400
350
^
y = 11.2x +3990
2
300
r = 0.949
250
200
150
100
50
0
0
200
20
40
60
80
100
120
140
160
180
Total Sampling Time (min)
Figure 7. Continuous sampling of TNT vapor by alternating two polyacrylate
SPME fibers at 20-minute intervals.
The rate of surface sorption of TNT on the fiber
Two fibers were used, so that while one fiber was
(k1) and the rate of desorption back to the vapor
sampling the headspace in the vial, the other was
(k1) is probably fast compared with the rate of
being desorbed and the sample analyzed by GC-
ECD.
diffusion of TNT from the surface of the fiber into
the bulk phase (k2), since vapor diffusion is many
The results for TNT are shown in Figure 7.
orders of magnitude greater than diffusion in a
While there was a slight difference in the mass of
condensed phase. In the case where TNT in the
TNT sorbed by one of the fibers compared to the
vapor is being replaced by sublimation from the
other, the general trend was a decreasing recov-
crystal, the following equation is relevant.
ery with exposure time. In this experiment, there
was no recovery time between sampling periods
k3
to allow reestablishment of equilibrium, and the
TNT (crystal)
TNT (vapor)
TNT in the vapor was being sorbed by the fiber
faster than it could be replaced from the solid
k1
TNT (fiber surface)
TNT source. The rate of depletion was slow, how-
k1
ever. For example, using the slope obtained from
the best fit of the data to a linear depletion model,
k2
the mass of TNT sorbed in the first 20-minute
TNT (fiber bulk).
exposure period was about 3.77 ng, while that
Since the mass of TNT collected in subsequent
sorbed in the second period was 3.54 ng. This was
exposure periods declines slowly (Fig. 7), the rate
only a 6% reduction, even without a recovery
of k3 must be slightly slower than the rate of diffu-
period between exposure periods. Thus, when
sion of TNT from the fiber surface into the bulk of
only a single 20-minute exposure period is used,
the fiber (k2). For our soil experiment, though, we
the depletion of the TNT in the vapor is insignifi-
do not envision that TNT that has migrated from
cant over the exposure period, and for practical
the buried crystals to the surface is present as
purposes, the vapor concentration can be consid-
crystalline TNT, but rather as TNT sorbed to soil
ered to be constant during sample collection.
particles. The rate of replacement in the vapor by
Sorption of TNT by the polyacrylate SPME
desorption from the soil particles is likely to be
fiber can be thought of as follows:
different from the rate of sublimation from crys-
k1
talline TNT.
TNT (vapor)
TNT (fiber surface)
k1
To evaluate the effect of this on the use of the
SPME fibers to sample the headspace above soil
k2
TNT (fiber bulk).
with sorbed TNT, an experiment was conducted
9
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