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9
solutions with low solubility for the analytes, and eluted in strong solvents.
Mixed solvent systems, including dichloromethane, often are themost effec-
tive for elution. Carbon cartridgeswill retainpesticides andPCBswhen intro-
duced as low volume (1mL) hexane extracts. They will retain the non-polar
sample interferences and release the chlorinatedpesticidesusingup to20mL
of a20%dichloromethane/hexanesolution*.However, cautionshouldbe taken
whenusing graphitized carbon to clean extracts for PCBanalysisbecause the
coplanar PCB congeners BZ#77, 81, 126, and 169 are retained and do not
elute using the above solvent. These congeners can be eluted using a 1:1
mixture of ethyl acetate and benzene.
*Due to the uniqueness and high capacity of graphitized carbon, all fraction-
ation and elution volumes should be verified from lot to lot.
SulfurContamination:Mercury, ActivatedCopperPowderCleanup
Sulfur also is a common contaminant in pesticide and PCB extracts, and it
produces a large signal on anECD. Sulfur can be removedusingGPC or one
of themany cleanup procedures listed inUSEPAMethod 3660.
Mercury added directly to the extract vial is probably the best method for
removing sulfur. (Note: Due to the hazardous nature of mercury, use caution
whileworkingwith this substance.) Adda fewdrops ofmercury to thehexane
extract. The sulfur is then converted from an organic soluble species tomer-
cury sulfide on the surface of the mercury drop, which appears as a black
powder. The hexane is then pipetted off and re-vialed. Repeat this procedure
until this reaction no longer occurs.
Activated copper powder also removes sulfur, but can react with some of the
chlorinated pesticides if the exposure time is too long. The first compound to
show signs of this reaction is usually heptachlor. The best way to use this
cleanupmethod is toapply theactivatedcopperpowder to the topof aFlorisil
®
SPE tube, so that the cleanup is performed as the sample passes through the
cartridge, resulting in minimum exposure time. Sulfur cleanup is not ame-
nable to the organophosphorous pesticides as several of them break down in
the presence of activated copper ormercury.
Analysis of Chlorinated Pesticides
Calibration
The instrument used for the analysis of pesticides and PCBs must be cali-
brated prior to performing quantitative analysis. The calibration should be
linear over a 16- to 100-fold concentration range. The calibration of 3 to 5
points includes analyzing a low-point standard tomeet the required reporting
limit, aswell as ahigh-point standard tominimize the need for dilutions. The
linearity check should containall thepesticidesbeing reported. Thenecessity
to verify linearity for all target compounds is important because the different
classes of pesticides (i.e.,
α
-BHC vs. methoxychlor) will cause differences in
injection port discrimination, chromatographic peak shape, or detector lin-
earity.
Aroclor
®
standards are mixtures of chlorinated biphenyls, called congeners.
The linearity of the PCB congeners is consistent from the monochlorinated
biphenyls todecachlorobiphenyl.Forsomemethods, runningcalibrationcurves
for three Aroclor
®
standards covering the entire analytical range (i.e., 1242,
1254, and 1260), followed by the analysis of a single concentration for each
remaining Aroclor
®
standard is sufficient. See pages 19-22 for common cali-
bration standards.
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Refer to page 19 for SPE
tubeordering information.
