Negative ghost peaks

[Tina]

I am running a GPC system for the analysis of different polymers and had no problems in the past. However, a few days ago I observed ghost peaks in my chromatograms. All the ghost peaks are negative and following a regular peak. The shape of each ghost peak is quite similar to the previous regular peak.
My chromatographic conditions:
·Mobile Phase: Tetrahydrofuran (stabilised with 0,025% Butylhydroxytoluol)
·Stationary Phase: 4 Columns, 300 x 7,5 mm with pore sizes of 100 Å, 500 Å, 10E3 Å and 10E4 Å
·Detector: Refractive Index
·Flow rate: 1 ml/min
·Temperature: 30°C
·All samples are solved in THF (mobile phase)

Does anyone have an idea of what is the source of my problem?
Thank you in advance!

Regards,
Tina

[Chris Pohl]

Tina,

Are you sure you haven't connected the column outlet to the detector so that the flow goes first into the detector sample inlet and then into the reference inlet? The behavior you describe is exactly what one would expect in this case.

[Uwe Neue]

Also a signal polarity switch could be the problem.

[Tina]

Dear Chris and Uwe,

Thank you for your quick reply.

Meanwhile I found the source of my problem.
The detector outlet line was squeezed. Due to the high pressure the detector ‘s purge valve opened and the sample flow went through both detector cells, the sample cell as well as the reference cell.
I removed the squeezed part of the outlet line and now my system is working properly again.

Regards,
Tina