Table 3. Initial hold time for soil vapor samples.
Table 2. TCE soil vapor equili-
bration and temporal variations.
Average and
Triplicate
standard deviation
Percent
Triplicate
Average and
values*
(mg/L)
relative
values
standard deviation
Time and condition (mg/L)
Stored
Time 0
difference
Time
(mg/L)
(mg/L)
1 day at 22C
23 1.7 26.0 2.0
22, 22, 25
12
First probe installation at 1005,
3 days at 4C
12 1.0
15 1.2
13, 12, 11
20
21 July 1997
3 days at 12C
11 0.58 15 1.2
2.1 0.10
11, 11, 12
27
1020
2.0, 2.1, 2.2
2.1 0.15
1025
1.9, 2.1, 2.2
* Triplicate values of stored vapor samples.
1.9 0.10
1155
2.0, 1.9, 1.8
2.1 0.06
1255
2.2, 2.1, 2.1
Table 3 shows the results for soil-vapor sam-
Second probe Installation at 0700,
ples that were held under different conditions for
29 July 1997
periods of 1 to 3 days. These findings suggest
Day 1
that, regardless of storage temperature, TCE can
21 1.2
0705
20, 22, 22
23 0.58
be quickly lost from the sample vessels. Vapor
0715
23, 23, 24
26 1.0
0730
25, 27, 26
losses are most likely attributable to the two
25 0.58
0800
25, 25, 26
holes in the Teflon-lined septum.
26 2.0
0900*
24, 28, 26
To reduce the approximately 10% loss of anal-
26 2.6
1300
28, 23, 27
22 1.7
yte per day of storage, as seen in these initial
1430
20, 23, 23
experiments, an alternative sample handling
Day 2
16 3.5
0700
14, 14, 20
procedure, using a water barrier to cover the holes
23 2.1
0900
21, 22, 25
in the septum, was assessed (Table 4). Including 0.5
* Triplicate soil vapor samples taken for
mL of water in the sample collection and analysis
holding time study.
VOA vial caused less than a 5% reduction in sensi-
tivity. This small disadvantage was offset, how-
there were large changes in soil-vapor TCE con-
ever, since the water present in the sample vial
centrations over the course of a day. As a result of
was useful for visually confirming that a vapor
these temporal trends, all subsequent samples
sample was being introduced by syringe during
were obtained between 0800 and 1400 to limit the
the transfer step. More specifically, the sample
influence of this potential variable.
vessel was tipped so that the tip of the needle
Table 4. Average responses (peak area) and standard deviations (values
in parentheses) for VOC vapor samples prepared in VOA vials with 0.5
mL of acidified water covering the septum puncture holes.
TDCE
CDCE Ben
TCE
Tol
PCE E-Ben
p-Xyl o-Xyl*
First set
Fresh standard
452
476
1307
409
1857
419
1617
1837
1723
No puncture
(4.5)
(3.6)
(38)
(0.6)
(32)
(4.6)
(12)
(23)
(29)
Fresh standard
440
468 1277
401
1837
413
1650
1770
1693
Punctured
(5.9)
(8.0) (29)
(6.0)
(55)
(8.1)
(17)
(20)
(15)
5-day-old standard
411
448 1223
364
1643
346
1387
1417
1423
Punctured
(8.5)
(5.1) (15)
(4.0)
(32)
(4.7)
(50)
(57)
(49)
Second set
Fresh standard
447
475
1287
405
1830
416
1640
1793
1720
No puncture
(3.5)
(2.3)
(5.8)
(3.1)
(46)
(3.2)
(50)
(21)
(27)
Fresh standard
444
473 1288
405
1847
413 1640
1770
1713
Punctured
(1.0)
(4.6) (15)
(3.8)
(42)
(0.58) (36)
(72)
(25)
12-day-old standard
383
427
1167
340
1543
319
1253
1240
1253
Punctured
(14)
(9.5)
(31)
(9.1)
(67)
(7.4)
(31)
(62)
(23)
Average daily rate
1.2
0.83 0.80 1.6
1.8
2.6
2.6
3.4
2.7
of percent loss
(0.1)
(0.03) (0.07) (0.4)
(0.5)
(0.9)
(0.9)
(1.2)
(0.7)
* Trans-1,2-dichloroethene (TDCE), cis-1,2-dichloroethene (CDEC), benzene (Ben),
trichloroethene (TCE), toluene (Tol), tetrachloroethene (PCE), ethyl-benzene (E-Ben),
para-xylene (p-Xyl), and ortho-xylene (o-Xyl).
6