HPLC Products

To understand low back pressure we need to establish why we have a back pressure at all, and what is a 'normal' back pressure for a given system.

Back pressure arises from resistance to flow. Mostly the resistance comes from the column. A little can come from a guard column (if fitted) and from the connecting tubing. The smaller the diameter of the tubing and the greater its length, the higher the contribution to the overall back pressure.

'Normal' back pressure for a given system arises from the type of column fitted, and in particular, its length and particle size, combined with the viscosity of the solvents used and the ratio in which they are used, the temperature of the lab and of the column heater (if fitted) and the flow rate through the system. Addition of buffers to the eluent increases the viscosity and hence increases the system back pressure. So for a given column, and a given eluent system, at a given temperature and a given flow rate, we should always get the same back pressure, or there abouts. This is what we refer to as the normal back pressure for the system. The problem being discussed here is when we use the column with the given eluent, temperature and flow rate, and we get a lower back pressure. It is indicative that there is a problem, but what has changed?

The column? Check that the right column is fitted, and the flow arrow shows it is fitted the correct way round. Too short a column, or too large a particle size will give a lower back pressure.

The eluent? Check that the eluent bottles have been filled with the correct solvents. If the eluent is premixed, check that the correct ratios have been used. Organic solvents generally have a lower viscosity than water, so too much of the organic component will give a lower back pressure. Using the wrong solvents can also have this effect, as would be the case if the wrong channels on a gradient pump were being used, or if the wrong solvents had been used to top up the eluent bottles.

The temperature? Check that the column heater has not been set to too high a temperature.

The flow rate? Check that the correct flow rate has been set. Some pumps have a button allowing you to pump at 1/10th of the flow, and if fitted, check that this has not been pressed. Too low a flow rate will cause a low back pressure.

A leak? This will also reduce the flow rate, even if the correct flow is set at the pump. Measure the drop rate into the waste bottle by collecting the flow in a 5ml measuring cylinder and timing for 1 minute. If it is low, look for evidence of a leak in the high pressure part of the system. ie the pump (fittings, piston seals) the tubing to the valve or autosampler, the valve itself, the tubing to the column, and the inlet connection to the column. See our Technical Note on avoiding leaks!