Standard solution and extracted solution : same concentration? Summary :
I am currently testing how a certain type of foam adsorbs HAP, PCB and OCP, and to this end I am depositing some on the foam, then extracting it. To know how much was extracted, I need to compare with a standard solution of HAP, PCB and OCP. The problem is that when I look at the chromatograms, the extracted solution's peaks' areas are about 10 times the standard solution's peaks' areas. I don't know where in the experiment I am doing something wrong.

Details :
I have a solution of HAP, PCB and OCP at 10 mg/L. I put 100 µL of this solution on the foam, so that the deposited mass is 1 µg.
The foam is ASE-extracted with acetonitrile, then the acetonitrile is rotary-evaporated. What is left in the round-bottom flask is then diluted in 1 mL acetonitrile. So at maximum I have 1 µg pollutants in 1 mL acetonitrile.
To this 1 mL I then add distilled water until I have a 20 mL solution. The concentration of this solution should at maximum be 50 µg/L, right ? (If all pollutants are extracted from the foam and none lost to evaporation.)

The standard solution is made by also taking 100 µL from the 10 mg/L solution (so 1 µg pollutants), and adding distilled water until I have a 20 mL solution. So this solution's concentration should also be 50 µg/L, right ?

But when I put the two solutions through GC-MSMS, the peaks' areas of the extracted solution are about ten times greater than those of the standard solution. (Not always ten, but it is usually the case.)

Have I made a mistake somewhere ? Are my calculations/manipulations wrong somewhere ?
The internal standard used is naphtalene d8, and in each 20 mL solution I added 10 µL of a 10 mg/L naphtalene d8 solution. (So its concentration is 5 µg/L : is it a problem that the standard is not at the same concentration as the pollutants ?)

The experiment was repeated several times, with different foam extracts and different standard solutions, and the results were the same. It is not a problem of contamination, as everything has been extensively washed with distilled water, detergent and acetone.

In case it is relevant : when a solution is done, I sample 4 mL that I extract with a SPME fiber, and then I inject into the GC-MSMS.

Thank you in advance, I am still testing and calculating, so any help is appreciated.



To tschoppi (since I don't have enough reputation to comment) : I haven't tried with another kind of foam, but it's a good idea (to see at which step the problem lies). I'll see what we have in the lab. I am also pretty sure the foam I used isn't contaminated, because it was given to us straight from the package, and I washed it three times with different solvents and analysed the wash-solvent (which was clean)(but I may check again to be sure).
I found the answer to my question, and I am going to share it just in case.
Apparently, when extracting the pollutants from the foam, I am also extracting some foam-specific materials that influence the pollutants' behaviour during the SPME part of the extraction.
I tested that by extracting the blank foam, then adding a standard amount of pollutants to the extraction solvent. This gave the same result as extracting a spiked foam, meaning higher peak areas than a standard solution.

In short : matrix effect from the foam. I don't yet know why or what, but it's a matrix effect.