ECD problem

[greenhouse gases]

We had two SRI 8610C GCs running analyses when the cylinder of make-up gas (10% methane in argon) ran out but the runs kept going.
We weren’t able to swap out the make-up gas cylinder immediately – it took about 2 weeks to get a replacement from our supplier.
During this two weeks period, the GCs were kept on (i.e. ECDs hot) with carrier gas running and the make-up gas still set as usual on the GCs but the actual cylinder was closed (i.e. no make-up gas going through but the make-up gas flow set pot was still at usual setting).
When we replaced the cylinder we found the following problem:
The two GCs that hadn’t been analysing samples at the time of the make-up gas running out are responding and behaving normally but the two that were analysing samples have now lost sensitivity to N2O. One of them showed peaks much smaller in area for our standards but now the peaks have disappeared. The baseline looks OK and the pressure pulse peak still shows up.
The other still has N2O peaks but is at about half the sensitivity. (Original area before problem happened for 100ppm N2O was 117535 which has dropped to 5000.)

We’ve tried adjusting the make-up gas flow rate but it doesn’t make any significant effect either way (higher or lower).

Anyone could suggest what we could try next? Thanks

[GasMan]

I am not familiar with the GC model that you are working with but am aware that the sensitivity of the ECD to N2O is dependent on the detector temperature. We run ours at 300°C, so just make sure that your detector temperature was not inadvertently changed during the GC down time.

Gasman