Extraction of PAHs from soil with sonication

[Melody.CO]

Hi,
My name is Melody and I'm new in this area. I been working in sample preparation since January of this year because for my thesis I have to quantify PAH's in sediment and soil. I read a lot of articles for extraction methods and the one that was most suitable for the supplies and equipment of my lab was ultrasound, nevertheless I been having a lot of trouble with this, and my recovery doesn't exceed 60%. Hope you can help me.

I did some preliminar studies and I found the following:
solvent: acetone
time: 20min
vol of solvent: 15ml
cicles: 3 times the above.

I use a liquid cromatographer HPLC with FD for the reading. My ultrasound bath have 35kHz and 285W. I collect the 3 extracts and concentrated by rotaevaporation to 1ml, and then I put it in a 5ml ballon with mobile phase.

Hope you can help me and tell me what am I doing wrong or what can I improve. Thanx in advance.

Melody

[Don Shelly]

Your extraction solvent could be too polar. What is your mobile phase? What is your sample size?

[AICMM]

Melody.CO,

First, I have to agree with Don that I suspect acetone is too polar. Second, if you are using an US bath, I found these to be woefully inefficient extraction devices. I would turn to a shaker table before I would turn to an ultrasonic bath. If you are talking a horn, all bets are off.

Best regards,

AICMM

[Melody.CO]

Thanx for the replies.

My mobile phase is Acentonitrile:water 75:25 and my sample size is 5 g. I did some test with Hexane:Acetone but it was similar with acetone alone. Some people tell me is my Ultrasound bath that have to low power or frequency. But I don't know.

Thanx a lot.

Melody

[Don Shelly]

Try mixing your soil sample with sodium sulfate anhydrous until it forms a free flowing powder. That should increase surface area and permeability. Use the 1:1 acetone/hexane. Add 1 ml of acetonitrile as a keeper prior to solvent exchange.

[echae]

Don't know anything about HPLC, but we use methylene chloride for GCMS (We used to use different ratios of MeCl2:acetone but recently switched to 100% MeCl2). Then we solvent exchange into hexane for the cleanup prior to analysis.

As Don stated, we also mix the soils with sodium sulfate.