Chromatography Forum

I currently use an HP 5890 with an FID to analyze free/total glycerol for biodiesel (FAME) using direct COC injection a Varian Ultimetal Select 0.32 mm x 15 m and a .53 mm x 3 m metal guard. I recently had a sample that was contaminated with high amts of triglycerides (which I understand do not silyate due to lack of free -OH grp) and, even though I have seen no carryover, think I need to trim the guard. I have run a total of 90 runs since the last time I trimmed it and 20 runs since I last saw the high triglyceride peaks.

If you need more tech information let me know.

What temperature maximum and what column are you running, and what inlet temperature? I need to use a specialty high temperature metal capillary with thin phase and high GC temperatures and high inlet temperatures to get triglycerides to elute.

Likely triglycerides never made it past your liner; I wouldn't trim the column unless you really feel there's a need.

Never trouble trouble until trouble troubles you

Hi Consumer Products Guy,

1) Inlet temperature starts at 50 deg. C tracking the detector temperature, which attains a maximum of 380 deg. C and is held there for 10 minutes.

2) As mentioned, column is an varian ultimetal (0.32mm) and it is intended for high temperatures (up to 400 C).

2) You mean the guard/retention gap? There is no liner since it's direct injection.

What makes you think you should trim the guard, do you have poor peak shape? Inconsistent peak area? I have the same model GC and column. I generally only trim my column when I see tailng peaks. If you start to see bad tailing, I have had very good luck with trimming a few inches of the guard column and then reversing the column and baking at 380C for an hour or so.

Have you tried the supelco or restek equivalent columns? So far I think the Varian is the superior, but just curious about other users opinions.

Then it sounds like: if you did get triglycerides on the column, just bake it at 380/390 C for an hour or two, with carrier gas flowing.

well i wasn't seeing any poor peak shape or anything like that... so it is likely the tgs were baked out during the actual run. since it holds 350 deg. C for at least 10 minutes at the end of the run i think all of them were baked out. i didn't see any carryover when i injected the next run (the next run coincidentally had no tg peaks).

but i ended up having to trim the retention gap anyway b/c i had to change the septa, and the end cut looked kind of flared to me. not that there were any leaks there but it looked flared so i decided to trim it.

since i'm a really lousy metal tubing cutter, i kept missing the score and doing a corkscrew around it instead of a straight score. so i ended up chopping off a foot before getting it right.

re-installed it now it works great but i'm out a foot of guard. good thing those are cheap.

well i wasn't seeing any poor peak shape or anything like that... so it is likely the tgs were baked out during the actual run. since it holds 350 deg. C for at least 10 minutes at the end of the run i think all of them were baked out. i didn't see any carryover when i injected the next run (the next run coincidentally had no tg peaks).

but i ended up having to trim the retention gap anyway b/c i had to change the septa, and the end cut looked kind of flared to me. not that there were any leaks there but it looked flared so i decided to trim it.

since i'm a really lousy metal tubing cutter, i kept missing the score and doing a corkscrew around it instead of a straight score. so i ended up chopping off a foot before getting it right.

re-installed it now it works great but i'm out a foot of guard. good thing those are cheap.