Steady increasing baseline on PDA


We are seeing some strange behavior on one of our PDA systems (on H-Class system, utilising a BEH C18 column, 50*2.1mm, 1.7um, flow 0.8 mL/min of 50/50 ACN/water). What can be the reason of this increasing baseline as shown below? The lamp was warmed up sufficiently.


  • Hi Martienvdm,We could use some more information to help you troubleshoot.Comments from Waters in house expert is as follows: -- the absorbance is increasing linearly to about 0.01 AU at 290 nm. water and acetonitrile are transparent at that wavelength (actually down to almost 200 nm) so it's unlikely to be coming from the stated 50:50 mixture of AN and water. since we have no history of the column -- or the system -- it's hard to tell where it's been  so ... it's time for some troubleshooting.  a. look for the obvious first. are the solvent bottles contaminated? how well primed were all of the solvent lines? are we sure that the solvents are what's claimed? this looks like a measurement of the system's dwell volume or gradient linearity  b. examine the reference spectrum from console you should be able to review the reference spectrum if 3D data was collected. . it's difficult to imagine how you get to 0.1 AU at 290 nm without seeing significant attenuation of the reference spectrum.  c. are you running a gradient -- flow or composition? I have not seen a baseline increase in a perfect line and drop back to the origin while the pump is at constant composition. are there additives in the mobile phase?  d. have they looked at the 3D spectra?  this is unlikely to be column contamination which usually shows peaks and such and isn't "smooth."  we have something of a mystery here and very little traceable information to go on.  cheers
  • Another colleague had the following suggestions:   Without a little more information, it's a bit difficult to troubleshoot. My best first guess is contaminated solvents. Either the solvents themselves or the reservoirs that hold the solvents. My second best guess is some type of electronic failure. The unwavering linearity of the drift is quite unusual. Typically, if drift is related to solvents or chromatography, there will be changes in the rate which produce some signal wander. The example appears to present an almost perfectly linear rate of change which may represent something that is actually failed. A relatively quick check of this would be to stop the flow and see if the linear drift persists. If it does not, the cause is most likely chemical/chromatographic in nature. If the drift continues unabated with no flow, my suspicions would turn towards an electronic failure which would require Waters Field Service to diagnose.