LC fundas: Dealing with high backpressure.

High backpressure is a chronic issue in HPLC. But then, how high is high?

To give you an idea:

For a C18, 150mm, 4.5mm ID column packed with irregular 5μm particles, at a flow rate of 1ml/min, water: acetonitrile (30:70) at 25 degs C, the normal backpressure should not exceed 1000 psi (or about 700 bar).

Under these conditions, anything more than 1000 psi should make you worry a bit. Sure, modern HPLC pumps can take up 6000 psi backpressure or more, but that’s no reason to push your expensive pump to its limits.

High backpressure increases the wear on piston seals, check-valves and other components, and will eventually result in higher maintenance costs.

Backpressure can be affected by several factors, to wit:

Mobile phase composition: Water and isopropyl alcohol (IPA) are considerably more viscous than acetonitrile. Try to limit the content of the more viscous solvent to 30%.

Temperature: Higher column temperature = lower backpressure. In general, higher column temperature will also improve peak separation, so it’s not a bad idea to experiment a bit. But try and keep it below 60 deg C.

Buffers: Precipitated salts from buffers are the primary cause of choked frits and high backpressure. Don’t forget to flush! I’ll tell you how to deal with choked frits in the next post.

Column packing particle size: 3.5μm can provide better peak resolution, no doubt, but at the cost of higher backpressure.

A quick way to reduce elevated backpressure is to disconnect the column from the detector, reverse it and reconnect it to the injector, and backflush with AcN:water::70:30 (or the mobile phase you’re using), at 0.5 ml/min for half an hour. That should take care of the problem for the time being.

Whether it’s your heart or your HPLC pump, don’t ignore high pressure.

Cheers … SKS