Propylene glycol with RID

[dorota]

Hello guys. I need your help.

Analyte: propylene glycol monopalmitostearate
http://library.njucm.edu.cn/yaodian/ep/EP5.0/16_monographs/monographs_l-p/Propylene%20glycol%20monopalmitostearate.pdf EP: 01/2008:1469
RID detector
Column: PLgel 5um 100A 600X7.5mm Agilent
Detector temperature 30C
Column temperature: ambient
Mobile phase: THF
Flow: 1ml/min
Injection volume: 40 ul
chromatograms:
[img=http://s30.postimg.org/4z4q96fzh/01_2008_1469.jpg]
1 How to expain the presence of this negative peak in the red and blue chromatograms?
2 Any suggestions to eliminate or reduce this negative peak?

[Consumer Products Guy]

Hello guys. I need your help.

1 How to explain the presence of this negative peak in the red and blue chromatograms?
2 Any suggestions to eliminate or reduce this negative peak?

1. Anything such as solvent from the injection passing through the detector that has lower refractive index than your mobile phase will produce negative peaks.

2. Use ELSD detector instead of RID.

If me: I would use high-temperature capillary GC after trimethylsilyl derivatization. And expect several peaks from the standard.

[dorota]

Thank you CPG for your comments (I am sorry for the delayed response but better late than never). I had to stick to EP, could not use ELSD or GC.
I thought that we had only THF in the instrument but it turned out that isopropanol was used for a needle wash. RI of isopropanol<RI of THF. When we changed the needle wash solvent to THF, we got a positive peak (see new pictures). I think this situation is better for integration and calculations.
http://pl.tinypic.com/r/28jyxap/9
I decided to share these results here because questions about negative peaks in RID are quite common:
viewtopic.php?t=13789
viewtopic.php?t=26128
viewtopic.php?f=1&t=26213&p=124896&hilit=refractive+index+negative+peak#p124503

[dorota]

Thank you CPG for your comments (I am sorry for the delayed response but better late than never). I had to stick to EP, could not use ELSD or GC.
I thought that we had only THF in the instrument but it turned out that isopropanol was used for a needle wash. RI of isopropanol<RI of THF. When we changed the needle wash solvent to THF, we got a positive peak (see new pictures). I think this situation is better for integration and calculations.
http://pl.tinypic.com/r/28jyxap/9
I decided to share these results here because questions about negative peaks in RID are quite common:
viewtopic.php?t=13789
viewtopic.php?t=26128
viewtopic.php?f=1&t=26213&p=124896&hilit=refractive+index+negative+peak#p124503