Refractive index: peaks followed by baseline dip

[bunnahabhain]

Dear colleagues
We try to establish the IP391(2000)/ASTM D6591 method fo aromatics in hydrocarbon mixtures as described by this application note: https://www.agilent.com/cs/library/appl ... 0743EN.pdf
We use an Agilent 1100 system and a Shodex RI-101 detector.
In our first experiments, we see the standard peak, followed by a dip in the baseline with about the same peak area:



The standard is dissolved in n-Heptane, the same batch as we use as eluent. Column compartment and detector are both set to 50 °C (We alseo tried 35 °C for both, same result).

Looking at the chromatogram in more detail, there is a repetitive pattern of fluctuations in the baseline:



I'm not sure if these effects are related, but maybe it is indicative to some expert. This seems to be related to the pressure and improved when setting compressibility to 120 (setting for heptane taken from the pump manual).

Thanks for any help,
Jörg

[JI2002]

Injecting a standard at a different concentration, e.g., 20 mg/L, to see if the magnitude of the dip doubles. If yes, the dip is likely caused by impurities in the standard. If not, it’s likely caused by contamination in the mobile phase. This can be confirmed by injecting air (empty vial) which should not show the dip.

Since the dip is observed after the peak of interest elutes, the dip is not expected to interfere with your analysis.

[Hollow]

This pattern looks like a worn plunger seal or check-valve to me.

One pump head/plunger is not sealing properlly so you have always a dip in the flow.
Increasing the flow (pressure) should make it worse and vice versa.