Coelution of Refrigerant samples using FID

[jwlee]

I am trying to improve the resolution of a gas sample that contains about 13 different HFC and CFC refrigerant components. Essentially my problem is this: I have 5 FID chromatographs, all shimadzu, all different models. I have a standard I use to determine ARF and RRF values. On 3 of the chromatographs, I am able to get good separation of all components with no coelution. However, one machine in particular has been giving me trouble, and I am unable to get two of the components to form separate peaks (they elute as one peak). I have tried everything I can think of, changing one variable at a time, and seen no changes. I even changed the column (identical packing and size just new). Besides carrier flow rate, temperature program, sample size, and column length, even increased sensitivity to make sure I wasn't missing the peak, what else can I try? The two components are R-152 (1,2-difluoroethane) and R-125(penthafluoroethane). Any help would be much appreciated.

Instrument: Shimadzu GC-17A
Column: Supelco Stainless Steel column with Porapak B 60/80, 1/8" x 24 ft,
Detector: FID

Thanks

[Johnny Rod]

Check out the AHRI (was ARI) method for R22 using packed column
http://www.ahrinet.org/search+standards.aspx
in standard 700. The example chroms are in AHRI700D. The column, in plain speak, is 1% SP-1000 on Carbopack B 60/80 meash, 1/8" x 24ft. This is a great column for pretty much anything in the world of refrigerants, and a 0.1ml injection gives a good balance of ppm sensitivity with narrow-ish peak width. I had a go with micropacked too but I can't remember how that went.

[jwlee]

Yea- that's the exact column I'm using, but I can't get separation between those two components of my AHRI standard.

[Johnny Rod]

You said you were using Porapak? The separation is close but does work. Are they small peaks or big ones you're trying to split?

[chromatographer1]

I would ask for another column made with an older lot of packing, or a different lot of packing.

There might be a difference in packing lots, either in the carbon, or in the mesh size distribution.

best wishes,

Rod

[Yama001]

My recollections of that column were that it can easily be damaged - at 24 feet, that column must require some fairly high operating pressures?