Anal. Chem.
1996, 68. 3431-3433
Effectiveness of Purge-and-Trap for Measurement
of Volatile Organic Compounds in Aged Soils
MinooD. F.
Askari,t
Michael P. Maskarinec,* StacY"M. Smith,* Paul M. Beam,li and
Curtis
C.
Travis*":'
Universityof Tennessee, Knoxville, Tennessee
37996,
EM451,
1()()()
IndependenceAvenue,
U.S.
Department ofEnergy,
Washington, D.C.20585, andChemical
and
AnalyticalSCiencesandHealth Sciences ResearchDivisions,
Oak
Ridge
National Laboratories,
Oak
Ridge, Tennessee 37830
The
U.s.EPA-recommendedmethodformeasurementof
trace levels ofvolatile
organic
compounds (VOCs) in
soil,
purge-and-trap, measures the readilydesorbable
organic
contaminants from soil pore spaces and
external
soil
suriaces.
It
does
DOt,
however, measure contamination
that
has diffused
into intemal
micropores
of soUmatrix.
Thus,
the purge-and-trapmethodmeasures onlya small
fraction of total soU contaminants,
especiaUy
in long·
contaminated. soUs,
where .....
90-99%
of contamination
may
be
in
the interior of the soUmatrix. We compared
three
methods
for
determination
of
VOCs in aged field
samples: purge-and-trap, methanol immersion, and
hot
solvent extraction. Hot solvent extraction proved to
be
muchmore effective
than
the
U.S.
EPA-approved purge–
and-trap
technique.
For
three
Iong-cootaminated
soils
containingsuchVOCs as trichloroethene, benzene, tolu–
ene,
chloroform,
methylene ch1oride, and
ciI·l
,1-dichJo–
roethyIene, recoveI)'from
purge-and-trap
ranged
between
1.5
and
41.3%
that
of bot solvent
extraction.
Our
data
show
that
purge-and-trap
may
DOt
be
the
best
methodology
formeasuring soUVOCs concentrations, particularly
in
aged soils.
It
is clear from
this
and
previous
studies that
the best
ovenill
choicefor
soU
VOCSmeasurements is hot
solvent extraction. These results also indicate
the
inef–
ficiency
of
purge-and-trapasamethodfor evaluating
vapor
extraction remediation technology. Our
resul1s
suggest
that
the
EPAshould review
the
useof
the
purge-and-trap
method for measuringVOCSconcentrations
in
soils,
A
aitical
requirement inthecleanupofcontaminatedsoilsites
is an accurate detennination of the nature and extent of soil
contamination. The primary U.S. EPA-recommended method
(EPAlSW-84fi..5030Aand 826OA) for measurement of volatile
organiccompounds (Voes)
in
soils
is-purge-and-trap,
I.2l.L
followed
by
gaschromatography/mass spectroscopy. Under this protocol.
organic-freewater containing internal standards and surrogates
ismixed
with
a soil sampleand heated to
40
°C.
An
inert gas
is
bubbled through the solution at ambient temperature, and the
vapor is
passed
through
a sorbent colwnn. where the volatile
components are adsorbed. After purging
is
completed, the
sorbentcolwnnisheatedandbackflushedwith
inert
gas
todesorb
.
lin~ity
ofTennessee.
' Chemical and
AnaJytical
Sciences Division. OR."'L
' t'.S. [)epartJllml
of
Entl%)·,
- Health
Scien~
Resean:hDivision. ORNL
m
Environmenw
Protection
Agency.
Cod,
of
Ftdmll Rtf'lII2/il1'U.
PaM ztil;
nlle
4(); U.s.
Government
Printing
Offlre; Washington.DC.1!/!14:Chapter
I. App. 1lI.
p
67
aul)·
1. 19941.
50003-2700(96100009·1 CCC: $12.00 (': 1996 Amencan Chemteal Soc.ety
the components onto a gas chromatographic column.
I
Use of
purge-and-trap
to
measureVOCs insoil is based on the assump–
tion that soilVOCs
rapidly
equilibrate
with
soil water. Recent
studies.2-~
however. strongly question
this
assumption and
indicate that soildesorption is
a
biphasic
process
with
an
initial
rapidsurface desorption followedby a much slower. diffusion–
limited,desorptivephase
from
the interior of thesoil
matrix.
Ht>-!l
The biphasic nature of desorption
casts
doubt on the widely
used.EPA-recommendedpurge-and-trapmethod. Whensoilhas
been
in contactwithVOCs for a long time period (aged soils).
VOCs diffuse
into
soil
micropores,
where they are unavailable for
purge-and-trap
measurement,
Except:
fora single studt involving
l.2-dibromoethane
(EDB),
the
impact
of
soil
aging-
on the
effectiveness
of
VOCsmeasurement techniques
is
largely
unex–
plored. The present
study
focuses on the effectiveness of three
commonlyused techniques formeasuringVOCSconcentrations
in
aged
soils:
purge-and-trap,
methanol immersion, and hot
methanolextraction. Sincevaporextractionisacurrentlypopular
technique for removalofVOCs
from
soils,we subjectedone soil
sample
to
air
stripping
to evaluate
the
effectofvapor extraction
on the extraction efficiencyofpurge-and-trapmeasurements.
EXPERIMENTAL PROCEDURE
Soils.
Soil
samples
were
obtainedfrom
three
geographical1)'
distributed
sites
with
a lQ-2().yearhistoryofVOCscontamination.
The Kentucky soil had
bigb
clay
content, with 100 ppb of
trichloroethylene. The Louisianasoil
was
a siltyloam.with3000
ppbofcis-l,l-dichloroethyleneand6000ppboftrichloroethylene.
The Florida soil
was
silty, fine to very fine sand, containing
methylene chloride at
240
ppb. benzeneat
2
ppb. toluene at 190
ppb. andchloroform at
2
ppb.
Sample CollectiOD.
Soil
samples were extracted with a
hoUow-stem auger and split-spoon sampler. Undisturbed soil
(2)
Steinberg.
S.
M.; Pipattllo.
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Pernm.
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AI18/yfical Chemisfry. Vol.
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No.
19.
OCtober
1. 1996 3431
