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4
Removing water from the dichloromethane with sodium sulfate is critical be-
fore the extract is concentrated to final volume. Dichloromethane can hold
approximately 11mL of water per liter. If water remains in the extract, it will
partition out of the extract when the volume is reduced. If this occurs, either
the dichloromethane will evaporate first, leaving only water in the collection
vessel, or a two-layer extract will form. In either event, the recoveries of the
analytes will be lower than desired, and the presence of water will interfere
with gas chromatographic (GC) analysis.
The best way to remove the water is to decant the dichloromethane extract
through granular sodium sulfate held in a funnel with a high-quality grade
(Whatman 541) filter paper or glass wool. Approximately 30g of sodium sul-
fate issufficient formost samples. Thisstepmustnot beskipped! Somemeth-
odsmay call for powdered sodium sulfate, but some analytes are adsorbed to
thesmallerparticles, soonlya10-60meshgranular sodiumsulfateor equiva-
lent should be used. It also is important that this material be free from or-
ganiccontaminants, so it shouldbepurchasedasACSpesticide residuegrade
in glass containers. If purchased in bulk packages where exposure to plastic
isan issue, bake inamuffle furnace. Tobake the sodium sulfate, spread it no
more than 1-inch thick into an appropriate container and place into amuffle
furnace, baking at 400°C for aminimum of two hours. After this time, place
the sodium sulfate intoa glass containerwhile still hot, and cap the container
to keep the material from resorbing contaminants from the atmosphere. If a
muffle furnace is not available, wash the sodium sulfate or extract it with
dichloromethaneprior touse. This technique is extremelywasteful of solvent,
making themuffle furnace preferable.
Liquid-Liquid Extraction
Liquid-liquid extraction offers unattended extraction once the samples are
ready and the solvent is added. Extraction is performedunder neutral condi-
tions and the recoveries are excellent for chlorinated pesticides. Set up of the
extractors should be done following manufacturer specifications. Due to the
extended contact time of the organic compounds with the glass surfaces, re-
active compounds could breakdown if these surfaces became contaminated.
Although, with the use of proper washing procedures, this is uncommon. As
with the separatory funnel technique, the use of granular sodium sulfate is
important to yield a dry dichloromethane extract.
Liquid-liquid extractors are available in two versions, conventional and accel-
erated. The accelerated type uses a hydrophobic membrane to separate the
aqueous and the organic phases, and the extraction time can be cut to
1
/
3
or
1
/
4
of the conventional extractor time. Thesemembranes are expensive and it
is important to analyze the cost versus the number of samples extracted to
determine if there is a benefit to using this technique.
Solid Phase Extraction (SPE)
Finally, SPE also is used for the extraction of pesticide and herbicide com-
pounds from aqueous samples (USEPAMethods 3535, 508, and 515).When
using SPE, it is extremely important to follow themanufacturer’s recommen-
dations on the use of the material. There are several manufacturers of C18
tubes and disks, which are the typical mediaused for these compounds, and
the extraction steps will vary somewhat depending on the manufacturer. In
general, the biggest drawbacks with SPE are the plugging of the disk or tube
with suspended solids and the breakthrough of targeted organics; therefore
this extractionmethodworksmost reliably if contamination levels and solids
are low. SPEallows very fast extraction times and low solvent volumes; it also
is easily used in the field.
SPE allows very fast extrac-
tion times and low solvent
volumes; it also is easilyused
in the field.
For ordering information,
seeproduct listings
beginning onpage18.
