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To understand high 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. Any restriction to flow adds to the 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 higher back pressure. This may be a sudden or gradual change. 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 long a column, or too small a particle size will give a higher back pressure.

A blocked frit or column blockage?  Any particles in the sample or the leuent will become trapped in the column. Some will become trapped in the inlet frit, but those small enough to pass through the frit become trapped in the column. No particles leave the column, so gradually the back pressure will increase. If it becomes too high, the column will have to be replaced. Provided that there is no peak distortion, so there is no indication of a void, try reversing the flow through the column, If the inlet frit is blocked, this will usually bring the back pressure back to normal. In which case the inlet frit should be replaced. Plan it first, and do it quickly. Opening the column can cause the bed to dry out, and it can easily become contaminated. If you do not have a spare frit, you can probably recover a good one from the bottom end fitting of an old column of the same type.

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 aqueous component will give a higher 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.

Immiscible solvents in the system. If a solvent change is made, and the old eluent is not miscible with the new eluent, you will find out why some people call this High Pressure Liquid Chromatography. Always use miscible steps. If a mistake is made (eg when changing from normal phase to reversed phase, or when using a new amino column without checking to see if it was supplied in acetonitrile:water or heptane) pump very slowly, for as long as it takes to clear through. This make take a couple of days. Alternatively you could try pumping THF or isopropanol, which are miscible with almost all solvents. But still pump slowly, because IPA has a viscosity of 2!

The temperature? Check that the column heater has not been set to too low a temperature. And check that it is turned on!

The flow rate? Check that the correct flow rate has been set. Too high a flow rate will cause a high back pressure.