Thermal Desorption GC/MS Internal Standard Addition

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

10 posts Page 1 of 1
I have problems with some samples. I do not have problems with calibraton curves or with Proficiency Test.
I have been using this tecnique for many years, but now I have recently introduced the internal standard for VOC analysis in air with ISO 16000-6. The method require the use of tolueneD8 internal standard.

I can use both VOC standards and TolueneD8 standard only as liquid solution, because I do not have the possibility to introduce them gaseous directly in the thermal desorber. Liquid addition of these standards is allowed by ISO 16000-6 but it is not specified if I have to add Internal standard solution to the tubes for samples, before air sampling of after air sampling.

Actually, I add internal standard solution after air sampling and then I put the tube in the desorber and I performe GCMS analysis. In the last months I saw that in some samples the peak of IS falls down. The Area decreases about 5-10-20 times respect to the Area of IS in the standard samples. Surely there is some trouble with sampling, but it is not my fault because sampling is performed by external people different from my company. We just perfom analysis.

I am asking you, If you have the same problem and when do you add internal standard solution to the tube. Do you add it before or after sampling?

NB: I add liquid standards to the tubes by using a Supelco flowmetere with inert gas. I always add 1uL of methanolic solution of multistandard as request by the standard ISO 16000.

Thank you very much.
Please write my also If you need suggestions for TD-GCMS.
I don't see that you are doing anything wrong. I suppose the best thing would be to have your IS in your air sample and collect it along with your analytes as you sample. That way, the treatment of all measured species is the same. However, that is very difficult so I would think the logical thing would be to add the standard after you have collected your sample on the tube.

For the phenomenon you describe, you say "some samples". Could it be that there is something wrong with those sorbent tubes? What if you save out those tubes that exhibit the phenomenon, add your standards to them, and calibrate again?
Thank you for your time and ideas.
Some samples means about 20 in the last 50-60 samples in the last months. This never happen with tubes used for calibration.
Impossibile that those tubes are damaged beacuse the same tube (unique code) was then sent to a new sampling outdoor and then it behave very good with IS.

I have just excluded these possibile reasons:
- problems with the TD-GCMS? no, because I done a PT scheme with IS addition in June with very good z score in 4 different tubes;
- problems with humidity sampled on the tube in field? No because before adding IS to samples in lab, I perform a dry purge with N2 on the fluximeter for 2 minutes.


If you have a good experience with TD tecnique I would like to keep in contact with you for further troubles and of course If you need some advice I can surely help you.
(I cannot write to you by the forum, my mail address is cristian.paparoni@analisicontrol.it)

Really thanks
On the tubes that had low recovery of internal standard, did they appear to have a high concentration of target analytes or possibly background signal that was very high?

If so, it could be that the adsorbent has already been saturated and is not holding the internal standard. Do you add the internal standard before flushing the tubes in dry purge?
The past is there to guide us into the future, not to dwell in.
Thanks for questions.
No background signal and no high VOC concentration. I perform the analysis both in SIM and Scan mode together and no troubles exist. it is not a problem with the tube beacuse I have re-injected the same tubes affected by IS decrease (by adding again IS to these tubes) and the peak of IS returned high. It is something with sampling but I do not know what.
My system is Markes TD-100xr. It has a very good leak-checking protocol that the sample tubes must pass before it processes/injects the samples. If there's a leak when the sample tube is under pressure at room temperature before desorption, it will kick it out and not waste the sample. It seems like you might have an intermittent leak in your sample path in your TD system.

I don't know how the PE system works exactly but I'm sure it is similar to the others. The Markes has a number of o-ring seals in the path from sample tube to cryotrap to GC column. If any is leaking, sensitivity will suffer. Could it be that all of your sample tubes don't seat into the sample path the same exact way, every time?
There are no leaks for these reasons:
- my Perkin Elmer has an electronic leak check for each tube s your Markes;
- I just changed the o-rings of the tube place and before the trap.
- The tube that show loss of IS, if reanalyzed does not show any peaks, so it means firt desorption (high temperature for many minutes) is quantitative.

What I really take care is to understand how to introduce liquid solution of internal standard in the tubes for field samples, in order to overcome this trouble. It is not so important for me to understand the cause of this loss of IS beacuse I am sure it depends only by the samplinf procedure (interference, acids in tubes, etc.) In fact I have also saw that most of the samples with Is loss has some strange peaks of acetic acid or butanoic acid (vey bad shape): even If the ret.time is different from my IS, maybe acids does affect the process of absorption of IS molecule during mu adding in the tube.

so, I am looking for a good feedback in the possibility to add my IS solution in the tube before my clients go to perfom sampling outdoor.
It is a very strange situation, I know
If you add the IS before you give the tubes to your customers, you will have to be absolutely sure that the customer collecting the sample on the tube does not exceed the break-through volume of your IS for the sorbent you're using.

This is a difficult problem.
Yes, but maybe it is the only way. I am searching for breakthrough volum of toluene D8 but I can't find any literature or official method with this information. Do you have any website or article that indicates this volume?

In the meanwhile I will try my self to estimate experimentally this volume for toluene D8.
Because it's heavier than toluene, I'd bet it's a little bigger than toluene. Do they pretty much coelute in your chromatography?

You probably will not be too far off if you just use the break through volume for toluene.
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