Where does these noise come from?

[jadexyj]

I try to setup a Water's HPLC, but see noise about 0.05 AU from 0 ~ 2.8 min, after 2.8 min, the noise is gone (UV absorption down to 0.00005 AU). I try different mobile phase, and clean the system overnight, but the noise is still there. Could anybody please give some hint where the noise came from? Thanks

[JMcK]

Is this isocratic? Does this happen when you run the analysis with no injection?

[jadexyj]

What I have tried uptil now:

I remove the column, replace with a union, isocratic (H2O:ACN 50:50), tried 2 flow speed, 1.0 ml/min and 1.5 ml/min,
and still repeating the same result: 0 - 2.8 min, I have relatively big noise (~ 0.05 AU), right at 2.8 min, the signal drop down to 0.0005 AU, looks like a thin line.

I will try the monitoring mode (is this what you mean without injection) next,

I also feel this is something related with injection, so I try to change the injection related parameters, so far I tried change the needle wash time (from normal to double), but result remain the same.

Is this isocratic? Does this happen when you run the analysis with no injection?

[DR]

Major mis-match between rinse solvent and mobile phase?

[ym3142]

did your detector have an event set from 0-2.8min, which is strange though?

[JMcK]

Major mis-match between rinse solvent and mobile phase?

Are you seeing this when you don't make an injection? Run an isocratic separation and inject the mobile phase. See if you see the disturbance. Try injecting 0, 10, and 20 uL. Are you seeing any differences?