Chromatography Forum

Hey guys
Have a scenario here
I have a Dionex P680 LPG pump with ASI100 Auto Sampler and PDA 100 Detector
My column is a Supelco Supelcosil with 15 cm x 4.6 mm x 5 micron dimensions.Flow rate of 1 ml/min with an isocratic method of 90:8:2
MeOH: H2O : 0.3% H3PO4
I'm running a method with two analytes- Octinoxate and Avobenzone
The wavelength changes from 310 nm to 357 in 5.65 minutes and here is the problem
I used to get 1.3 Slope for Octionoxate and 0.95 for Avobenzone and my curve for both of these guys was coming out near to perfect with 4 point calibration with R square value of 0.9999.
After about 2900 injections on the same column my slopes have changed to 0.95 and 1.3 respectively.
My assay numbers are not off the chart- There are still in range- But I have paid attention to my slopes all the time since I have started this method and I see a change- Although my numbers are coming up "ok" , but I don't like the slope change.
Any ideas what could be happening here? Could it be the column?

Thank you in advance

Could it be the column?

Yes. If the peak shape is changing due to column age, the integration parameters may be applied differently. Easiest way to tell is to replace the column. 2900 injections is a reasonable lifetime for a column.