Negative peak in LC

[jameslizh]

I am analyzing iohexol with isocatic (MeOH/H2O/acetic acid) using 1100 Agilent (DAD) at 254nm, phenomenex Luna column. For high concentraion of standard, good peak at 11.6min. But for low concentration standard, got negative peak at 12.2min. I used all LC grade of solvent. acetic acid is ACS grade (but used another bottle with LC/MS grade of acetic acid, same results). Ran blank, pretty clean and no peak. Look for advice why. Thanks.

[tom jupille]

One possibility is that something is in your sample (or blank, for that matter) that absorbs more strongly at the reference wavelength than it does at 254.

Check to see if the reference wavelength is set on the DAD. If it is, turn it off and see what happens.

[jameslizh]

thanks. Tom. Tried that and the negative peak is still there. Developed another method using new column and mobile phases, now the negative peak is gone.

[tom jupille]

I'm glad you got it fixed. I'll go out on a limb and speculate that you had a "system peak" with just enough of an RI shift to show up (but thats a very long limb! ).