PAH / DRO by hexane/acetone microwave/ultrasonic extraction

Discussions about sample preparation: extraction, cleanup, derivatization, etc.

10 posts Page 1 of 1
Is anyone doing PAH in soils using the 1:1 hexane:acetone mixture either with method 3546 (microwave) or 3550c (ultrasonic) methods? I'm thinking about trying it for PAH analysis via method 8270. I'd try to use my 5890/5971 GC-MS and a DB-5ms (19091S-433) 30m x 0.25mm x 0.25um column with helium carrier.

I've never done any of the 8270 analytes but I am starting to send out quite a few samples for PAH and I'm thinking PAH might be a nice subset to get some experience with the method. I want to use the hexane/acetone extraction because I'm trying to keep methylene chloride away from affecting my 8260 methods.
Normally when using this method you exchange into methylene chloride before injection, which takes a bit of doing since it boils so much lower than the other two solvents. I also have found exchanging into ethyl acetate to work very well and it works great for doing solvent effect injections since you can have the oven start at 70C instead of needing a far cooler starting temperature.

PAH compounds are fairly easy to get to work on a 5 phase column, just remember you need an injection temp of around 275C to get them to transfer quickly to the column, lower temps work but you get more tailing in the last eluting peaks.

We are getting ready to try the microwave extractions ourselves, we have done the sonication in the past with good results.
The past is there to guide us into the future, not to dwell in.
Too bad mineral oil would probably elute right in the PAH range. Back when I did pesticides and PCB's in the early 1990's by sep funnel extraction, I used to add a mL of isooctane which contained mineral oil (I think it was maybe 1 mL mineral oil per Liter of isooctane). This worked great for volume reducing the methylene chloride for injection. Even if it went dry, the pesticides and PCB's were still there in the mineral oil residue, and could be picked back up with fresh solvent.
James_Ball wrote:
Normally when using this method you exchange into methylene chloride before injection, which takes a bit of doing since it boils so much lower than the other two solvents. I also have found exchanging into ethyl acetate to work very well and it works great for doing solvent effect injections since you can have the oven start at 70C instead of needing a far cooler starting temperature.

PAH compounds are fairly easy to get to work on a 5 phase column, just remember you need an injection temp of around 275C to get them to transfer quickly to the column, lower temps work but you get more tailing in the last eluting peaks.

We are getting ready to try the microwave extractions ourselves, we have done the sonication in the past with good results.
I can see why exchanging to hexane is a problem since its the acetone/n-hexane azeotrope boils at 49.8C and the vapor is 41% hexane 59% acetone. I'll try and find some ethyl acetate azeotrope data. Sounds like ethyl acetate is not too harmful to have in a volatiles lab.
LALman wrote:
James_Ball wrote:
Normally when using this method you exchange into methylene chloride before injection, which takes a bit of doing since it boils so much lower than the other two solvents. I also have found exchanging into ethyl acetate to work very well and it works great for doing solvent effect injections since you can have the oven start at 70C instead of needing a far cooler starting temperature.

PAH compounds are fairly easy to get to work on a 5 phase column, just remember you need an injection temp of around 275C to get them to transfer quickly to the column, lower temps work but you get more tailing in the last eluting peaks.

We are getting ready to try the microwave extractions ourselves, we have done the sonication in the past with good results.
I can see why exchanging to hexane is a problem since its the acetone/n-hexane azeotrope boils at 49.8C and the vapor is 41% hexane 59% acetone. I'll try and find some ethyl acetate azeotrope data. Sounds like ethyl acetate is not too harmful to have in a volatiles lab.


I think the only time I have to report ethyl acetate is if the client asks for a full appendix 9 list so not often I worry about it. Evaporating in the hood takes care of most of the fumes and the high boiling point of the ethyl acetate makes it easy to exchange. I also like the lower toxicity of the ethyl acetate versus methylene chloride, I just wish more EPA methods would allow it.
The past is there to guide us into the future, not to dwell in.
I've got some environmental grate EtOAc from Alfa Aesar and it seems to be completely clean for DRO work. I've decided to start with DRO and then move up to doing PAH as my expertise improves.

James_Ball Did you make your own dry down setup? Also what vendor do you favor for SPE prep for DRO in water? There seem to be a lot of offerings and I did not see the point of 12 or 24 port setups until I saw the Restek #26250 Sample Delivery System and realized how such a setup could work.
LALman wrote:
I've got some environmental grate EtOAc from Alfa Aesar and it seems to be completely clean for DRO work. I've decided to start with DRO and then move up to doing PAH as my expertise improves.

James_Ball Did you make your own dry down setup? Also what vendor do you favor for SPE prep for DRO in water? There seem to be a lot of offerings and I did not see the point of 12 or 24 port setups until I saw the Restek #26250 Sample Delivery System and realized how such a setup could work.


The last couple manifolds we bought were the Supelco Visiprep glass block units, 24 position. With 6ml cartridges we normally use ever other port staggering the front and rear so you have clearance to adjust the cartridges, especially the ones with the tabs on the sides like syringes. Any C18 or equivalent should work, whether Restek or Waters or Phenomenex who makes the polymer StrataX cartridges. The polymer ones are more forgiving if it goes dry during the extraction as the C18 silica ones are difficult to get the flow started again if it happens that the tube in the bottle is not on the bottom and you end up pulling air in early.

I was working with a dry down setup I cobbled together with a hot block and some needles mounted in a plexiglass manifold, but we now use the N-Evap 24 position water bath the needle purge and it works so much better, and easier to see the volume of the sample as it evaporates. I normally evaporate down to about 1ml then add 2ml more of EtOAc and finish down to just under 1ml before bringing to final volume.
The past is there to guide us into the future, not to dwell in.
What sort of sample sizes can you extract using the 6 mL SPE columns? Are you using the 500mg beds and are you using some sort of adapter to let you pull from 100-1000 mL bottles? I really have no idea how much SPE bed is required for typical groundwater samples.
LALman wrote:
What sort of sample sizes can you extract using the 6 mL SPE columns? Are you using the 500mg beds and are you using some sort of adapter to let you pull from 100-1000 mL bottles? I really have no idea how much SPE bed is required for typical groundwater samples.


We are going by method EPA525.3, which should also work for groundwaters. We use 500mg packing in the cartridges and you can find adapters and tubes from several suppliers to allow the cartridge to draw directly from a 1L bottle of sample.

From EPA525.3

"Use of a transfer tube system (Supelco “Visiprep”, #57275 or equivalent), which transfers the sample directly from the sample container to the SPE cartridge is recommended."

I believe we bought ours from Restek or UCT ( https://www.unitedchem.com/product-category/spe )

When using the transfer lines, fill the cartridge full of DI before inserting the adapter so you will have water in the cartridge after the air in the line has pulled through, it makes them flow better than if a slug of air passes through the cartridge.
The past is there to guide us into the future, not to dwell in.
Thank you for the information and suggestions.
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