Normally it should not matter. However, in the case of extremely high affinity of IS to quartz fibre filter /compared with the other components in the sample/, and thus slower  extraction of adsorbed IS, the mode of addition of IS might matter a bit.

It may also happen, that components of your sample may be absorbed better /stronger/ than added IS.

In the both mentioned  cases a larger amount of extraction medium should be used.

 Normally, chosen IS often represents a compound with similar chemical and physical properties to compounds in the sample, and so the complete extraction of all components is not a problem.

If you are not sure which way of IS addition is better, make preliminary tests with the same amounts of added  IS -  prior to, and after the extraction. Determine IS in both cases, and select that whith higher reproducibility.  

Note:  The best might be to follow the analytical protocol concerning your airborne analyt /containing the same compounds, sorbent and extraction medium/ .

Dear Dr. Solàr, I agree in general with your consideration. May be that an added IS on the filtre is not really adsorbed in the same way as the analytes that have to be determined. Further an IS must be added in solvent solution, so that dissolved IS -at all different - from captured analytes.

In my opinion the best way might be to have an IS suitable for controlling the GC/MS analysis and that is added just before injection (in other words after preparative steps) and taking it in account when calculating results; an additional IS can be used just to control the whole analytical process, from extraction-sonication and rotary evaporation to GC/MS intrumental determination, by calculating its recovery in every analysis and if it fall in the acceptability criteria that you have outlined. Best wishes. MV. 

Thank you Dr solar.

Thank you MV.......