Chromatography Forum

I am using GC-FID to analyze the concentration of various hydrocarbon compounds in the oil contaminated site. I plotted standard curves for decane, octane, dodecane, hexadecane and eicosane for aliphatics and BTEX, napthalene, acenapthene, and butyl benzene for aromatics. One thing that I have observed is that aromatics has twice or more than twice the area of the aliphatic components. Thus, aromatics have response factor twice that of the aliphatics. I wonder what is the factor that contributes to this difference.

I would suggest checking your calibrations. Hydrocarbons with just C and H from C6 upwards have similar FID response factors within 10-15% or so, aromatic or not.

Evaporator,

How are your standards prepped, mole/mL, volume/volume, weight/volume?
It is usually the case that equal moles of carbon on different molecules respond basically equally and the range of compounds you are using sort of rules out peak width issues if you are indeed using area rather than height...

Best regards.

I am working on mass/volume.
I dissolve aliphatic compounds in hexane and aromatic samples in dichloromethane and prepared in six six different concentrations, 1000, 500, 200, 100, 50 and 10 mg/L of each compound. I have a correlation of 0.99 or better for all samples. It is just that the response factor for all alipathics is around 900-1100 while that of aromatics is about 2000-2500. I repeated it thrice but I got the same results.

By the way, I am using area.

Dear Evaporator,

It is possible that you have a unique FID.

There is no need to put your aromatics in DCM separately. Put your hydrocarbons in DCM as well as the aromatics.

Then see what happens.

Readjust or check your fuel air mixture in your FID and make certain the flows are correct.

Your results are not typical.

best wishes,

Rod

Evaporator,

Because you are dealing with mass/volume, your results are especially intriguing. I would be very interested to hear if these change when you switch to all one solvent, DCM being the better as Rod suggests. What is your starting temperature on your oven, injector temp, and column dimensions?

The only other thing I can figure from your post might be something about the response factor??? But if the areas are way different, one would expect the same from the RF???

As others have noted, for a properly-operating detector, the FID responce for liquid HCs above carbon number 6 is purely a function of the relative C and H content.

From memory, Benzene usually has the highest response, but still only about 120% of the n-alkane, and Toluene about 107%, and the aromatic response difference decreases with increasing carbon number.

Your problem is most likely related to the different standard solvents selected, and it's important to use the same solvent for your samples and standards.

The use of different solvents for different analytes would require internal standards to provide correction factors, which is not very efficient.

You should be following one of the many standard methods that exist for analysng TPH from contaminated sites, especially if the data is to be reported to a client or regulator.

From memory, the US EPA has several, as do some US states, some countries, and there is the European Eugris www site as well. Many newer ones will use MS detection, but there are still FID methods around, which a WWW search should identify.

Those methods specify the standard and sample solvents, which may be DCM, or a CS2/DCM mixture - if solubility is an issue.

Please keep having fun,

Bruce Hamilton

I am using Inertcap 17 MS column, 0.25mm IDx 30m and 0.25 um thickness. The detector and injector temperatures are 350C. The oven program is 4C for 5 min and 10C/min to 220C. The areas are entirely different. Aromatics have almost twice the area of the aliphatics. I will try to check the difference between the solvents that I use- hexane and dichloromethane.

Many thanks.

At 4C initial oven temperature you should be getting very good refocus on column phase. I am no expert (Rod?, Bruce?) but this should negate any solvent effects from the flash.

Still would be interesting to see aromatics and aliphatics at same concentration in same solvent.

Best regards.

I tried performing the same experiment using hexane and dichloromethane and I got almost similar results. I found out that the problem could be in the auto sampler. It could be that the injection volume when I ran my aromatic standards was twice than that when I ran my aliphatic samples.
Thank you for help and suggestions