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8
Contamination from Double-Bond, Triple-Bond, or Aromatic
Compounds: Sulfuric Acid Cleanup
Sulfuric acid will add to nearly any double-bond, triple-bond, or aromatic
compound, producing a compound that has the general structure shown in
Figure 1. The only compounds that do not undergo this reaction are those
with single bonds, or compounds that are stabilized by groups, whichmake
themultiple bond inaccessible to sul-
furic acid addition. This reaction can
be used to convert nearly every com-
pound found inpesticide andPCB ex-
tracts, from organic-soluble com-
pounds to aqueous-soluble com-
pounds. The resulting organic phase
then can be removed and concen-
trated, resulting in a much less con-
taminated extract. It is important to
note that many of the pesticides will
undergo this reaction, so this cleanup
can only be used for PCB analysis.
Toperformsulfuricacidcleanup,place
the hexane extract in a vial about 3-
to 4-times the volume of the extract.
Add an equal amount of 1:1 sulfuric
acid, cap the vial, and shake for a few
minutes.Mostof thecolorwillbe trans-
ferred to theaqueous (bottom) layeras
the reactionprogresses. Allow the lay-
ers to separate either by standing or
centrifugation. Using a glass pipette, quantitatively remove the hexane (top)
layer and transfer toaKDor a concentrator vial. If thehexane extract still has
significant color, repeat the steps until no more color is exchanged into the
aqueous layer. Once the extract has been transferred to theKD or concentra-
tor vial, reduce it to final volume.
Following sulfuric acid cleanup, primarily only hydrocarbons, sulfur, some
chlorinated pesticides, and PCBs will be remaining.
General Contaminants: Carbon Cleanup
For many years, activated charcoal has been used to separate target com-
pounds from sample matrix interferences. Past problems have included lot-
to-lot variability of thematerial itself, as well asmanual column packing in-
consistencies, which often resulted in variable elution patterns. Labs were
required to test eachnew lot of charcoal, and thencorrect the requiredelution
solvent volumes.Over the lastdecade,developmentof anew, chromatographic-
grade graphitized carbon has provided a more consistent product with pre-
dictable elution behavior. This carbon material has far fewer contaminants
than charcoal and also is available in commercially prepacked cartridges,
which further increases performance reproducibility.
Graphitized carbon tubes have a unique elution pattern characteristic com-
pared to Florisil
®
, alumina, and silica gel tubes; and have higher sample ca-
pacity incomparison toC18 tubes. Ingeneral, carbonelutespolar compounds
first, then nonpolar compounds. For this reason, carbonmakes a very good
absorbent to remove nonpolar matrix interferences from sample extracts.
Graphitized carbon is a versatile, nonporous adsorbent, which retains or ex-
tracts a variety of compounds. The extraction system may be adjusted to
retain and elute aliphatic, aromatic, polar, and nonpolar analytes. For opti-
mal recoveries, compounds of interest should be applied inweak solvents, or
Figure 1: Sulfuric acid will add to nearly any double-bond,
triple-bond, or aromatic compound, producing a compound
that has this general structure:
Over the last decade, develop-
ment of anew, chromato-
graphic-gradegraphitized
carbonhas provided amore
consistent productwithpre-
dictableelutionbehavior.
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