Chromatography Forum

Good afternoon,
I am having some trouble in one current method we use to quantify some anti-infeccious drugs from plasma samples. We use an Acetonitrile:Phosfate Buffer gradient by 25 minutes at 215 nm, witch results in a perfect separation of peaks. Our system is an Elite LaChrom and the coullum is an Waters Sunfire. The method had a normal drift of baseline, as the acetonitrile proportion rises up, at max 0,005 mUA, witch we accept as rasonable.

Last week we exange the collum to try a new method on an Waters Atlantis, and when we put the Sunfire back in the system and calibrate the anti-infeccious drugs method, the drift get up at max of 0,2 mUA, witch is incompatible with our LLOQ.

What could be causing this?

If you have not already done it, prepare fresh mobile phase (particularly the phosphate buffer). Microbial growth in aqueous buffers can give rise to that sort of drift. While you're at it, check the needle rinse solution in the autosampler.

Try purging the column and system for a few hours with acetonitrile and then run a few blank gradients. It's possible that something may have bled off the other column.