Negative peak in HPLC

[crystal1314]

Instrument : Shimadzu SCL-10A system
Column : Supelcosil LC-NH2 250 X 4.6 mm\
Flow rate : 1.2ml/min
Temp : 30
Detector : RID 10A
Mobile phase : 75 : 25 MeCN : ultrapure water
Diluent : 75 : 25 MeCN : ultrapure water

I'm using this system to detect d-sorbitol. After the solvent peak, a negative peak appear just after d-sorbitol peak. This negative peak is absent when I ran the system in Jan.

This negative peak is absent when I inject the mobile phase.

[carls]

Negative peaks with refractive index detection (RID) are relatively common. In your case, is the sample dissolved in mobile phase removed from the mobile phase reservior (recommended). If not then there's likely something in your sample diluent. If so the negative peak is from a component in your sample.

An RID "sees" everything (i.e universal detector) but some components have higher RI than your mobile phase while others have lower RI. This gives rise to positve and negative peaks.

Hope this helps

[crystal1314]

Yes. The sample is dissolved in mobile phase. After the sorbitol peak being eluted out, a negative peak immediately appear beside it. Both postive & negative peak having almost the same area.

Is it due to the column?