Table 1. Microtip readings of TCE
vials were prepared for high- and low-level meth-
and gasoline standards* in ppmv
ods of analysis. Vials containing 10 mL of Type 1
with and without soil present.
water, acidified with 0.25 g of NaHSO4, were pre-
pared for low-level analysis, while ones contain-
TCE
Gasoline
ing 5 mL of MeOH were prepared for high-level
10 g soil† no soil
20 g soil† no soil
analysis. Grab samples, preserved in MeOH, were
10.8
18.7
8.9
17.5
analyzed after we transferred a 0.1-mL or smaller
11.1
19.0
8.0
20.4
volume of the extract to an autosampler VOA vial,
11.6
19.5
9.3
20.8
containing 10 mL of Type 1 water, for HS/GC
11.1
17.8
6.9
21.4
11.0
18.7
analysis. The 22-mL VOA vials used for this proce-
12.1
17.4
dure, and those prepared for the low-level sam-
11.3
16.7
ples, were compatible with an automated HS/GC
11.0
17.9
analysis system (Hewitt and Lukash 1996). In gen-
11.5
18.3
9.8**
10.0**
sistent with proposed Method 5021.
4.3††
For the second field trial, a Model 580B OVM,
6.9††
furnished by the site investigators, was used for
* 4 L of stock standards (0.53 mg TCE/
on-site total VOC vapor analysis. The working
mL or 1.1 mg gasoline/mL).
standards were periodically analyzed throughout
† Weight of soil, thus concentrations
the day between the collection activities. As with
were approximately 0.2 mg total VOC/
the surface collections, approximately 10 g of soil
kg.
was analyzed using the rapid soil vapor analysis
** Held for 24 hours prior to analysis.
†† Held for 48 hours prior to analysis.
method. Depending on whether the response was
greater or less than that for the 0.2-mg TCE/kg
working standard, we transferred grab samples to
Table 2. Microtip readings of TCE (ppmv) in work-
ing standards* with and without soils of various
vials prepared for the high- or low-level methods
moisture contents, under different environmental
of analysis.
conditions.
In-lab
Direct sunlight
Coldroom
RESULTS
23C
35C
1C
Temperature
Table 1 shows the response of the Microtip HL-
Background
1.3
0.0
2.1
2000 to working standards when the VOA vials
No soil
17.7, 17.8
14.8, 14.6
16.2, 16.6
were either empty or contained 10 or 20 g of the
native soil. These working standards were
Soil (10 g)
10% moisture
8.5, 11.0
8.4, 8.4
6.7, 7.4
allowed to sit for 24 hours or 12 days prior to
20% moisture
9.6, 10.5
8.3, 8.1
7.4, 7.2
analysis. The instrumental response to the work-
saturated
11.4, 12.3
8.4, 10.0
10.8, 10.9
ing standards, made with the site-specific soil, was
* 4 L of stock standards (0.53 mg TCE/mL).
well above the laboratory background readings of
0.5 to 1.0 ppmv. As anticipated, the presence of the
slightly lower values established for the stan-
soil matrix had a pronounced effect on the instru-
dards placed in direct sunlight at 35C may have
mental response. The near-surface soil used for
been caused by pressure buildup and subsequent
these site-specific working standards has a mois-
ture content of 15 5% and an organic carbon con-
vapor loss from the VOA vials. There also appears
tent of 1 0.5%. In addition, Table 1 shows the
to be a small dependence on moisture content. In
general, this method of preparing and using
response obtained for working standards that
working standards for establishing a decision
were held for 24 hours or longer prior to analysis.
marker appears only slightly dependent on both
The reduced response for these samples was
soil moisture and the meteorological conditions
caused by vaporization losses, since aluminum
at the time of analysis.
foil fails to form a hermetic seal with rigid surfaces
Tables 3 and 4, respectively, show the results
(Hewitt and Lukash 1996).
obtained during the first and second field trials,
The results of the robustness study are shown
in Table 2. While not always a factor of 10 greater
along with values obtained for site-specific work-
ing standards for TCE. In the first trial, locations
than the background, the responses for the work-
that had been established to be above and below
ing standards in soil were fairly consistent. The
5