Retention Time Variation

[akshata]

Hello,
I am using a 5890 Series II GC for analysing the reduction of tar content over time using catalysts. I am trying to quantify H2,CO2,CO,N2,CH4 and toluene concentrations. I have installed an EPC(Electronic Pressure Controller) and we have two columns Agilent ( Plot Q and the Molsieve) in series. We also have a sampling valve for online sampling and we are using the concept of Column isolation to separate out the toluene and carbon di oxide . The problem i am facing is that there is no reproducibility of the retentiontimes. There is a slight variaion and the pattern i have observed is that the retention time becomes less as the day progresses. This is because the flowrate of the carrier becomes higher as the day progresses. Also the other problem is that as the concentration of the toluene varies the retention times also varies. I would really appreciate if any one could suggest any reason and a solution to my problem!! thanks

[Ron]

Have you actually measured the flow rate at the column end to verify the flow is increasing? If the flow is actually increasing then the EPC is not functioning properly and needs to be replaced.

I noticed that you are using a Molsieve column. These columns tend to retain water, and if they are not baked out the retained water will affect retention times. Do you have water in your samples, and if so what is the maximum temperature you are heating the column to?

[akshata]

Hey ROn!
Yes I have actually measured the flowrate towards the end of the day. Also we checked our EPC by inserting an external pressure gauge and we have found that our EPC works perfectly fine. As far as water goes....we have not yet injected any water into our samples and in our experiments we do plan to use water however we plan to vapourise it and there are several reactions which will take place and ensure that water gets converted into H2.