In Sigma-Aldrich's FAQ regarding SPME fibers:
https://www.sigmaaldrich.com/analytical-chromatography/sample-preparation/spme/faq.html
You can read that the technique is quantitative, as long as proper internal standards are used, and under the question: "How do I quantify results from manual SPME?", it can be read:
Use internal standards that are similar to your analytes. Internal standards help to offset extraction variables. Standard preparation is important. Prepare standards at 1-2mg/mL solvent, then spike 0.05-5uL of the standard into the water sample prior to the extraction.
Right, I can do that. So then I'll have 0.05-10 ug of internal standard… in my sample.
But my wish would be having this known amount, be it 0.2 ug or 1 ug or 3 ug… adsorbed onto my SPME fiber.
Instead, what I see by GC-FID is a peak whose area is strongly dependent on the extraction time. A part of the IS remains dissolved, another part may go to the walls instead of going onto the fiber, and this is trouble when wanting to measure extraction kinetics.
I simply do not know how much internal standard I got in my fiber at the time of the injection. And I fail to see how to make it possible.
Hello Roberto,
I think that quantitative analysis via HS-SPME is a big concern. I think that one cannot expect that the specific amount of compound will be absorbed on fiber.
My idea is, that after preparation of the sample, with known amount of IS, we need to leave the sample in the extraction conditions for establishing the liquid-gas equilibrium, and thereafter carring out the extraction process. Then, the extraction should be repeatable, so one may estimate that the share of particular constituents in gas phase is proportional to the share in liquid phase. That's why IS should be similar to analytes, regarding the Raoult's law. Nevertheless I prefer to leave a resault as a % of share in a volatile profile of sample, and for quantification carry out the liquid injection.
Kind regards, Jacek
Add same amount of IS to each calibration level samples and same amount to the analyzed samples. If you obtain good linearity for calibration everything is ok - it is not important what is the partition coefficient - you get reproducible results - that is all you need.
Regards
G
Hi Roberto.
If you are using a FID for detection, then I would not use an internal standard at all. Signals of detection can vary (even for very similar compounds) and especially the recoveries of extraction can be very different for the same conditions.
One critical aspect is for you to ensure that your extraction conditions are such that there is an equilibrium (or very close to equilibrium) in your extraction system. In these conditions you will either not see any signal variations with increasing extraction periods or see small ones. Check that you reach such conditions and simply use standard addiction for quantity control of your samples. SPME is not an exhaustive extraction technique, thus it it critical that you are working in conditions as close as possible to equilibrium if you want validation of the method by sets analyte/matrix. Some advices for you to achieve fast extraction equilibrium conditions: tune temperature, agitation, salinity conditions, pH (depending on your analytes and matrix), consider DI or a combination of DI with HS and, naturally, the time.
Cheers!
Mario,
" If you are using a FID for detection, then I would not use an internal standard at all. Signals of detection can vary (even for very similar compounds) and especially the recoveries of extraction can be very different for the same conditions. " - I don't see any problems - this is what is determined (the lets say relative responce factor or calibration curve for internal standard) during calibration. Application of IS is the best choice - you need to select a proper internal standard - that's all.
regards,
GB
It's just that the use of non-isotopic labeled IS when SPME is used is a waste of time, even though it can be kind of standard. This is simply because there is no accurate way to: i) determine recovery; ii) transfer any type of recovery factor to your analytes. If the same compound is not used, then there will be differences about solubility, volatility, and adsorption to fiber. If accuracy is the concern, no point is using IS with a compound different from the analyte.
Cheers!
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