holes as the entrance to the VOA vial, while a sec-
RESULTS AND DISCUSSION
ond set was made by transferring the spike to an
Table 1 shows the results of the laboratory per-
open vial, which was then quickly capped.
formance study assessing the overall operation
of the probe, including the handling and analysis
of a vapor sample. The 91% or greater efficiency
ANALYSIS
measured for this process suggests that this soil-
The soil-vapor samples and discrete soil matrix
vapor probe and the approach to sample hand-
samples were analyzed using a headspace (HS)
ling are very likely to be quantitative. Further-
autosampler (model 7000, Tekmar-Dohrmann,
more, the precision of subsample collection is
Cincinnati, Ohio) coupled to a GC (model 8610-
very good, as is shown by the less than 10% rela-
0058, SRI Instruments, Torrance, California)
tive standard deviations for the triplicate VOC
equipped with a 15-m, 0.53-mm-diam. MXT-1
measurements.
capillary column (Restek Corp., Bellefonte, Penn-
sylvania) and photo-ionizationflame-ionization
Table 1. Comparison of analyte responses
sequential detectors. Before the VOA vials con-
(PID peak area) taken from three separate
taining an aqueous soil slurry were placed onto
working standards (VOA vials) and from the
the HS autosampler, they were shaken for 2 min-
similarly prepared standard (VOA vial attached
utes to completely disperse the soil and to help
to end of soil vapor probe) sampled through
them attain equilibrium. Prior to analysis, the
the soil gas probe.
autosampler heated all of the samples to 40C for
Transfer
20 minutes.
Working
Soil
Percent
efficiency
standard
vapor probe
rsd*
(%)
Analyte concentrations for the soil matrix sam-
362 6.4
329 9.5†
ples were established relative to aqueous head-
TDCE
2.9
91
space standards prepared by adding small (less
326 10.4
305 12.7†
CDCE
4.2
94
280 4.4
254 17.0†
than 10-L) quantities of a MeOH stock solution to
TCE
6.7
91
autosampler (22-mL VOA) vials containing 10 mL
* Relative standard deviation.
† Corrected by a factor of 1.27, because of both a
of organic free water. Analyte concentrations for
smaller vessel (21 vs. 22 mL) and subsequent dilution
the soil-vapor samples were established relative
of analyte caused by purging with approximately 4
to vapor working standards. The vapor working
mL room air prior to and while collecting the tripli-
standards were prepared by transferring 4 L or
cate vapor samples.
less of a MeOH stock standard, consisting of
weighed amounts of TDCE, CDEC, and TCE, to
Table 2 assesses the waiting period necessary
empty autosampler vials. This approach to instru-
after the probe is installed to obtain a soil-vapor
mental calibration for soil-vapor samples is con-
sample that is in quasi-equilibrium with the in-
sistent with the works of Hughes et al. (1992).
situ concentration of TCE. Review of these data
Instrument settings for the autosampler and gas
suggests that between 15 and 30 minutes is
chromatograph and information concerning the
appropriate. However, it should be noted that,
performance of this method for establishing VOC
after a wait of only 5 minutes, greater than 80% of
concentrations in soil have been previously docu-
the maximum TCE concentration was obtained.
mented (Hewitt et al. 1992, Hewitt 1998a). Values
Therefore, a reasonable estimate of the soil-vapor
for both the soil-vapor concentrations and dis-
concentration could have been obtained without
crete soil are only reported with two significant
delay. Caution, however, should be taken when
figures, based on the uncertainty associated with
dealing with analytes of lower vapor pressures
sample handling, analysis, and analyte spatial
than TCE, and when dealing with fine-textured
variability.
or very moist substrates (e.g., clays).
The soil-vapor collection and handling method
The concentrations of TCE in the soil vapor
used in this study required the transfer of 0.500-
appeared to be somewhat influenced by the time
mL of sample to a 22-mL container. With this
of day that the sample was collected. To better
approach, a detection limit of at least 2 ng/mL is
understand this phenomenon, more attention
anticipated for TCE. In general, this same detec-
should be given to meteorological parameters
tion limit also applies for many other VOCs that
(e.g., ambient temperature and barometric pres-
can be photoionized and would be even lower for
sure) in the future. The intent of this experiment,
those compounds that respond to electrolytic con-
however, was not to quantify the potential influ-
ductivity detectors.
ence of meteorological variables, but to see if
5
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