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GC - using liquid standards/calibration curve for gas samples

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sas1995:
Hello, I am very new to chromatography so I am still learning how to obtain data. Our GC was bought on a budget so no training. I am wondering if a calibration curve using liquid standards can be used to determine gas sample concentrations.

Thanks!

marquis:
Normally, just injecting the materials gives way too strong a peak.  The materials need to be diluted.  The best way to approach this is to look at the samples or materials you will be testing.  They will require standards.  If your calibration department requires nothing else, start with the standards required for your test methods.  Do you know what columns will be needed?  By the way, what columns will you be using and what are your detectors and injectors?  Thanks.

rjb:

--- Quote from: sas1995 on February 07, 2024, 09:02:18 AM ---Hello, I am very new to chromatography so I am still learning how to obtain data. Our GC was bought on a budget so no training. I am wondering if a calibration curve using liquid standards can be used to determine gas sample concentrations.
--- End quote ---

It's wonderful when companies make the decision to cheap out by deciding to forego the training isn't it?! I have been in a similar position, but despite all the stress that is no-doubt coming your way, you're going to learn a lot very quickly - which is good - but mostly out of necessity!

The answer to your question really depends upon what you're trying to achieve, the nature of your sample, what level of accuracy you require and whether your sample is truly a 'gas' phase sample, or whether you're attempting to run headspace samples and want to know the amount of analyte in a liquid or solid sample.

In an ideal world, X amount of injected analyte should yield Y response from whatever detector your GC uses and this should be reproducible. Moreover, as the amount of analyte X is changed, the response Y should also change and should (hopefully) do so linearly allowing you to produce a calibration 'curve'. In an ideal world, whether your analyte is a 'gas', a liquid or dissolved in a solvent should make negligible difference under the proviso that the same amount of analyte is transferred onto the column and reaches the detector.

This is however where our problems begin... Ignoring the (probably very slight) differences between liquid and gas samples when it comes to retention times (no big deal) due to differences in residence times, there's this nasty little thing called the matrix effect which can cause us all sorts of problems, especially when analyte concentrations are low. When you inject your analyte dissolved in a nice clean solvent (as was implied), active sites (silanols and metals) within the liner (and possibly elsewhere in the system) tend to bind up (or degrade) some of the analyte molecules and these never make it to the column. This means that your calibration curves are based upon less analyte reaching the detector than was intended. This is not a problem under the proviso that the same process happens to the same extent with your actual samples. However, in the real world, samples contain other stuff (the matrix) which might interact preferentially with active sites, meaning that more of your analyte molecules (which in your standards were otherwise bound or degraded) make it to the detector. This could be a considerable source of inaccuracy, but depends very much upon concentration, the nature of the sample matrix, inlet conditions and perhaps even inlet residence time (which would differ slightly between liquid and gas samples). For this reason, calibration conditions and calibration samples should be as close as possible to those used for real samples and it is not unheard of to make use of matrix spiking as part of calibration or using a matrix spiked samples as part of a QC process.

Does all this matter? Well it depends upon what you're doing and why... If this is part of a research project (rather than you providing a proper analytical service, especially one that might have legal ramifications), then it might not be a big deal, you can always discuss it as part of your findings as a source of inaccuracy or attempt to quantify it. Otherwise I think you might like to do a bit of research to find out what others have done before you...

Kind Regards

R

p.s. If you're using headspace and not using 'gas' samples then that's going to need a whole new response!
 

Babcock_Hall:
It might be worth drawing a distinction between the method of absolute calibration and the method of internal standardization (also called the method of indirect calibration).  The treatment of these subjects in an older book Basic Gas Chromatography by McNair and Bonelli was very clear.  In the latter method the peak area of the analyze over the peak area of the internal standard is plotted on the y-axis.

sas1995:
Marquis, rjb, and Babcock_Hall thank you so so much for your responses. :)

Marquis:
Yes, I dilute the compounds in a solvent to make the standard solutions. I get nice peaks doing so. Our GC is an SRI 8610C, with a mid polar capillary column, on-column injection port, and FID/FPD detectors. We are measuring battery electrolyte solvents (ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, etc.) and their decomposition products to quantify emissions. So, the carbonate solvents are liquid or solids under ambient conditions, which is why I having been preparing liquid samples, but some are also volatile and will produce gases during our our processing with them, which we would like to quantify. The decomposition products are mostly all toxic gases - which is its own issue since we do not have an MS detector and you can't purchase these products to make standards.

Rjb:
It certainly is not ideal! We are a small start-up so money is scarce. I am definitely learning a lot - I did some GC in my undergraduate studies, so things come back to me also here and there. Thank you for the encouragement. Eventually, we will be sampling emissions out of our pilot process, which is what I am trying to mimic right now. Our injection method is manual (another cheap out), so what I am doing right now is using temperature conditions similar to our pilot process (which may decompose and/or volatilize the analytes), and sampling the headspace. Is this even a proper process? Really, I am just trying everything to learn more and obtain responses to present species. I have had no such luck with this method, but I had broken our gas tight syringe so I was using some cheaper ones. I ordered some more gas tight syringes, so I think we will get better results this way but I will just have to wait and see. The measurements don't need to be super accurate, since these numbers don't need to be reported (it is for our own understanding). I will also be taking some liquid samples, which I am not concerned about determining results.
Would you mind explaining how you know when you are experiencing a matrix effect?

Babcock_Hall:
Thank you for the book suggestion, I will definitely get this and try comparing these methods!

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