Chromatography Forum

Anbody have any ideas on how to prevent o-terphenyl from shifting into C19 on an FID alkane run?

Generally when this happens I install a new RTX-5 column, but I know there has got to be some steric hindrance going on and I would like to get more time out of my columns without significant changes to my aquisition method. I'm already getting 6 mos ~ 2000 injections, but I'm wondering if I can't do better.

Alkanes 8-40, pristane, phytane. OTP surrogate and 5 alpha androstane as a quantifying IS. ~69 min run.

Hmm - that is a pretty long run time; perhaps it is needed to get the C17/Pristane and C18/Phytane separation? (BTW, it seems that separation is easier to achieve with a 0.32 mm ID column rather than a 0.25 - annoying. An MS rather than an FID seems to make it harder as well - likely the vacuum at the outlet screws things up a bit).

I currently use the following: RTX-5 column, 30m X 0.32mm X 0.25um film. Hydrogen carrier at 6.2 cc/min constant flow (sometimes when the C19/OTP gets bad dropping this to ~ 4 cc/min helps). FID make up is 10 or 20 cc/min of Helium.

2uL split injection 3:1 (the crappy 6890/7890 inlets seem to have trouble with splitless and early eluters, otherwise I would have stuck with splitless).

Temperature program: 0.5 min at 60, 30 deg/min to 315, hold 3.5 min. These conditions give me no trouble with C19/OTP, but back when I needed the pristane/phytane stuff routinely, I believe I needed to slow that down to maybe a 25 min run.

I can get a year or more out of a column unless I get a lot of clean sediments that deposit a lot of solid material - that seems to be the worst matrix, since I cannot dilute out the garbage. The C19/OTP is never a complete baseline separation; the valley height is around 2 to 5 percent max between them. I tend to change the column due to baseline rise issues rather than separation concerns.

Hope that helps out.

Thanks for the reply Yama. I do have to separate pristane/phytane from the alkanes, and unfortunately, I can't break legacy with my aquisition method. I've got a significantly shorter run, but I can't use it for the current flywheel project.

Generally I can separate the C19/OTP as you said, but when OTP begins shifting, I switch out my columns, and I was wondering if anybody had any magic tricks to lengthen column life.

I appreciate the input.