Chromatography Forum

I am using PDA detector. My base line is always going negative so I am not able to see my peak..what is causing that? it is the flow cell or something else.

What kind of PDA do you have? What wavelength are you measuring?

Is this an isocratic or gradient separation? What are the mobile phase components?

In some situations, a negative drifting baseline is normal. In other cases, it could mean that one solvent ("A" if this is a gradient) is contaminated.

Why do your peaks disappear? Do you use a strip chart recorder for data collection?

MK - can you supply a example chrom?

See my earlier postings about strange baseline effects with a faulty flow cell on my UPLC system.

I am using Accela PDA(by thermofisher). My mobile phase is 70:30 Water:MeOH (isocratic).Flow rate is 0.2ml/min. Base line starts at 0 and then goes down to -2000-5000 and then I have normalise( auto intensity) to see the peaks.

wave lenght I am using 220 and 254.

What I tried to ask is whether your data are lost when you don´t see the peaks while the chromatogra is running. I am just curious about what people hook up to a relatively modern machine like a PDA to do data analysis/workup.

-2000 to -5000 mAU is a lot.
accela models are pretty new so your instrument should be still under warranty logically.
first approach that i would take is use the vendors PQ test for drift.
if the test fails and the procedure uses water then there are 2 reasons for the failure.
the PDA itself or the water. make sure that you use good quality HPLC grade water for the test if it is required in the procedure.