triacylglycerol analysis - poor resolution

[jcraven]

Hello,

In the past I have achieved excellent resolution of milk fat triacylglycerol peaks using a high temperature polarizable capillary column (Restek MXT-65TG, 30m long). However, the resolution has diminished after a number of major (installed a new FID jet and installed a new column - Restek Rtx-65TG, 30m) and minor (install, uninstall, reinstall, repeat 'til completely frustrated then try again) adustments. Ironically, it appears, that the more I tried to fix it, the worse my problem became. This led me to search the internet for clues to my problem - and that's how I found this forum.

In this forum, mtarrien (July 9, 2004) reported a similar problem, subsequently, on July 15, they reported that the issue had been resolved by using all new FID jets. Their conclusion was that graphite (from the ferrules used to hold the column in place) was accumulating on the internal surface of the FID jet and inhibiting elution or resolution of triacylglycerols in the sample.

In addition to replacing the FID jet I am considering making some other changes to my FID. Instead of installing a standard capillary jet I may install a high-temperature jet (suitable for simulated distillation). Since, high-temperature jets have larger orifices than standard capillary jets and may be less prone to graphite buildup. Additionally, I will most likely switch from graphite ferrules to aluminum ferrules (alumiseal ferrules from Restek).

I wonder if any of the readers of this board have similar or related experiences and can provide some guidance on these matters. Your input would be greatly appreciated.

Method details:
instrument: HP 6890 with split/splitless injector and capillary adaptable FID
column: Mxt-65TG and Rtx-65TG (Restek)
temperature program: injector and detector at 370C, oven temperature - 1 minute at 80C, 30C/min to 240C, 4C/min to 365C, held at 365C for 5 minutes.

Thank you,

John

[Bruce Hamilton]

If it worked OK before, I'd try to fix what has broken, rather that change.

Can you give us examples of what has changed - are peaks retention times different, are the peaks broader or asymmetric, are they much larger in area etc?. How do they change during a run.

I would suggest testing all of your detector gas flows, to ensure that they remain constant and are optimised for the column flow. I'd also be testing the column to ensure that it still has good performance, peak shape, resolution, etc. If the peaks are broader, are of similar area, and retention time, and the gas flows OK, the column or injector may have seen oxygen, so check your carrier gas purifiers.

I'd be checking the the peaks areas, shapes and retentions are similar to what you had before, if they aren't, I'd be looking at your injector and sample solutions, as well as the detector gases.

Personally, I'm always a little dubious about issues like graphite from ferrules into FIDs, or ends of columns not being cut square. Provided people cut a cm or two of the column after inserting a new graphite ferrule ( holding the column end at 6 o'clock, not 12 o'clock - as I frequently see ), I've never found it an explanation for poor chromatography performance, only sudden noise spikes..

I suspect degradation of the outer polyimide (or whatever ) coating
on the column causes more noise issues at high temperatures.

Please keep having fun,

Bruce Hamilton

[JI2002]

It seems like you are using same method as in Restek products book. But there is one thing different: you are using split/splitless injection instead of cold on-column injection.For high boiler compounds, it tends to have discrimination problem with split/spliless injection, maybe that's why cold on-column injection was used by Restek. For an Agilent GC injector, even it's set at 370C, the temperatures at the top and the bottom of the inlet are well below 370C during the injection, depending on the starting oven temperature, in this case, oven starts at 80C, the bottom of the inlet could be only 250C. This can cause tailing and disappearing of high boilers. Do remember to have isolation cup around the injector. Just a suggestion, this won't explain why you had good resolution before.