Peak Area confusion

[vxfx]

Hi,
I'm a bit confused. What is it that the peak area represent? COrrect me if I'm wrong, but it is the integration of concentration (mass/volume) over time. I want to find how much methane is present relative to my other gases (ethane, propane). So, can I just use the peak area value immediately? or I still need to find the volume, mass or concentration?

Before, I used the peak height to find volume; based on linear equation of the standard, I use the peak height I got for my samples to find the volume. However, my prof says it is not that accurate and peak area is better.

I'm thinking, I can use this method again to find volume.
i.e plot peak area vs. volume for the standards.
then if R^2 is close to 1, I will use the linear equation to find volume for my samples because I can substitute the peak area I have into the equation. I can assume a similar relationship right?

Thanks

[larkl]

What type of detector are you using? You could use an area% report to get approximate concentrations with FID. I wouldn't recommend it, you're better off using a calibration standard and calibrating. If this isn't an option, you can try using effective carbon number response factors (for FID). See paper by Scanlon and Willis.

[chromatographer1]

There is no correlation between peak area and mass, or volume for that matter with GC. For gas samples a pHID comes close to a 1:1 relationship for volume for some samples. But for other detectors especially a TCD or FID the difference in molecular response can easily vary by an order of magnitude. This is why calibration samples are REQUIRED for process analyzers. Search the archives here and you will find postings about this topic.

Good luck,

Rodney George
consultant

[carlos.teixeira]

Dear vxfx,

As already written before, normally there are no direct relation between area and amount when we are using GC. So, you should use STD injections for quantification.

Have a nice elution!!

Carlos Teixeira

[SkylineGTR112]

If you are using an FID and the mixture you are analyzing is all hydrocarbons, the area percent can roughly correlate to weight percent. But if your using a different detector (TCD, PDID, HID), it won't really transfer over as anything due to different chemical properties or what is being tested.

[chromatographer1]

SkylineGTR112's post is not correct for the shorter hydrocarbons. With HC of more than 6 carbons the response is approximately the same per mole weight as SkylineGTR112 indicated.

But methane ethane propane have quite a lower response with a FID or TCD !

best wishes,

Rodney George
consultant

[vxfx]

Thank You everybody,
I was a little confused about the calculations I have to do for this.
I used GC-IRMS to test my samples, and I am not too sure about detectors and all.

I do injected in house standards first, Scotty reference. I did my plots for that with Amp 44 vs. Peak Area to check on linearity. SO far, they all correlate very well.
I also use NGS1 and 2 (from NIST) and so, I need to compare the value of these gas samples to the known, published values from NIST. They reported the volume, but now I need to find the volume first.

Do you know any book or article on this? I tried to find, but to no avail
I hope some of you can help me with some of the background reading on this.

[Suresh Seethapathy]

Vxfx,

Are you using GC-IRMS? You are interested in analyte quantitation in your sample or determining isotope ratios of target analyte? If you are using GC-IRMS, does your system also have a FID or TCD?

GC-IRMS uses a detector called Faraday cup. It simply counts the ions (after separation by the magnetic sector) hitting it. More than one faraday cup is present in the GC-IRMS so as to be able to count different ions (for eg. carbondioxide ions with C13 and C12 isotopes).

Using GC-IRMS for quantitation is certainly the most expensive way! Confirm the above questions and i can send you some excellent publications on GC-IRMS.

Cheers,
Suresh.

[Ron]

If the instrument is an isotope ratio mass spectrometer all the hydrocarbons are converted into CO2, so there is no compound effect on response. Since you are basically counting carbon atoms, so you can determine concentrations using peak areas divided by the number of carbon atoms in each molecule to determine relative concentrations.

If you are using an instrument that is a GC-IR-MS, then the situation is different. This is an instrument that uses an infrared detector in series with a mass spectrometer. In this case response is compound dependent.

[vxfx]

Hello Suresh, thanks for your reply.

"Are you using GC-IRMS? You are interested in analyte quantitation in your sample or determining isotope ratios of target analyte? If you are using GC-IRMS, does your system also have a FID or TCD? "

- YES, I am using GC-IRMS. I am interested to find the volume % of methane, ethane and propane. I have their isotope ratio values already. However, I don't think my system has FID or TCD.
- I do hope you can send me some publications. I am trying to find the volume % based on NGS 1 and 2 standard that I injected. I tried to calculate them already, but I am not sure if it is the right way to do it.



Hello Ron,
My samples are converted to CO2. Can I just use the calculation as you mentioned? For example, the peak area for methane in a 70uL sample is 6.833 and methane has 1 carbon atom. So, I just divide 6.833 with 1? But I have NGS1 and 2 standards. I think I have to use the published volume% in order to find the volume% of the unknown samples.

To answer your next question, it is not GC-IR-MS.

Thanks everybody. You guys helped me a lot to understand this.