Chromatography Forum

Hello,

I'm new to chromatography, but got some strange FID results on our new varian 3800 GC with FID, PFPD and TCD.

I was running some JP-5 fuel, which is high flashpoint military jetfuel. I ran it straight with a 20 split through the FID and got a beautiful chromatogram. However, I wasnt sure which hydrocarbon was which, so I added some pure octane into a vial of JP-5.

I put about 1 mL of JP-5 in there, and somewhere between 0.25 and 0.5 mL of octane - I didnt measure it.

On the orignal chomatogram, the peak which I later identified as C8 was very small, only about 2000 - just high enough to see, as the other higher hydrocarbons in there were all right around 3.5 million.

On the sample that I doped with octane, the method was exactly the same. In this one, we got a large peak which was C8 - 6.5 million. However, the other peaks for other hydrocarbons were exactly the same height as in the first chromatogram, and just as beautiful. We overlaid the chromatograms, and other than the C8 peak, the chromatograms laid EXACTLY on top of each other, it was perfect.

So my question is, say I diluted the JP-5 by 20 to 30 percent... Shouldnt the peaks for the other non-c8 hydrocarbons have decreased by some similar factor? If I was running a PFPD and diluted a sample, the peaks would reduce proportionately... but it didnt occur in this FID test.

Do hydrocarbons register in a different way in an FID detector, that given a split ratio and temperature profile, hydrocarbons show up with a specific height peak regardless of concentration???

Or did I just dilute at exactly the right ratio to get a nice peak without effecting the peaks? I dont see how that could be possible.

Thanks for any insight you can give!

JMH

If anything ever comes out exactly the same in chromatography then it is not the chromatography that is doing it !!

Check your data processing - I suspect that somewhere you have told the software to make one of the jet fuel peaks full scale on the chromatogram (it might call this normalizing). It will do this blindly no matter how big the peak is, and all of the other jet fuel peaks will look as if they are the same size as before becuase the components are all still in the same ratio even though you diluted the sample with octane.

Peter

I suspect Peter has identified the issue, but you should be able to tell from the total peak area of each injection whether the detected areas were similar.

Also, examine the area % of your JP5 peaks ( excluding your octane ), to see if that profile is still consistent between the injections. You may be seeing some discrimination because your octane is acting as a volatile sample solvent.

If the diluted sample has a total area that's larger than expected, you may just be seeing the effect of the increased volatility, reduced viscosity and reduced surface tension contributions from the added octane.

Bruce Hamilton

as it turns out, everything stays the same, octane or not. more or less the same area, same times, etc. It appears to be something to do with the relative amounts of each substance (maybe they are in a ear perfect ratio to not effect the original sample)? It is not a softwareissue, ive checked it multiple times over, and compared peak integration numbers.

Thanks!

JMH

Is this effect reproducible ?, in other words if you inject pure jet fuel again, and spike it with octane and do another injection, do the jet fuel peaks stay the same size ?

The jet fuel peaks will always have the same relative areas compared to one another, because adding the octane dilutes all of them by exactly the same amount.

Also (with a few exceptions) changes in concentration would not effect retention times.

So the only puzzle here is that the jet fuel peaks stayed the same height when the sample was spiked with octane. If this only happened once I would put it down to poor injection volume repeatability.

Peter

nope, fully reproducible!

Now we start with the possible but wildly improbable ......

With more octane you get a pressure surge in the inlet, or better evaporation of the sample, that transfers more sample to the column. What happens if you inject fuel and spiked fuel with a split ratio of 50:1 or 100:1 ?.

What is the inlet temperature, and does changing the inlet temperature make any difference ?

Could you post the two chromatograms - I just love looking at things that cannot possibly happen !!!

Peter

Here is a picture of what Im tallking about, both jet fuel and diesel. Ill get more info on the equipment and method later:

Jet with no C8 doped in:


jet with C8 doped in, note the peak height of the actual fuel components is still the same as before:


Same deal for road diesel:




Thanks again!

JMH

HI JHM

Having seen the chromatograms I think that I know what is going on.

The major peaks on the chromatogram are overloaded (in other words there is too much of each compound for the column to elute it as a symmetrical peak). This gives peaks that rise slowly and come down very quickly, giving an asymmetrical "sharks fin" profile. If you blow up the chromatogram on the software you will see this more clearly.

For the present discussion the important thing about overloaded peaks is that their heights are only weakly dependent on analyte quantity, with changes in quantity they get wider or narrower rather than taller or shorter.

Check out the peaks with octane added, they look a bit narrower to me, can you confirm this by blowing them up on your software ?.

Overloading also means that you are losing a huge amount of resolution - hence the hump of uresolved minor components that sits under the peaks. This in turn is also compromising the integration.

Try increasing the split ratio to 100:1 or even 200:1 - you will then see a forest of sharp, symmetrical, resolved peaks with heights that do change when you dilute the sample.

Peter