picric acid (g/g) = 91 (g/g-ABS unit)
Field screening in water
(Final ABS @400 nm 0.5
Initial ABS @ 400 nm) (Fig. 7).
Selection of ion-exchange material
and extraction and elution conditions
The computation of the MDL was based on the
Initial tests with the basic ion-exchange materi-
replicate extractions of seven 22-g samples of lo-
als indicated that the Anion membrane retained
cally acquired sandy loam that had been fortified
picrate from water but the Alumina-A cartridge
with 5 g/g of picric acid. After a few minutes
did not. Thus development of the screening
settling time, 30 mL of the acetone extract was fil-
method for water utilized the Anion membrane.
tered using a 0.45-m Millex SR syringe filter, then
A 2-L water sample fortified with 2 g/L of
diluted with 30 mL of reagent-grade water. The
picric acid was extracted with the Anion mem-
diluted extract was then passed through an Alu-
brane in 20 minutes. The membrane turned yel-
mina-A cartridge using a 60-mL syringe at 10 mL/
low. Microscopic examination of a cross section of
min. The cartridge was rinsed with 3 mL of metha-
the membrane showed that all of the picrate was
nol followed by 3 mL of acetone using a 10-mL
retained on the top surface of the membrane. One
syringe. A 10-mL aliquot of 2% H2SO4acetone was
5-mL aliquot of 10% H2SO4methanol was added
then passed through the cartridge at 5 mL/min
to the extraction apparatus and allowed to soak
using a 10-mL syringe. The eluent was collected
the membrane for three minutes. Vacuum was ap-
through another 0.45-m Millex-SR syringe filter
plied and the extraction solution pulled into a test
attached to the tip of the cartridge. The initial ab-
tube placed beneath the membrane holder. This
sorbance at 400 nm was recorded. An additional
procedure removed most of the yellow color from
5-mL aliquot of unacidified acetone was added to
the membrane, producing a colorless solution of
the extract. This dilution resulted in a visible de-
undissociated picric acid. Some picrate remained
crease in the yellowish humus-derived color of
in the ring of membrane that was clamped be-
the initial extract. Then 5 mL of reagent-grade
tween the top and bottom of the extraction appa-
water was added and the resulting deeper yellow
ratus. This could be recovered by removing the
color measured at 400 nm. The resulting MDL was
reservoir top and pouring a few milliliters of ex-
1.3 g/g.
traction solution on the surface of the membrane,
A field screening method that includes a quan-
allowing that to soak, then pulling it through the
tification step requires a daily check standard for
membrane by vacuum. This procedure required
validation. An aqueous, unacidified standard at
some care and did not appear to be worthwhile
10 g/mL of picric acid is stable for at least two
for a field method. The additional recovery was in
weeks. A simulated soil extract is prepared by di-
a more dilute solution than the original 5 mL of
luting a 30-mL aliquot of this standard one-to-one
extract, reducing the overall absorptivity of the
with acetone. This mixture is then extracted and
combined extracts. An addition of 5 mL of reagent-
eluted according to the method. The absorbance/
grade water produced the maximum yellow color
cm at 400 nm should be 0.56 0.08.
from the picrate ion.
A breakthrough test was performed using re-
0.8
agent-grade water fortified with 20,000 g/L of
picric acid. The run-through was measured using
Soil (g/g)
RP-HPLC. No breakthrough was observed when
0.6
2 L of water at this concentration was extracted.
Potential interferences
The same yellow TNT transformation products
0.4
Water (g/L)
that were retained by the Alumina-A (two isomers
of amino-DNT and 3,5-DNA) were also retained
by the Anion membrane. A 5-mL methanol wash
0.2
removed these from the membrane without elut-
ing any picrate.
Humic acids produced the same problems in
the water method as in the soil method. Humic
0
100
200
Picric Acid in Soil and Water (RPHPLC)
material was retained by the Anion membrane,
Figure 7. Calibration curves for soil and water methods.
then eluted by the acidified solvent. Fortunately
7
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