As opposed to high back-pressure, it is relatively simple to figure out why the back-pressure on your HPLC system is below normal.
By “normal” backpressure, one means that you know what the usual backpressure for a given LC column under given operating conditions is.
Since we already know that the usual backpressure on a C18, 150mm, 4.6mm ID, 5µ column, for AcN:water:;70:30 at 1ml/min at 25 deg C is about 700 to 1000 psi, a backpressure lower than 600 psi requires your attention.
The first suspect is a trapped air-bubble inside the system, especially in the pump piston head. An air-bubble will cause the back-pressure to drop and fluctuate.
Disconnect the column and purge out your HPLC thoroughly. If the backpressure is still lower than normal, it means there’s a leak somewhere.
High-pressure sites, i.e., any fittings and seals before the column are most likely to leak. Check-valves, piston seals and injector fittings, in that order, are what you should be looking at.
When you locate the leaking fitting, do not overtighten it. You can cheat a bit, and use a small piece of teflon tape around the fitting. That should take care of it.
If the threads on the fitting are damaged, you have to replace it. Wherever possible, I use Fingertight fittings, and I’d suggest you try them out too.
Cheers … SKSrinivas.