Chromatography Forum

Hi everybody
I lately had a problem with an in house method for preservatives in herbal liquid extract ( potasum sorbate , sodium benzoate , methyl paraben sodium ) respect to their elution sequence , ion pair method , mobile phase consist of phosphate buffer pH 2.5 with 0.095% tetrabutyl ammonium hydrogen sulfate , acetonitrile and THF ( 90:5:5 %) respectively , because herbal extracts are very complex samples we have to increase K` for complete peak purity ( risk benefit concept ) and fortunately it fits for all types of herbs thyme , fennel , eucalyptus , guava , tilia we produce , we are using it for about 8 months now , we tried it on schimadzu system and two different waters systems with two different models it worked well but lately I had a drifting noise of the base line in all runs ( each run is about 50 minutes , back pressure is stable along runs ,column c18 250 x 4.6 mm flow rate 1.7 mL/min wavelength 235 nm and notice that this drifting appeared on two different systems with two newly maintained and calibrates dual wavelength detectors I hope you can help me

In an isocratic method with very strongly retained "junk", it's not unusual to see baselines drift up and down as 'CRUD" (Chromatographically Retained Undesirable Debris) gradually elutes. I suspect that what you are seiing is simply *very* strongly retained and hence *very* wide peaks.

If that's the case, then purging the column with a strongrr eluant should clean it up. You will have to spend the time to re-equilibrate the column afterward.

I agree with Tom. Reverse the column and clean with a strong reverse phase organic solvent like 95% IPA dissolved in water.

Thanks alot for replying ,
It appears a part of my problem , the rest is that column got so old it makes higher back pressure than usual and lately base line shows drifting noise , firstly I thought it might be the detector or mobile phase impurities but then it is the column