I have a problem with glucose/fructose/sucrose/sorbitol analysis.

HPLC: HP1050
Column: Alltech 700 CH Carbohydrate analysis
Detector: RI HP1057

Negative peak appears at 17 min. just before sorbitol ( it does not interfere with g/f/suc peak.

Since water is the eluent, I've injected three blanks 1.HPLC grade water 2.&3. water from two different deionizers. The negative peak appears on all the blanks, of the same size - additional but rather small negative peaks appeared for the first blanks earlier in the chromatogram.

I will be grateful for any hints and pointers to a solution.


One thing I am going to try is to separate the negative and sorbitol peaks, so thet I can do the sorbitol quantitation, but this is not a good solution.

This could be a problem with deggasing. If you are using online vacuum deggaser - stop it and try again. Deggase samples/blanks.

This question brings to mind a memory. Many years ago, I used RI a lot for determination of initiators in a polymer. The solvent was tetrahydrofuran and there was always a negative peak in th chromatograms. It did not seem to make a difference if we used new, old, stabilized, unstabilized, etc THF. We also got the peak just with solvent injection. I always assumed that it may have had to do with a side-effect of the injection (similar to a pressure peak in GC) or something like that. We could not do anying to change it, although the degassing should definitely be tried.

I've used the Bio-rad Aminex 87C column for a similar application with water as the mobile phase. I've seen a negative dip around 17 minutes due to low levels of methanol in the sample. Maybe you are picking up low levels of a solvent in your system or injector.

Are u sure it is not a peak because of load-inject turning of injector?