UPLC-MS QTOF pressure drop every 15 seconds

[chemist23]

Hello,

I am having a problem with pressure and retention time with Waters UPLC QTOF system.

Retention times are about 1-1.5 min late.

Pressure is at ~11000 psi (3000 psi higher than normal) and drops about 3000 psi every ~20 sec interval consistently. I've already ruled out the problem stemming from the LC, since this pressure issue does not exist when the MS is turned off and only the LC is on. Once the MS is on and flow is going through the sample probe, I begin to see this pressure issue.

Any advice on how to correct?

[James_Ball]

If it is just whether the MS is powered on or not, I would look at the position of the divert valve. Is it switching or partially switching and causing a blockage in the flow?

Or possibly the heating of the needle causing a change in flow. Not many things in the MS are involved in the flow that would cause restriction.