[blackcat]

I saw a shift in retention times for GPC for the same sample over months, especially after the GPC has been down and then repaired.

The shift is like 1 minute. Is this normal? Any reason?

[wildfyr]

Sounds like something is up with the column. 1 min is a very large shift.

Do you have a standard you can check?

[blackcat]

Sounds like something is up with the column. 1 min is a very large shift.

Do you have a standard you can check?

Nope. But the GPC has been down and fixed before. Maybe this is why

[DrCMS]

I would say a 1min change in RT for the same sample over the span of months with the down time and repair in the middle is hardly any change at all for GPC.  We see changes in reported Mn, Mw etc of >10% on the same instrument on the same sample over the span of days.  We see differences to our customers results of much much more than that.  We no longer report just the Mn or Mw but report instead the difference from an agreed sample that both sides hold and run side by side with every product sample.  GPC is very very variable.