Table 5. Retention times for explosives-related compounds
by gas chromatography.
Retention time
Retention time
(min)
(min)
Analyte
DB-1 column RTX-200
Analyte
DB-1 column RTX-200
2,4,6-TNT
7.01
11.37
1,4-DNB
3.97
7.76
2,3,6-TNT
7.13
11.37
1,3-DNB
4.23
8.16
2,3,5-TNT
7.54
12.17
1,2-DNB
4.41
8.71
2,4,5-TNT
7.78
12.53
2,6-DNT
4.44
7.83
2,3,4-TNT
7.98
12.93
2,5-DNT
4.89
8.43
RDX
8.07
13.38
2,3-DNT
5.18
9.22
3,4,5-TNT
8.62
13.87
2,4-DNT
5.28
9.05
4-ADNT
9.14
12.54
3,5-DNT
5.50
9.46
3,5-DNA
9.24
13.00
3,4-DNT
5.79
10.25
2-ADNT
9.59
13.38
1,3,5-TNB
6.61
11.52
* The capillary column was a DB-1, 6 m 0.53 mm i.d., 1.5 m. Operating para-
meters were: initial column temperature of 100C (hold 2 min), ramp to 200C
at 10C/min, then 20C/min to 250C and hold for 5 min. The injection port
and detector temperatures were 250C and 300C, respectively. The carrier gas
was hydrogen with a constant flow rate of 12 mL/min. Analyte identities were
confirmed by analyzing a number of SPME samples on an RTX-200 capillary
column, 6 m 0.53 mm i.d. using hydrogen carrier at 10 mL/min.
Analyte identities were confirmed by analyz-
event, the fiber was thermally desorbed in the
injection port of the gas chromatograph (250C)
ing a number of SPME samples on an RTX-200
capillary column, 6 m 0.53 mm i.d.; the carrier
to assure that it was clean. To sample the head-
gas was hydrogen at 10 mL/min (Table 5). It
space, the aluminum foil was punctured by the
should be noted that whenever SPME fibers con-
needle housing of the fiber and the fiber was
taining TNT were desorbed into the hydrogen
extended into the headspace for a sorption period
carrier gas and separated on either the DB-1 or
of 20 minutes. The opening in the screw cap was
the RTX-200 columns, we observed peaks ascrib-
just the size of the outer diameter of the SPME
able to 2ADNT and 4ADNT (2-amino-4,6-dinitro-
holder and helped hold the assembly during
toluene and 4-amino-2,6-dinitrotoluene). Subse-
sampling. Later in the study, this sampling time
quent research revealed that these transformation
was reduced because the concentration of some
products of TNT were being produced by reduc-
analytes in the headspace increased to the point
tion of TNT by the hydrogen carrier, presumably
where the amount collected over the 20-minute
because of catalysis on the hot metal protective
sampling period maximized the detector signal.
needle of the SPME device. These amino reduc-
After the sampling was complete, the fiber was
tion products were eliminated when the carrier
withdrawn into its protective needle and the
gas was changed to helium.
needle was removed from the vial. The outside of
the needle was wiped with a tissue wetted with
acetonitrile to eliminate any vapors sorbed to the
RESULTS AND DISCUSSION
metal and inserted into the injection port of the
gas chromatograph. The GC-system consisted of
Qualitative nature of headspace signatures
The signature chemicals observed in the head-
an HP 6890 gas chromatograph equipped with a
micro-electron capture detector (ECD). The cap-
space samples above military-grade TNT were:
illary column was a DB-1, 6 m 0.53 mm i.d., 1.5
1,2- and 1,3-dinitrobenzene (DNB), 1,3,5-trini-
m. Operating parameters were: initial column
trobenzene (TNB), the various isomers of dinitro-
temperature of 100C (hold 2 minutes), ramp to
toluene (DNT), and 2,3,6- and 2,4,6-trinitrotolu-
200C at 10C/min, then 20C/min to 250C and
ene (TNT). 2ADNT and 4ADNT were also
observed, but, as mentioned above, we subse-
hold for 5 minutes. The injection port and detec-
tor temperatures were 250 and 300C, respectively.
quently determined that these two compounds
were being formed by reduction of 2,4,6-TNT in
The carrier gas was hydrogen with a constant
the presence of the hydrogen carrier gas in the
flow rate of 12 mL/min. Retention times for the
injector of the gas chromatograph (GC). We did
target analytes are presented in Table 5.
5
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