Mobile phase as sample

[bunnahabhain]

Are you sure what you see is not dissolved air? Try to purge your eluent sample in the autosampler vial with inert gas (helium is best, argon will also do) and see if the peak gets smaller. If you don't have helium or argon, purge with pressurized air, the peak shoud increase in this case. Or inject 5 µL air from an empty vial and see if the retention time is the same.

[greg07]

Maybe I'll try degassing sample vial in ultrasonic bath? And what you thing about the basic question/problem with mobile phase methanol/water 4:1:

mobile phase as sample must give no peak

I thought so, but the client showed me an example that I described above

Regards

[RyanH]

greg07 you thought right. If you inject a "sample" of the same mobile phase you are using to push it through how could there ever be any peak because of that assuming no contaminants?

If you post some chromatograms I could be more certain, but what I think you are seeing is a peak caused by air being injected or coming out of solution. Whenever you mix different solvents that have not been degassed small air bubbles can come out of solution and show up in your detector. You've got a few options that might fix this. Degass your mobile phase, degass your sample, and/or hook up a back pressure regulator set at about 80 to 100 psi after your detector. This will help to collapse any small air bubbles that would show up as a peak with very short retention time on your chromatogram.

[tkubowicz]

Hello

hook up a back pressure regulator set at about 80 to 100 psi after your detector. This will help to collapse any small air bubbles that would show up as a peak with very short retention time on your chromatogram.

Backpressure 80-100bar is quite risky - for Agilent DAD with Max-Light cartridge or FLD you can easilly damage detector cell. It is better to use Peek tubing with smaller diameter (it will give enough backpressure and low risk of cell overpressure)

Regards

Tomasz Kubowicz

[dblux_]

Backpressure 80-100bar is quite risky ....

Fortunatelly he thought of "psig"

[tkubowicz]

Backpressure 80-100bar is quite risky ....

Fortunatelly he thought of "psig"

Ups sorry...I think I'm blind in the morning
Anyway...comment about detector cell still worth to mention

Regards

Tom

[greg07]

Thank you very much for your helpful comments, but I add that all the measurements are amazingly repetitive. Air bubble problems would be more or less accidental. Next time, I'll add more chromatograms.

@RhyanH - take a look at the chromatograms shown above

Regards

[Rndirk]

How does the spectrum of the unknown peak look like? Any features?