Chromatography Forum

Hi,

Can some one pleaseassit with the following:

Is there a simple sample cleanup method for the following groups of analytes-pesticides PCBs, aliphatic hydrocarbons in sediment,water and biological tissue. At present I extract my sample in 1:1 hexane acetone, concnetrate and solvent exchange into hexane( 5ml)and then cleanup as follows:


for PAHs/aliphatics, I take a 1ml aliquot and I use silicagel SPE tubes(20g/5ml) and elute the 1st fraction with hexane 20ml and the PAH fraction with 20ml DCM. I concentrate the samples to 1ml and inject onto a Varian Saturn 2000. The problemis I get very poor recoveries for the aliphatics and some of the PAHs, and the chromatogram looks strange.



For pesticides/PCBs I take a 1 ml aliquot of my 5ml extract and using a florisil SPE tube, I get fraction 1 which is in hexane and is supposed to contain the PCBS and some pesticdes, and F2in DCM which should contain the rest of the pesticides.Again I get poor recoveries.

I am using a CRM Nist 1941b to check my methods.

Is my sample preparation incorrect? Can anyone please help mewith this.


Thanks

Ruben

Ruben,
I have done the cleanup procedures you describe many times, including on SRM 1941b. 20 gram silica gel cartridges will give you plent of capacity before you have a problem with overloading. Even so, run a check solution through your process and make sure you know where your target analytes elute. When we start a cleanup procedure here we run our blank spikes through and fractionate our eluent as it comes off the cartridge so that we know how many mLs of solvent it takes to elute each target. That way if you can check to see if a target is "close to the edge" and will be prone to loss with matrix shifts. It is also improtant to have the solvent the same as in your samples. Running a test in a similar blank matrix helps too.

It has been a couple of years since I ran it but if I remember correctly, SRM 1941b contains near % levels of sulfur and that can affect your analysis.

Dear Steve,

Thank you for the reply. It is appreciated. Do you think you could provide mewith details of your method of cleanup?

Most applications speak of fractionation, e.g PAH and aliphatics, or Chlorinated pesticides and PCBs, never all of these classes. Is there such a method?

Thanks

Ruben

Ruben,
Sorry I haven't been back sooner.
I don't know of a single column method that is set up for all your target analyte groups.
The silica gel column will work for separation of aliphatics and PAHs. In the early 90's I did it following sw-846 method 3630.
http://www.epa.gov/epawaste/hazard/test ... /3630c.pdf
Later I adapted that to smaller cartridges using the same ratio of elution volume to mass of silica gel. Using 100% DCM for the 2nd elution solvent elutes more polar compounds than does the 40% dcm in pentane listed in the method. I have used both during fish extract cleanup. The pentane is better than hexane for solvent blowndown, especially if you are looking for relatively volatile targets, e.g. dichlorobenzenes.
For your cartridges process a blank matrix sample spiked with all your target anayltes. Start with 10 mL of hexane, then collect the 11th mL, the 12th mL, etc. on out through the 25th mL and analyze each separately. Then elute with dcm and collect them as individual 1 mL fractions. If you don't see all your targets, it is time to stop and consider what went wrong.
Do something similar for the Florisil cleanup.
The sw-846 method 3620 is a more recent update;
http://www.epa.gov/epawaste/hazard/test ... /3620c.pdf
(The lauric acid test is a throwback to the really old days.)
It would be helpful to know if your recovery problem is a result of analytical interferences or losses in your cleanup steps.
Try a sulfur removal method, copper if you have it. Sediments can be nasty.

Steve