Chromatography Forum

Hi,

Im using a varian 3800 with PFPD and FID. I am getting great analysis of jet and diesel fuels with my FID, with nice, even-spaced peaks in jet and diesel. However, no matter how cold I start the column (I have tried down to 30C), and how slow I ramp up the temperature (Ive gone down to 1C/min) I cant get a similar 'hump' with the nice even spaced peaks like I can with diesel and jet fuel.

Perhaps it has to do with the types of compounds in the fuel, but from what Ive seen, as soon as I start heating up, I get a big peak and then justthe little ones in a mess on out, no real shape to the overall chromatogram, like you get with diesel or jet fuel. Ive tried injector temperatures from 160-320C, column starting temperatues from 30-80C, and I run the detector at 300C.

I dont recall the length ofthe column off hand, but it is between 15-25m.

so, any way to get nice, even spaced peaks and a good overall shape for a gasoline chromatogram, like you can get with diesel and jet fuels???

This is more academic than business-oriented, so I dont need any particular official mthod - just something that will work well. Im sure there has to be a way to speciate C3-C6 without being in one big hump, or a couple peaks so close that there is no resolution. Perhaps I dont have the right equipment...

Ive seen some references to lighter HC fuels being run with an injector and detector temperature of ~200C, but a constant oven temperature of 80C. What is the benefit of doing it this way, as compared to running a temperature ramp?

Any help would be appreciated.

Thanks!

JMH

What carrier gas are you using? If you want decent peak separation, you need to use optimum flow for the carrier gas that you are using. What do yu mean by 'hump'? Can you post a chromatogram for ideas?

Regards

I think you will find your problem is related to column not GC conditions. I guess you have a thin film short OV-1 type column (15m with 0.1um film ?) and with the high amounts of low MW Hydrocarbons you do not have enough phase in the column to retain the peaks enough. Either a longer column and/or a thicker film should help. Post the chromatogram and or column details if you need more info.
Regards Richard

What is the stationary phase in the column ?

Gasoline = petrol and it is mainly centred around C8, so you should see very little propane (C3) anyway.

Peter

My assumption from your description is that you may be using a 15m 0.25mm ID 0.25mm film column.

Since Petrol is composed of C5 to C8 hydrocarbons primarily, you will not get much separation using a short thin film column.

You should be using a 50-100m 0.25-0.32mm 0.5-1.0µm film Equity-1 (Supelco - my employer, see bulletin 868A on our web site) or comparable capillary.

If you wish to verify propane content (added in winter months for improved starting characteristics) you need a long capillary or a moderate length packed column. (3-5m 10% SP-2100 on Supelcoport or for greater retention and separation the same on Chromosorb P-AW fused silica lined steel or glass column)

The Bulletin 868A should give you an idea of the capillary column you should have and the different components found in Petrol.

best wishes,

Rodney George
Senior Research and Development Scientist
Gas Separations Research
Supelco
595 North Harrison Road
Bellefonte, PA 16823

814-359-5737 voice
814-359-5459 fax
rgeorge@sial.com

Thanks for all of the replies. Here is about the best I can do right now with the FID for gas, compared to diesel:





Thanks again!

JMH

Hello

The petrol samples I've looked at (in the UK) have butane as the lightest component in the winter months and pentane in the summer.

I use a PLOT Alumina with Na2SO4 deactivation and helium carrier.

Usually no problems with C2 to C9 separations

Regards

Pete

Thanks for sharing your analysis.

Your chromatogram of gasoline is quite typical.

As you can see it does NOT resemble the analysis of diesel fuel, but then it is not supposed to be.

No worries, mate !

best wishes,

Rod