Chromatography Forum

I have 2 analytes that give a significantly lower concentration on one column when compared to the other column. These analytes are in a standard mixture, and the other analytes’ concentrations are the same concentration when compared on both columns which is expected. The columns are basically the same except for a 0.05 micrometer difference in film thickness. Does anyone know what could be the possible cause(s)?

Is this in samples compared to standards, or are you comparing the absolute responses on two different columns?
What are the detectors?

I’m comparing an analyte concentration from a sample that’s ran on both columns. One column serves as a confirmation.
I’m using ECD.

Are these 2 analytes more volatile than the others? It'd be happening because the interaction with the film.... Although the difference in the film is so small....

Are you also seeing peak broadening, more so on the column with the signal suppression? Hydrocarbons have little response to ECD but can suppress detector response.

If it's only two analytes out of the mixture that have different responses, there may be chemistry going on. What are the two problematic analytes and some of the other analytes in the mixture that aren't problematic? Are you running with two inlets or splitting one inlet to two columns? How different are the responses?

The problematic analytes are alachlor and heptachlor epoxide. I'm using two inlets. On the front column, I get 10 ug/L of of alachlor, but the back column shows 1.4 ug/L. Based on quality control testing, the higher concentration of alachlor is expected. Heptachlor epoxide gives me 1.1 ug/L on the front column, but 0.53 ug/L on the back column. In this case, the concentration on the back column is expected based on quality control testing.

Have you swapped stuff around? Swap injection ports and columns/detectors? Swap injection port/columns and detectors?

Has this always been the case or is it something new? How old/new is the problematic column?

I plan to swap the lines for the inlets and detectors to determine if it's an issue with the column or the detector. Thanks for the feedback!

Swap just one. If it were me, I would swap just the inlets. If you swap both inlet and detector, you don't isolate the problem all that well. The inlet is where I would be looking anyway...

Best regards,

AICMM