Dear Raquel,

your question is not completely clear, but seems to me , you are speaking about discrimination of hepta- to octa- CDD/Fs, right? Is is a chromatographic problem, which has nothing in common with HRMS.

If you are using SSL injector, frequent exchange /cleaning of liner will fix the problem.

As far as I remember ( I have been out of dioxin bussiness for ages :-) ), the best results (-the smalest discrimination of hepta-octa- CDD/Fs) were obtained with "double taper" SSL liners, but it is not easy to clean them.

If you want to run Thermo DFS (I operated MAT 95XP in the past), you must have money to buy good brand liners (e.g. blue Restek or SGE), forget for resilanization!

hope it helps a bit, kind regards

Tomas

Hi Tomas, thanks for kindly answering my question. I've changed the liner and got much higher recoveries as you commented!

I still have a problem: although recoveries of isotopic labeled internal standard are getting close to 100% after changing the liner, native dioxins, furans and PCBs still have low recovery, which doesn't make sense to me... Any suggestions on where the problem might be? the study matrix is ​​the pesticide 2,4-D. Thank you in advance.

Hi Raqel,

chromatographic discrimination of hevier analytes occurs when deposit of oily residues from samples builds up in the liner.

Do you inject herbicides without any cleanup?

regards, Tomas

Hi Tomas,

I'm actually new to this type of analysis. I have not received adequate training and Iam trying to learn how the processes work. I was given the following procedure for herbicides:

- extraction with dichloromethane

- multilayer silica column (silica gel, basic silica and 22% acid silica)

- separation of PCBs and dioxins in a basic alumina column

With the change of the liner I noticed that all recoveries for internal standard were back to ~100%, but the fortified natives, for both PCBs and dioxins (and in this case, it is a problem for all groups, tetras to octas) are not recovering with a minimum value of 70%. In fact, the greater the concentration of fortification, the lower the recovery rate. Could it be some cleanup loss?

Thank you.

Hi Raquel,

13C12 internal standards have (almost) identical physical properties, compared to 12C12 (native) compounds. That is the reason why isotopic dilution is so great method for quantitation, you just measure the ratios of native/ISTD…

Taking my first sentence in account, I do not understand your statement “ ISTDs are OK, but native analytes are low…”

I have no own experience with analysis of pure phenoxyacid herbicides (we analysed some concret and plaster from a plant after Seveso-like crash). In the real world, there are many difficult matrices on which you cannot obtain >70% recoveries (eg. minerals used as feedingstuff additives or sorbents for removing of PCDD/F from flue gas…) and I think 70% is not a dogma.

How about to make an attempt to obtain a suitable reference material or a sample with a known PCDD/F content and practice with it?

regards, Tomas

Hello Tomas,

Thank you very much for your kind considerations. I am giving my best to improve the results in lab.

Regards,

Raquel.

Could You describe the problem a bit more in detail?

What is the procedure You are working on (Matrix, extraction, cleanup)?

From the second half of the question I imagine You are talking about 13C labelled internal standards only, is that correct?

I very much suspect Your cleanup is not tuned in well enough, so You get too dirty lines too rapidly.

Indeed I always kicked out liners after a weeks use in 24/7 operation with well functioning extractions and cleanups.