[a confused chiral girl]

Hi everybody 

I have some trouble understanding what's going on chemically behind the steps I'm doing for an extraction on foods for PFC analytes (PFOS, PFOA etc etc)

To start off the extraction, I add 8mL of 100% ACN and then sonicate it for half an hour. What does the ACN do to the analytes and why need 8ml and sonicate it for so long?

Then I centrifuge it at 4000rpm for around 12 minutes, and what am I trying to separate here? Because after I keet the supernatant, I evaporate it to near dryness which means I'm trying to get rid of the ACN (since that's the only solvent I've used so far)? why do I need to concentrate the extract and then dilute it to mL with ultrapure H20?

Then after conditioning the cartridges from SPE-WAX, I load the sample into the cartridge, and wash the cartridge with 4mL of 25mM NaOAc (pH4) Is this to recovery the analytes from the cartridge?

Then I elute with 4mL MeOH, why is that? because after this, I need to elute with 4mL 0.1%NH4OH in MeOH....so why not just use the latter one instead of eluting with just plain MeOH first?

Before putting it into the instrument, I evaporate it again before putting in int. stds. Then I use 30% ACN in H2O to dilute the extract again. Why do I do this since I already wanted to evaporate the ACN in the very beginning?

Thank u in advance!! 

[MOTOBALL]

1.  you add 8 mL MecN to EXTRACT (i.e. dissolve) the analytes ( I suspect it it 8 mL per gm of food); 0.5 hr sonication to FULLY extract.

2.  Centrifuge to separate the non-dissolved, solid food components from the dissolved analytes in the supernatant.

3.  Take to dryness so that the MeCN solution does not pull the analytes through a subsequent SPE step. (SPE = solid phase extraction)

4. Re-constitute in pure H2O so that the analytes will then be loaded in a tightly focussed band at head of subsequent SPE cartridge.

5.  Elute non-wanted (and possibly interfering) ionic, components from column with 25 mM NaOAc.

6. Ditto for covalent compounds with MeOH

7.  Elute analytes of interest with 0.1% NH4OH/MeOH.

8.  Concentrate to dryness, so that  ~99.9% MeOH does not elute analytes (without any separation) straight through the subsequent analytical (HPLC ??) column.


9.  Terminology: you are not diluting a dried residue with aq. MeCN, you are re-dissolving.  Use Water/MeCN (3:7, v/v) to )load the analytes  as a tightly focussed band on head of analytical column.  A tighly focussed band will give sharp bands of analytes upon subsequent chromatographic development (HPLC; TLC)


There IS a method to the apparent madness !!!!