GC6890 problem with peak area

[PeryDem]

Hello,

Working with seperation of gases and have Porapak N and Molsieve columns in series with a injection valve with TCD detector.

The problem started when I changed the Porapak N column to a new one and since then the calibration of the method have not been approved since the CO peak area is decreasing (beginns with high peak area and goes down continuously without stabilizing). RSD for the CO is 2 % and it should be 0,7%. The other components in the sample are okay.

Have changed the Porapak N column again and the Molsieve to a new one without any success. Have checked leakage in the system couldn't find any. Have replaced the injection valve and the switching valve and stil doesn't work.

I have a old GC and when I run same (almost same) method in that one it works so it can't be the carrier gas or the calibration gas.

How can I solve the problem?

[aidnai]

Did changing the columns have any effect?

Are these packed columns? who are you getting them from and is there a conditioning procedure?

The fact that only CO is acting weird does indicate to me the issue is in the column(s).

One other thought -- I guess you've got the porapak, then a dean's switch, then the molesieve (I've used this setup myself to get permanent gases along with CO2 and C1-C3ish). With the new columns, is the dean's switch activating before all of the CO can get onto the Molesieve? Worth checking the timing of everything I think...

Good luck,
Aidan