How much of a difference do you see? The question is better phrased as whether there is a difference in detector response time due to fluctuating nebulizer gas pressure, since the elution time of a compound off the column is unaffected by the gas pressure. Many ELSD systems have an internal regulator to guard against fluctuations in the gas supply pressure.

Try running compounds that also have a UV absorbance and see if the peak alignment changes between the detectors.

Thanks Jack. The retention time shift is around 1 min for subsequent injections. There is no pressure fluctuations with the LC Column. That is why, I was suspecting with the Detector side.

Hi Jessin,

I would check my system for solvent leaks. Then I would check my LC pump. Does the pump deliver the correct amount of solvent? If not, I would check the check valves (inlet and outlet valves). Retention time shifts are very often due to defective pumps. If the column is defective, you can see this from the peak shape or lack of retention of your compounds. What about the equilibration of your column? And last but not least: Is your column oven working correctly, i.e. is the column temperature constant?

Regards

Markus

Do they shifting back and forth or drifting to just one direction, are you observing increased retention times or loss of retention at every subsequent injection? Depending on your answers we may say that it is a column equilibration problem or defective pump or oven indicator as Markus Christ indicated. I also think that is not the result of the nebulizing gas fluctuation, since you say approx. 1min retention shifts are being occurred during analysis. It seems to be more chromatographic rather than the molecule detection configuration.

Thanks

As mentioned above, I would look for a chromatography issue. Without knowing your method, we are "shooting in the dark". You mention a 1 minute time shift- is the compound eluting earlier or later? Is it possible more equilibration time is needed, assuming a gradient is being run.

A change in the gas pressure on an ELSD will have no significant change on retention times. *I say no "significant" change because poor quality nebulization may result in small changes to peak shape leading to poor quality signal being integrated. As long as the S/N is good, retention times will be stable using a well developed HPLC method (area counts are another matter...). *Disturbances to the atomization / nebulization process may and do result in signal changes, amplitude, not Rt.

E.L.S.D. operation and use requires a lot of training. These detectors require a lot of optimization and tuning to use and they are best used by intermediate to advanced level users (because the detectors require so much "tweaking", internal cleaning and specialized training to setup, tune and use to obtain accurate data from them).

Retention Time variations are the result of other issues, not the detector. Here is a link to a free training article (taken from the classes I teach) that may help you troubleshoot this concern.

• "HPLC Retention Time Drift, Change, Variability or Poor Reproducibility. Common Reasons for it"; https://hplctips.blogspot.com/2015/11/hplc-retention-time-drift-change.html

• "HPLC Retention Time Drift, Change, Variability or Poor Reproducibility. Common Reasons for it"; https://hplctips.blogspot.com/2015/11/hplc-retention-time-drift-change.html