ness: consistent accuracy despite inevitable varia-
Initially, we thought that these compounds were
tions in the sample matrix. The experiments per-
adsorbed back onto the matrix, since rapid dechlo-
formed here included four soil types, and several
rination processes are not likely under these con-
of the most frequently identified constituents at
ditions and no degradation products were observed
hazardous waste sites (Plumb and Pitchford 1985),
(i.e., increases in CDCE, or formation of tri- or di-
two of the more likely pathways (vapor and aque-
chloroethanes or vinyl chloride). This assumption
ous transfer) of vadose zone contamination, and
was based on the hypothesis that, with time, tetra-
short (days) and moderate (weeks) analytematrix
glyme caused the clay lattice to swell, opening sites
residence times. Of the sample preparation meth-
where chlorinated compounds, such as TDCE,
ods tested, solvent extraction with MeOH best ful-
CDCE, TCE, and PCE, could be preferentially
fills the above-mentioned criteria. Recovery of
sorbed. However, this theory could not be substan-
VOCs from these laboratory-fortified soil samples
tiated by introducing MeOH (20%), sonicating, or
with MeOH extraction was found to be quantita-
adding CaCl2 to the sample to extract or exchange
tive for all soils tested (Table 5), and it ultimately
sorption sites with these chlorinated compounds.
achieved the greatest analyte concentrations for
Thus, destruction through an alternate path, or a
vapor-fortified samples (Tables 6 and 7). In the case
chemical transformation, remain distinct possibili-
of the vapor-fortified soil samples, where concen-
ties.
trations could only be operationally defined (spiked
The other solvent tested, PPG, which was used
levels were not controlled), several days of room
in only one experiment, failed to show extraction
temperature (22C) extraction with MeOH provid-
efficiencies equivalent to those of MeOH (Table 7).
ed the maximum values. The only surprising re-
Moreover, tetraglyme achieved significantly high-
sult was that, with the CR-B soil, the recovery of
er concentrations than PPG with five of the nine
VOCs continued over a very long period (29 days,
analytes for the extraction period that was less then
Fig. 3). This slow extraction kinetics is consistent
2 hours and beyond.
with the recent findings of Askari et al. (1996).
The low vapor pressure of tetraglyme and PPG
Therefore, the extraction kinetics of VOCs from soil
make these solvents far less likely to interfere with
with MeOH can be an important variable.
instrumental analysis procedures, and these sol-
Of the two other solvents studied, tetraglyme
vents do not raise the flammability, toxicity, and
often failed to show extraction efficiencies equiva-
regulatory concerns associated with MeOH. A
Method Detection Limit (MDL, Federal Register
lent to those of MeOH (Tables 57); however, the
1984) study was performed using seven 4-g soil
results were generally better than those achieved
subsamples, each spiked with 44 ng/g of nine
with the HS techniques. In two cases MeOH and
VOCs and allowed to equilibrate for 2 days. These
tetraglyme achieved similar results for some of the
aqueous spiked soils were then extracted with 8
extraction periods used with the Ott and Ft. E soils.
mL of tetraglyme. MDL values obtained are pre-
In general, the discrepancies between the tetragly-
sented in Table 8. To achieve this level of detection
me and MeOH, which had been previously ob-
for HS analysis of soil extracts, 1 mL of the solvent
served by Jenkins and Schumacher (1987), were
extract was transferred to a VOA vial (22 mL) and
found to increase with percent organic carbon in
9 mL of NaCl-saturated solution was added just
the test matrix, and with analyte octanolwater par-
before the vial was capped. With this preparation
tition coefficients. In general, solvent extraction was
and analysis procedure, the soil sample only expe-
independent of the laboratory-spiking method and
riences 20 dilution (a 2-fold dilution during ana-
matrixanalyte residence time. However, an unan-
lyte extraction from the soil matrix and a 10-fold
ticipated finding was the loss of chlorinated com-
dilution of the extraction solvent during the analy-
pounds from the tetraglyme extraction solution in
sis step). With MeOH, often only a 0.100-mL ali-
the presence of a soil with a high clay content (Fig.
quot of the extraction solvent can be taken for analy-
1, 2a, d, and f).
Table 8. Method detection limit (MDL) study of soil extracted
with tetraglyme.
MDL (ng/g)
TDCE*
CDCE
Ben
TCE
Tol
PCE
E-Ben
p-Xyl
o-Xyl
7.25
8.97
4.53
14.4
7.72
10.4
9.72
14.1
12.5
* See Table 2 for full names.
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