I was wondering if anyone can tell me what is the problem with my hplc. During my analysis runs, The chromatogram dips below zero (using a UV) and there seems to be peaks in that region. Are these considered to be negative peaks and what can I do about it.Sometimes the baseline dips between -0.02 to -0.4 Au. The main peak that I am interested in sometimes reaches an intensity of 1 or above and if I expand the chromatogram I see peaks. Is it baseline noise??

A little more information would help, mobile phase, gradient or isocratic, wavelength, analyte, sample matrix, sample dilution solvent, column, etc.

Sorry, my fault was dont be clear
and explain the problem. I need
to separate in one run: sulfadiazine, sulfamethoxine, sulfamerazine,
sulfamethazine, sulfamethoxazole, sulfamethoxipyridazine, sulfanilamide,
sulfamethizole, sulfaquinoxaline, sulfathiazole and sulfisoxazole. In
conditions to separate all sulfas at pH 2.0, sulfadiazine is not retained in
the column and have tailing in the others. I try Hypersil ODS 5 µ, 4.6 x 200
mm and Hypurity Elite C18 250X 2.1 mm 3µ. Mobil phases: PO4H3 0.02 M /ACN
gradient 10 to 20%;
H2O+ 0.02 % TFA / ACN same gradient. I try pH 3.0 and 5.4 but the problem
persist. I think to do a pH gradient but I not have
references about that.

excuse me, I commit a mistake: the sulfa that is not retained is sulfanilamide

To get the sulfanilamide retained, you should start the gradient in water instead of 10% organic. This should be compatible with Ye Olde Hypersil ODS, but may not work with the Hypurity Elite. If you can't get it to work on Hypersil ODS, try a modern packing with an embedded polar group such as SymmetryShield RP18.

Dear Sir,
I will be obliged if you send me information to separate different Rhamnolipids on HPLC.
Thanks,
Raza, NIBGE.