FAME analysis-------------abnormal peak broadening

[Zydrunas Stanius]

Dear Colleagues,

I ahve a peoblem with FAME analysisi with GC-FID.

Peaks become abnormaly broad somehow suddenly, and I cannot get it solved back to normal state.

Small description:

1) FAME peaks become very broad with Shark fin shape front shoulder. And it goeas as a system, where higher the carbon number larger the broadening. Solvent peak (hexane) is very normal.
2) Changed several columns and teh effect remains.
3) Cleaned all what is possible - FID, SPL. Changed liner, FID nozzle.
4) Run another kind of sample - some alcohol mix and alkane mix. all is normal.
5) Done splitless analysis to eliminate assumed split valve defect. Nothing much changed.
6) As there are to FID's on instrument checked with second with seperated electronic gas controler, same problem remains.
7) Checked the same sample and same column on another but same model GC, all is working.
It seams that somewhere and somehow some kind of specific adsorption causes a problem. But how it was working for few years without problems?


Maybe someone has any kind of similar problems with FAME's? Or know any trick how to check wether it's detector or injector problem.

thankyou in advance

Zydrunas

[BZK]

If you have tested the same sample/column with other gc's and all works fine I think that a possibility could be "cold spot" into injector or detector side (probabily detector). If you note that the broadening increase with carbon number an incorrect (low) temperature of FID heater block can be the cause.
Try to measure the temperature of detector as first.

Regards.

Roberto Barcarolo, Italy

[Peter Apps]

Number 5 is the deal breaker. If a splitless injection looks the same as a split injection you probably have a problem with the split - most likely a blockage in the split line or a blocked split vent filter. You will still get flow out of the split vent because the flow is bypassing the inlet.

Are you seeing in increase in peak area compared to when the peaks were narrow ?

Peter

[Zydrunas Stanius]

Dear Peter,


thanks for your comment.

higher carbon number FAME peak areas are hard to measure, nearby peaks gave one unresolved waywing shape. But it seems that areas of peaks must be much larger especialy for higher carbon compounds.

We are using Shimadzu GC2010 instrument. All gases are controlled automatically. Split was one of the first things checked, by measuring flow out of a split vent. And it was quite near to set value. But I am interested in your sentence "You will still get flow out of the split vent because the flow is bypassing the inlet". I am not sure how it could be. Split flow setting valve in Shimadzu GC is after injector/inlet, after split filter, just right to split outlet. But in other instruments setup is maybe different.



Zydrunas

Number 5 is the deal breaker. If a splitless injection looks the same as a split injection you probably have a problem with the split - most likely a blockage in the split line or a blocked split vent filter. You will still get flow out of the split vent because the flow is bypassing the inlet.

Are you seeing in increase in peak area compared to when the peaks were narrow ?

Peter

[Peter Apps]

Hi Zydrunas

Unless the GC2010 differs from all of the other current models from the major manufacturers it will have a mass flow controller upstream of the inlet, with a back pressure regulator downstream of the inlet. It is the back pressure regulator that controls the flow through the column by controlling the pressure in the inlet, while the mass flow controller sets the split ratio by controlling the flow of gas into the inlet - obviously the split flow is the total flow minus the column flow and septum purge. To turn the split on and off there is a solenoid valve that diverts flow past the inlet directly from the mass flow controller to the back pressure regulator. If there is a blockage in the split vent line close to the inlet body the flow will also be diverted, so you get a splitless injection. The solenoid valve might also be stuck on splitless. In either case you still get flow out of the split vent. Sadly there is no way to trouble-shoot this by measuring gas flows.

I would be surprised if there is not a diagram of the inlet plumbing somewhere in the users manual.

Peter