Chromatography Forum

Hello,

I am using a 5890 series II GC with FID to analyze toluene and TCE with manual head-space injections. The column is an hp-5ms 30x.32x.25. My method is isothermal at 120 deg with column pressure of 60 kPa and flowrate of 30mL/min N2.

I am shooting each sample in triplicate and the area will be 1200 ( which is what it should be) then 2 minutes later it will be 9000 and the peak will be 30 seconds long. Sometimes, it will do it 3 injections in a row, or sometimes it will go 2 hours without doing it. Not sure what the issue is.

The flow-rate and and column pressure are set by manual dial. The flowrate is very touchy usually (brush the side and it will adjust), but has recently and sporadically become much less sensitive. It almost seems to stick

Thoughts on what to try to fix the problem?

Thank you,

are you saying the column flow is 30mL/min?

Yes, I believe so

The flow rate of 30 ml/min is the total flow through the inlet, of which about 1 ml/min is going to the column, giving a 30:1 split. It sounds as if the flow controller is faulty and that the wide peaks are from unintentional "splitless" injections. If you measure the flow out of the split, does it fluctuate ?

Peter

Peter,

Yes the split does appear to fluctuate. I switched it to a splitless method since my sample is relatively low concentration. The column pressure will not stay constant for a any flow rate and the total flow rate still seems to drop or surge. Any thoughts on what to try next?

Agilents suffer from contamination of the split line and vent trap - you could try cleaning or replacing the liner and replacing the trap. Otherwise you have to suspect a faulty EPC, which would need to be replaced.

Peter