Clean the filters in the solvent bottles - it helps sometimes

Beyond it you would require a technician I suppose - if purging does not help.

Hi Evelin Bell,

you please check the following points one after another and verify the pressure.

Step :1

Sonicate the solvent input channels filters (filters at the beginning of the solvent line- in the bottle) .

Step :2

Remove the GE valves( valves at the pump) clean. then verify 

Step :3

Flush the detector lines with syringe.

Step :4

if it is not resolved, Finally change the pump valve and verify and confirm the reason.

If this is not still resolved, call for instrument service person.

I hope  this helps.

Regards,

Bujji Reddy.

With Waters brand HPLCs, the check valves sticking (clogged, worn, dirty or have air bubble inside) are the most common reason for a loss of prime. Be sure and fully DEGASS your mobile phase before using the system (this is the 2nd most common reason for loss of prime. Using helium sparging is the best method so you should be OK there). Clean the check valves in an appropriate solution that will dissolve any salts, particulate matter or residues which may have caused them to clog. Sonication is best, but you can also "swish" them in a beaker filled with solution too. You may need to replace them too (they WEAR OUT and are normal consumable parts so you should have spares ready for use). As mentioned, the solvent pickup filters also require regular cleaning and/or replacement too. If they are obstructed or clogged, then the pump(s) may lose prime or show cavitation (= no flow). Always test the flow with back-pressure on the system (~ 500 psi or more). The HPLC is designed to operate under pressure, not open to the atmosphere. You can use a restriction capillary for this purpose (anyone using an HPLC should have one available for basic troubleshooting/servicing),

Another reason for a sudden drop in pressure is that you may have a leak somewhere or air in the system. Check for loose fittings. Be sure and check the system plumbing and connections very carefully (a clean system is easier to find leaks on then a dirty one).

You also implied that when using Pump channel 'A', your flow was unstable, but Pump 'C' was OK. If I have interpreted your statements correctly, then in addition to check valve issues, you may have a problem with the GPV unit. Flush and clean the gradient valve with IPA to restore performance [Link to article on this topic which may be helpful: "HPLC Gradient Valve / Proportioning Valve / MCGV Leaks. How to Identify Them"; https://hplctips.blogspot.com/2021/08/hplc-gradient-valve-proportioning-valve.html ]

You may also find these two general articles useful in troubleshooting these types of problems,

• "Diagnosing & Troubleshooting HPLC Pressure Fluctuation Problems (Unstable Baseline)". http://hplctips.blogspot.com/2014/01/diagnosing-troubleshooting-hplc.html
• Also: "The HPLC Restriction Capillary; Troubleshooting, Qualification and Running Without A Column:"; https://hplctips.blogspot.com/2018/04/the-hplc-restriction-capillary.html