Estimating the Total Concentration of
Volatile Organic Compounds in Soil
A Decision Tool for Sample Handling
ALAN D. HEWITT AND NICOLE J.E. LUKASH
INTRODUCTION
thought to be contaminated with VOCs at levels
Volatile Organic Compounds (VOCs) are the
higher than 0.2 mg/kg are prepared by extraction
most frequently identified constituents in the
(dilution) in methanol (MeOH), i.e., the high-lev-
designation of environmental hazardous waste
el method. In contrast, samples thought to have
(Plumb and Pitchford 1985). Leakage of petro-
concentrations below 0.2 mg VOC/kg are ana-
leum fuels from underground storage tanks and
lyzed using a direct purge procedure, called the
improper disposal of spent chlorinated solvents
low-level method.
have contaminated large areas throughout the
A second, and greater, challenge is that VOCs
world. Because of the toxicity of many VOCs
in soil samples fail to maintain their concentra-
associated with these products, detection and
tion integrity if they are not collected and han-
quantification in all environmental media are of
dled with limited disruption and exposure and if
utmost importance. The vadose zone, that region
preventive measures are not taken to limit biolog-
between the ground surface and saturated zone,
ical degradation of aromatic compounds. It is
is a particularly difficult medium to characterize
well recognized that the current protocols recom-
because these contaminants either partially or
mended in Method 5030 (SW-846, U.S. EPA 1986)
mainly exist in a vapor phase. Therefore, the
often result in a 90% to more than 99% loss of
vapor phase must be accounted for along with
VOCs from soil samples prior to laboratory analy-
matrix sorbed and liquid phases to achieve an
sis (Urban et al. 1989, Illias and Jaeger 1993, Lewis
accurate quantitative assessment.
et al. 1994, Hewitt et al. 1995, Liikala et al. 1996).
Since the beginning of the cleanup programs
To minimize losses through volatilization and
mandated by Superfund and the Resource Con-
biodegradation, in-vial sample collection and
servation and Recovery Act (RCRA), gas chro-
analysis Methods 5035 and 5021 have been pro-
matography/mass spectrometry (GC/MS) Meth-
posed for the third update of the SW-846. Method
ods 8260 and 8240 (U.S. EPA 1986) have served as
5035 is titled Modified purge-and-trap and extraction
the major laboratory procedures for identifying
for volatile organics in soil and waste samples. Meth-
and quantifying VOCs in soils. One of the chal-
od 5021 is titled Volatile organic compounds in soils
lenges of coupling a sample collection and hand-
and other solid matrices using equilibrium headspace
ling protocol with this instrumental method of
analysis.
analysis is that GC/MS has a limited range of lin-
For these in-vial methods to be effective, dis-
ear response (two to three orders of magnitude).
crete "grab" samples must be transferred directly
Typical soil concentrations can range over seven
to vessels with hermetic seals that already con-
orders of magnitude (percent levels to the current
tain the appropriate dispersionextractant solu-
levels of instrumental detection, approximately
tion and preservative for the chosen method of
0.005 mg/kg). In particular, high analyte concen-
analysis (Hewitt and Lukash 1996). Therefore,
trations are problematic since they can degrade
samples must be handled differently from the on-
the performance of the mass spectrometer detec-
set of collection, depending on anticipated VOC
tion system. To cope with this limitation, samples
concentration. This requires that the regulators or
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