Impurity in HPLC Water

[munish]

Dear All,
I am running Agilent 1100 for impurities in mannitol (sugar). Column temp: 85 C, Flow Rate: 0.5 ml/min, Column: Carbohydrate column, Injection Volume: 20 uL, Mobile Phase: DI Water, Diluent: DI Water, Detector: Refractive Index.
I see a negative peak at around 17 mins. It also occurs in blank.
When I reduced the injection volume of my blank from 20 uL to 10 uL the area of this peak reduced by almost half.
When I reduced the flow rate by two times, the peak Retention Time increased by two times.
When I ran DI Water from a different source, same peak appears.
It is a very small negative peak but I want to justify its existence.
Any comments of what could it be? Impurity in water? Something sticking to my injector loop? or something inherent to RI detector?
Thanks!!

[DR]

Has the water been degassed?

[munish]

I ran degassed water too. Still the same peak shows up.

[bisnettrj2]

What are your column dimensions?

[munish]

Column Dimensions are:
300 x 7.8 mm,
9 um particle size,
Aminex HPX-87C

[bisnettrj2]

Based on your flow rate and column dimensions, that dip could be noise from your injection valve. Essentially, t0 noise. Do your peaks elute after the dip?

EDIT - take what I say with a grain of salt - I've only worked with traditional RP-HPLC - others may have more knowledge of the specific application and/or detector.

[munish]

My peaks elute before and after this dip. My sugars elute at 12 min, 19 min, and 24 min. This dip is at 17 mins. If it was just a noise would its peak area change proportionally with injection volume?

[Alexandre]

Could you please run blank without injecting anything? In some systems you can run a gradient without injector it will just profile your column and mob. phase. Do it few times to see trend and let us know.

[HPLCaddict]

Based on your flow rate and column dimensions, that dip could be noise from your injection valve. Essentially, t0 noise. Do your peaks elute after the dip?

EDIT - take what I say with a grain of salt - I've only worked with traditional RP-HPLC - others may have more knowledge of the specific application and/or detector.

I'd say it doesn't depend on the mode of chromatography - the retention time of the dip is suspiciously close to the theoretical dead time. This would mean that one of the analytes actually elutes before the void - some kind of exclusion phenomenon?
Anyway, Alexandre already suggested the thing to do - looking at a blank run...

[cody84]

Needle wash solvent match the mobile phase?

[munish]

Yes both needle wash solvent and mobile phase are DI Water. Its an isocratic run. No gradient.
I will try some zero injection volume runs too.

[munish]

I injected 0.0 uL injections four times. I still get this dip in each run. The area of this negative peak is lesser than with DI Water and also its varying in every run.
Thoughts????
Could it be the air in the injection-loop that enters the line during the injection?

[bisnettrj2]

Is the valve switching from load to inject, even when doing zero volume injections?

[Alexandre]

Some instruments can run flow bypassing the loop; it is different to 0.0 injection volume. Can your instrument do this? It gives a lot of good information.

[munish]

Yes, the valve is switching from load to inject, even when doing zeor volume injections.