Methyl C1, Ethyl C1, Diphenylamine, 2-4 DNL, DI or SPME?

Discussions about GC and other "gas phase" separation techniques.

6 posts Page 1 of 1
Hello,

Forewarning, I am a 1st year graduate student using GC/MS for the first time. My thesis revolves around the determination of these organic volatile compounds at a known amount on cotton fabric.

I have run into a problem where I cannot seem to find any published articles (or dissertations, or even a thesis) on a method of extraction using direct injection. It seems when it is done, it is more so using headspace and a SPME fiber. So far I have run known standards of these compounds using direct injection using methanol as a solvent and the peaks come out fine.

From what I can gather, direct injection gives a greater limit of detection than headspace SPME preparation. This is all in the context of forensic trace analysis for organic gunshot residue. Why is SPME so common? What are the advantages it possesses over direct injection?

If it helps, I am on an Agilent 6890N system. Any input into this subject, as well as any links to relevant articles on direct injection vs SPME, or research involving known amounts of these compounds (or organic volatile compounds in general) on fabric would be greatly appreciated.
What are "methyl C1" and "ethyl C1" "DNL", "DI". This is unfamiliar nomenclature to me. The only things that I recognize are diphenylamine and SPME.

I did a simple google search and this popped up:

https://pdfs.semanticscholar.org/45eb/d ... 032e87.pdf

SPME is easy, solventless, you actually concentrate your analytes in the coating on the fiber so detection limits are generally much lower. You don't need to change the inlet on your GC (other than changing to a small-diameter glass inlet sleeve made for SPME). Many, many advantages. I use it almost exclusively.

Good luck.
Thank you, I can't believe I missed that!

Ethyl Centralite, Methyl Centralite, (DNL) Dinitroluene, (DI) direct injection.
Thanks. I've never heard of those molecules before. Interesting.

I found this one too:

https://pdfs.semanticscholar.org/76c0/f ... b0d55b.pdf

They're pretty heavy for SPME (at least volatiles sampling). You might get away with SPME if you can heat the cotton fabric hot enough to desorb them from the cloth in a reliable fashion. Since your matrix is cotton (great water affinity), you might soak the cotton cloth in water to create a water-cotton-vapor equilibrium for your sampling. You can try to immerse the SPME fiber in the water to perform your extraction. I've never had great success with immersion SPME like this but it seems that others have made it work.

Direct thermal desorption of the volatiles from the cloth might be the best way to go. You'll need a thermal desorption unit to make it work (Markes or Gerstel).

The premise involves putting a known mass of cotton fiber in a sample tube and placing the tube in the sampler. My system is by Markes. I'd put a piece of the affected cotton in what Markes calls a sample tube and cap the ends. Once in the sampler, the sample in the tube is placed in an oven and heated (you choose the temperature) while carrier gas sweeps the molecules desorbed from the cotton fabric onto a very narrow cold trap inside the sampler. After a prescribed amount of time, the analytes on the cold trap are desorbed into the GCMS system for analysis. You'd create a standard by adding a known mass of your analytes to a known mass of cotton fiber (sort of like a standard-addition calibration).

Another way might be to slurry the cotton with water and stir it with a pdms-coated stir bar (Gerstel "Twister"). As the analytes are liberated from the cotton by the water, the "Twister" extracts them from the water. You need a thermal desorption system for that sampling device as well.

Markes and Gerstel have a host of app notes on their respective websites. You might poke around a bit there to see if you can find something they've done that might suit your needs. Lots of times, they're not exactly what you need but you see something that gives you and "ah-ha!, I can modify that technique and make it work for me."
Interesting one.

So you are using GCMS

In addition to the useful suggestions by Rb6banjo (I am also a fan of SPME)

1. Prof. Janusz Pawliszyn, the inventor of SPME, has some very useful books on the theory and practise of SPME - recommended reading to give you a better background and understanding

https://uwaterloo.ca/pawliszyn-group/pu ... shed-books

2. My instinct would be just to keep it simple, using the equipment that you have, with direct injection after extraction with say, DCM/methanol in your initial studies. You have said that methanol works as a solvent. Just try it and see :-)

3. From my experience of analysing volatiles from cotton fabric it is highly adsorptive unfortunately from an analysis point of view:-(. (It is basically cellulose) .
On the other hand, that may well be an advantage for my suspected aims of your study.
Cotton/polyester adds a new dimension.

4. Also. Cotton fabric has many residual volatile impurities from production/ washing/post washing/exposure etc. So, although it may sound obvious, in the case of cotton fabric it really is essential to run a blank control extraction of a piece of identically sourced fabric with the same history when doing your studies. Luckily, you have the advantage of using SIM

Interested to hear how you get on.
Regards

Ralph
Ralph is correct. I got caught up in "what I'd do with the equipment I have". Certainly, if you don't have access to a TD system, the best and easiest option is to simply extract the cloth with a solvent and shoot it.

In addition to the selected ion monitoring option for this project, the molecular weight of your analytes is in your favor as well. You can probably concentrate your extracts down to 100-200 µl (evaporation of the solvent) to get some extra sensitivity.
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