No expert of this pump type. I try a shot anyway: A sudden pressure drop indicates a leakage. During the injection process the autosampler switches the sample loop from column to filling-with-sample and back to column. Therefore, I suggest focussing on the injection valve. Check the connections and try again. If it did not help, clean the valve (disassambling, ultrasonic bath) and check the relevant parts for scratches.

Btw: How severe is the pressure drop (in %)? If you cannot find the reason and the chromatography is reproducible you may also try to change the "min. pressure" value (look into the manual) of the derivatizer pump.

Thanks Karsten for answer. About the leakage, could leakage exist even there is no leakage indication or leakage massage on the instrument?

The HPLC pressure falls to 0%

The derivatizer pump switch off at less than 500psi and could not changed.

Wow, a complete pressure loss... OK, in this case forget the "min. pressure" manipulation at the deriv. pump. ;)

The leakage detector is usually located within the column oven. Any leakage outside won't be recognized. Use a paper tissue and look for moisture at/around the injection valve (specially the connections of the sample loop).

A very brief drop in pressure is normal after an injection. This is due to the injection valve making and breaking the flow connection from the loop to the main path of the HPLC.If the injection volume is very large (or takes a long time to complete), this can happen, but it would indicate poor method quality as the injection should be fast, not slow so as to not disturb the flow and baseline. BTW: If you have not primed the system and lines, prior to starting the run, air can cause a wild pressure change since gases easily compress, but liquids do not. You would need to purge the air from the system to fix this.

*A "large" drop in pressure with shutdown could be due to a low pressure alarm being set in your system which is triggering the shut down. Check your software parameters to insure that no low-pressure alarms are set. Check operation of the injector without using a sample, under steady conditions, switching the valve from mainpass to bypass (inj volume set to zero) to observe what happens. Repeat with a full loop injection of mobile phase and compare the results. The valve rotor may be deeply worn or grooved resulting in the stream bypassing the port. Standard valve maintenance would be needed in that case.

BTW: Since the pressure drop seems to start after the injection, check that the post column derivatization pump is correctly plumbed and configured (event settings) during the method. An incorrect configuration of the valve could direct the flow to waste or perhaps backflow to the post column reagent pump. Either of these might result in a pressure loss in not configured properly. The Post Column system should be removed and the HPLC system tested/verified for correct operation. The post column system should also be tested offline too to confirm it is operating properly. We see a lot of these systems fail due to lack of maintenance.

Shamsun, that is very rude. Please do not hijack another forum member's post from 4 years ago with your own, unrelated question/post. Please follow the rules of good etiquette. Please delete your previous post and create a new one, in a new thread, and please include more details about your intended HPLC method too.