Baseline drift

[Laji]

Hi
Hi I am running HIC LC under the following conditions:

HP- 1100 LC system
Tosohass (TSK-Gel-Butyl-NPR 2.5 um )
Detector: FL
Ex 278 nm
EM 350 nm
Column load 10 ug/mL
Pre-column filter with 0.5 um stainless steel FRIT
Moble phase A - 1.8 M (NH4)2SO4/0.1 Sodium Phosphate Dibasic, pH 7.0
Moble phase B - 0.1 Sodium Phosphate Dibasic, pH 7.0
Flow rate 1 ml/min
From time to time I am noticing a downward drift in the basline. I have noticed it only when a certain lot of amm.sulphate is used to prepare Mobile phase.[/b] I am forced to believe that the culprit is ammonium sulphate used and it is lot to lot dependent. Now I am not sure what in amm sulphate could cause this or if there is something else that I am overlooking.
If you have any insight to this problem, please let me know.
Thank you,
Laji

[HW Mueller]

Your column accumulates fluorescing or light scattering substances which are washed out with the mobile phase that give the drift.
How do the different mobile phases, H2O, ammonium sulfate, phosphate solutions look like (where is the baseline?) if pumped through the detector without the column, how do these compare with column effluent?

[Laji]

That is a good thought. I have never tried it. I do know that it is some impurity in the (NH4)2SO4 material that is used for the Mobile Phase prep. This conculsion is based on my observation that, everything else remaining the same, but changing the Ammonium Sulphate leads to this drift. It is very temporamental and lot to lot specific---everything else remaining unchanged.
L