I have to measure by GC (MS of FID) different types of refrigerants (CFC) in gas samples without pure analyte or external standard in different mixtures.
For example, R404a contains 3 different compounds and some impurities.
44% +-2% R125
52% +-1% R143a
4% +-2% R134a
Maximum 0.5% impurity (air, other CFCs etc.)
I don't have Certified reference materials for these compounds and I don't have a known amount standard mixture or pure compound to do standard addition.
How can I quantify the exact amount of each compound?
Thank you.
You can't reliably quantify an unknown if you do not have standard reference material and can establish a calibration curve. At best, you can make some relative estimates based on your peak areas.
Hi David, if you know this impurity are detectable in your detector you could use area percentage for determine percentage concentration in your sample. You describe some possible compound present in your sample, I think best choice is MS detector, but you need evaluate both option.
In Chromatography, if you have to quantify a compound, you need reference standard. You have to create a calibration curve using methods like internal standard or external standard. But in case you have no standard, you can use the area % method wherein you will get a relative percentage of each compound in your sample. Even though it is relatively a less accurate method, it is still being used widely, especially in the absence of a standard.
As R.Girijan Menon mentioned the only reasonable solution here is to use % area method but taking care of relative response factors RRF's (some call it normalization).
In simple words FID (and MS) will respond differently to the same amount of different compounds. E.g. 1 ng of R134a will produce the peak of area equal to, let's say 1 000 000 and 1 ng of R143a will produce the peak of area equal to 1 100 000. So we can see the FID (or MS) is more sensitive to R143a. This must be taken into account when calculating the result.
The RRF values can be found in the literature.
I bet this approach would be much more reliable than external calibration if you are sure that the content of impurities is low (below 0.5% as you wrote). Why, because preparation and analysis of each calibration standard (in gaseous phase in particular) would introduce significant uncertainty to your final result.
Thank you for the answers,. Ifound a company who can make CRM for refrigerants, but not in the same concentration. I will get 0.1 % concentration for each analyte, so I need to measure the RRF value and after that I can measure the concentration in the gas mixture.