adapted to run at this low flow rate. I know that optimal flow rate decrease with internal diameter and increase when particle size decrease but I don't exactly know how to estimate it.
Is there a rule or an equation to calculate optimum flow rate depending on the column?
May I estimate the back pressure induced depending on the flow rate and the column dimensions? Are there equations available to calculate it?
What following XBridge columns will operate at optimum flow rate with a back pressure < 5000 psi (with 95% water)?
250x3mm, 5µm
250x4.6mm, 3.5µm
150x4.6mm, 3.5µm
150x3mm, 3.5µm
150x2.1mm, 3.5µm
100x2.1mm, 3.5µm
75x4.6mm, 2.5 µm
50x3mm, 2.5 µm
In your opinion, which of these columns will give the best resolution with my HPLC considering its void volume (650µl), large flow cell and pressure limitation?
At which flow rate?
Thanks in advance for your comments.