The column temperature was maintained at 28C
toward the heavy isotope of nitrogen. Two advan-
tages to measuring the δ15N values of TNT in the
with a column oven.
groundwater are: 1) groundwater samples are
GC-IRMS analysis
easier to obtain than subsurface soils samples, and
Stable carbon and nitrogen isotope analysis of
2) groundwater is a well-mixed system relative to
nitroaromatics in solution was conducted with a
the soil through which it passes. A disadvantage
Varian Star 3400 CX gas chromatograph (GC)
is that the concentrations of TNT may be too low
to obtain reliable δ15N values.
(Varian, Harbor City, California), equipped with
a Finnigan Magnum ion trap mass spectrometer
Regardless of the mechanism, changes in the
δ
15N values of TNT in groundwater toward
(ITMS) and a Finnigan Delta-S isotope ratio mass
spectrometer (IRMS) (Finnigan Corporation, San
heavier N would indicate that some form of
Jose, California) (GC/ITMS/IRMS). The analytes
attenuation is ongoing.
were separated on an SPB-1 fused silica capillary
column (15 m 0.32 mm i.d., 0.25-m film thick-
Objective
ness) using a temperature program of 100 to 250C
The objective of this work was to evaluate the
at 8.3C/min. The effluent from the column was
feasibility of using 13C and 15N stable isotope
split, with 10% going to the ITMS for peak identi-
analyses as an analytical tool for monitoring natu-
fication and the remainder to the IRMS where the
ral attenuation at explosives contaminated sites.
analytes were combusted in line at 940C to CO2
Laboratory studies were conducted to determine
if changes in the δ13C and δ15N values for TNT in
and N2 for isotope analysis. For δ15N analysis, the
solution and the δ13C values for the soil carbon
capillary transfer line was submerged in liquid
could be measured as TNT proceeded through its
degradation and attenuation processes. Field stud-
ies were conducted using contaminated ground-
standard for carbon was Pee Dee Belemnite. The
detection limits were 1 g C and 3 g N, and the
water from Louisiana Army Ammunition Plant
(LAAP) to determine whether changes in the δ13C
precision was 0.2‰ for δ15N and 0.1‰ for δ13C.
be measured as a function of time or distance as
Dual inlet IRMS analysis
the TNT plume migrated through the soil. Field
Soil samples were analyzed isotopically by a
studies were conducted using soil samples from
modified Dumas combustion method that con-
LAAP to determine if the δ13C values for the soil
verts organic carbon and organic nitrogen to CO2
carbon changed over time as a result of exposure
and N2 for mass spectral analysis (Macko 1981).
to and attenuation of TNT.
Soil samples were placed in quartz tubes with Cu
and CuO, evacuated, and sealed. The quartz tubes
were then heated to 850C at a rate of 450C/hr,
kept at 850C for 2 hours, and cooled to room tem-
METHODS AND MATERIALS
perature at a rate of 600C/hr. The slow cooling
cycle ensured that any oxides of nitrogen were
Instrumental analyses
decomposed to N2. The CO2 was separated from
HPLC analysis
N2 by cryogenic distillation. The N2 gas was then
Sample extracts for RP-HPLC (reversed phase
analyzed on a Nuclide 3-60-RMS. In turn, CO2 gas
high-performance liquid chromatography) analy-
sis were diluted 1:4 (v/v) with water and filtered
was analyzed on a Finnigan MAT 252 IRMS
through a Millex SR 0.5-m filter (Millipore Corp.,
(Finnigan Corporation). The standard for nitrogen
Bedford, Massachusetts). The chromatographic
was atmospheric N2 and the standard for carbon
system consisted of a Spectra Physics 8800 pro-
was Pee Dee Belemnite. The detection limits were
1 g C and 3 g N, and the precision was 0.2‰
grammable pump (Spectra Physics, Inc., San Jose,
for δ15N and 0.1‰ for δ13C.
California) operated in isocratic mode, a Spectra
Physics Spectra 100 variable wavelength detector
set to 254 nm, and a Dynatech LC-241 autosampler
Calibration experiment
(Dynatech Corporation, Baton Rouge, Louisiana).
A series of standard solutions of standard ana-
The analytes were separated on an LC-8 (150 3.9
lytical reference material (SARM) grade TNT
mm) column (Waters Corp., Milford, Massachu-
(Army Environmental Center [AEC], Aberdeen
setts), eluted with a binary eluant of isopropanol/
Proving Ground, Maryland) were prepared in aceto-
water (15:85, v/v) at a flow rate of 1.4 mL/min.
nitrile (AcN), with concentrations ranging from
4
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