My injection peak for a new analysis is far to large in the negative region, what can I do to reduce/eliminate this? Mobile phase is MeOH/H2O and I am injecting sample made in degassed mobile phase. Injection volumn is 20ul. Help!

I think more data would be helpful. What do you mean by injection peak? Does this peak occur when you inject just mobile phase? Where in the chromatogram does it occur? What does far to large mean? What are the analysis conditions, detector sensitivity settings, sample type and concentration? What kind of system are you using?

Obviously, the more details you can provide, the better.

Best Regards,

Michael Hinsberg

http://www.hinsbarlabs.com

How is the sample being injected?Using a manual
injector without a bypass,will interrupt the flow and cause a peak.

How do you know that the degassed mobile phase used for making the sample has the same composition as your mobile phase?

Thanks for your comments. By referring to injection peak, I mean the negative peak @ 3.2 minutes into my 50 minute analysis, which corresponds to the system void volume of 3.2 minutes. The peak does occur when I inject just mobile phase. Injector is a manual Rheodyne 8125 with 20ul loop. The peak pegs out the Waters 410 DRI set at a sensitivity of 32 with a scale factor of 20. Reducing the sensitivity does not work. Mobile phase is methanol/water with vacuum degassing (yes I've tried helium also). The pump is a HP1100 binary. Furthur comments are appreciated. Thanks

I am still not yet convinced that the "mobile phase" composition is the same between the stuff that you pump through the column and the stuff that you are injecting. Let me try again!
How are you degassing the mobile phase? Are you doing this on-line or off-line? Where do you get the stuff from that you are injecting? From the mobile phase container? If you are getting it from the mobile phase container and you are using on-line degassing, you got the answer to your puzzle. For example, you could collect the stuff that is coming out of your column and inject it side-by side with the stuff that you have in your mobile phase container and see if there is a difference. If this is not the source of the difference, I would investigate the sample vials and the syringe that you use to load the injector.

Mike,
If you just move the valve from load to inject, not making an injection into the loop,just
operating the valve,do you still see the negative
peak?

Could you be looking at a temperature effect? If
the temperature of the sample volume is very
different from that of the mobile phase you could
conceivably get viscosity effec