Hi Lamis,

I would try another extraction technique with NMP as extraction solvent assisted by microwave at 130°C. The article compares different extraction techniques.

Article Selection of an efficient extraction method for the determin...

Regards

Markus

Markus Christ thank you so much for your suggestion.

Hi Lamis,

I would contact Ludwig Gruber from the Frauenhofer Institute (Ludwig Gruber ). This guy has more experience with PAC then me.

Regards

Markus

Add the native compounds of PAH before exctration

Lochan Singh

Markus Christ thank you for that .

Yanxiao Zhou I have spiked standard of 16 PAH for checking the efficiency.

Markus Christ I fear that microwave assisted solvent extraction is designed for solids (soils, plants, sludges, sediments...).

To avoid naphthalene evaporation, it may be advisable to stop DCM evaporation before dryness, for example by keeping the residual volume to about 1 mL, then transferring into the 2 mL volumetric flask, rinse the evaporator flask with aliquots of less than 0.5 mL, and complete to 2 mL.

Jean-François Gal thank you so much for your reply.

Dear Lamis Javid

For quantification you have to add a internal standard/chromatographic standard whose concentration is known to you. add that internal standard just before the injection of analytes. then compare the peak area of the internal standard and the analytes. Do the same for the analytes in control or blank and compare the ratios of area of internal standard and analytes in sample and control to calculate the recovery.

when detection limit is determined the S/N ratio should be 3:1. so make the solutions of PAH of different conc. and inject in GC-MS. till u get peak for analytes to determine the LOD. Another one is by calculating the standard deviation.

Regards

Dear Lamis,

1) are you sure that the double recoveries of heavier PAHs are not caused by the background (have you checked your blank effluent, your evaporator background...)?

2) considering only 16 EPA PAHs, there are huge differencies in their volatility. As proposed earlier (Dr. Gal), the evaporation of PAHs cannot be conducted to dryness... After reduction of volume of extraction solvent, we add 1 ml of isooctane ( a solvent in which we inject PAHs to GC). The final evaporation is performed under gentle stream of nitrogen in a small, conical flask to approximate volume of 1 ml. IMHO, Turbovap is a better choice for evaporation of light PAHs.

3) for mass spectrometric determination, I recommend isotope dilution quntitation method ( that is, extraction standards are spiked to the sample at the very beginning). Then, you measure ratios of native/labelled PAHs and the exact final volume of your extract is not neccessary to obtain a good result. The best internal standards are 13C -labelled (produced by CIL, MA, USA). There are deuterated PAHs out there as well ( Chiron, Absolute ...), they are cheaper, but less stable...

hope it helps a bit,

kind regards, Tomas

As Tomas said: The best way to determine the concentration of individual PAH with GC-MS analysis is isotope dilution which means the use of isotope labeled internal standards that are added to the effluent before extraction. You can use a "syringe standard" which is added after sample cleanup to determine the recovery.

52 ml of DCM + PAH for 20 ml effluent is far too much solvent. For most effluents a liquid-liquid extraction (LLE) with n-hexane (4 : 1) will work well. To determine the extraction efficiency perform 3 LLE steps.

If you evaporate your sample to dryness a loss of naphthalene is very likely. If you use LLE with hexane this step is not always necessary.

To unterstand why you have double recoveries of heavier PAHs we need to understand your way of quantification e.g. internal or external and what standards have been added at which time.....

I used External Standard for the analysis. Also, I didn’t completely dry my samples , I let them dry to 0.5 ml.