I'm developing a method to measure SF6, CFC-11, and CFC-12 in air samples using a Shimadzu GC-2014 with an ECD. I have a calibrated standard of SF6, and a mix of CFC-11 and CFC-12 Based on what I've read in the literature, I'm trying a 6' packed mol sieve 5A precolumn to separate SF6 and the CFCs from O2, and then backflushing out the O2 once the SF6 and CFCs come off the column. I then separate the CFCs using a 10' packed Carbograph 1AC column. The SF6 and CFC concentrations are high enough so that a purge and trap is not necessary.

The first problem I've experienced is that the MS-5A is failing to separate the O2 from the CFCs. Based my understanding of how MS-5A works, the CFCs should be too large to be retained by the column and should elute before O2, but this doesn't seem to be happening. I've baked the column out for ~12 hours at 235 C, but that didn't seem to help.

The second problem is that either (a) the carbograph is failing to separate the CFCs or (b) one of the CFCs is not eluting, as I see only one peak after SF6 and O2. I've let the analysis run for up to 15 minutes without seeing a second CFC peak.

If anyone has any experience with these gases and these columns, any advice would be very welcome.

Thanks.