Hi Sarika. You should separate the DCM phase from Methanol phase before concentrating the sample (only organic phase, DCM). And, in my opinion, you have to repeate the extraction procedure at least three times, adding the DCM phases together.

Martina

thanks martina...........but i had repeated ultrasonication thrice still i am not getting the results............

You are not seeing the alkanes that you spike after the filtration, directly before you inject into the GC?

i am not able to get alkanes in any way,neither those which my sample contain nor which i have spiked on............pls help

If the sequence of operations is exactly as you wrote it, you'll end up with a MeOH solution, which is very, very bad for both GC chromatography (difficult polar solvent) and your analytes (unsoluble in MeOH) - you said that some peaks do show up, so I'll assume your GC/MS is ok (how's the chromatography of the spike alone?). Try a pure DCM extraction, which is equally effective and let us know. If you really need a more polar cosolvent I suggest using acetone instead of MeOH.

thanks salvatore.......u r rite but After DCM extraction only,i am not able to get the alkanes......

If you want to analyse only alkanes, you should extract them from filters by mixture of hexane/DCM 1/1. I dont understand why do you use MeOH because this solvent is polar and suitable for extraction of polar compounds such as saccharides, acids etc. In other words, you extracted too many organic compounds and the reason can be in the huge background. Then you have to cleanup your sample, for example by fractionation on column with silicagel.

You could read the article: Krumal et al., Characterization of organic compounds in the PM2.5 aerosols in winter in an industrial urban area. Atmospheric Environment 105, 2015, 97-108.

thank you sir..........could you please mail me this article on [email protected] or send here on researchgate.It would be a great help.