Page 264 - Advances in Forensic Applications of Mass Spectrometry - Jehuda Yinon

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ion source was used. Additives tested included ammonium nitrate, trifluo-
roacetate (TFA), and chloroacetonitrile. Nitrate adduct ions were more
intense than TFA or chloride adduct ions by a factor of 6–40. The base peak
in the negative-ion mass spectrum of TNT, with 1 mM ammonium nitrate
–
in the mobile phase, was at m/z 226 due to the [M – H] ion. For samples
–
as large as 500 ng, a nitrate adduct ion [M + NO ] at m/z 283 with a relative
3
intensity of 5% was observed. The base peak in the mass spectrum of 4-ADNT
–
was at m/z 259, [M + NO ] , while in DNT no nitrate adduct ion was detected.
3
Additional experiments in ESI with the additives ammonium fluoride, bif-
23
luoride, chloride, bromide, and iodide showed no formation of adduct ions
in nitroaromatic compounds, probably due to the high electron density in
their rings.
ESI and APCI in the negative-ion mode were compared for the analysis
of explosives, using a Hewlett–Packard 5989B LC/MS mass spectrometer. 24
Standard solutions of the investigated explosives, which included 1,3,5-TNB,
1,3-DNB, TNT, 2-ADNT, 4-ADNT, 2,4-DNT, and 2,6-DNT, were prepared
in a methanol–water (50:50) solution. Figure 6.3 shows the APCI and ESI
mass spectra in the negative-ion mode of TNT. In 1,3-DNB, 2,4-, and 2,6-
DNT, no ions were detected in the ESI mode. The APCI mass spectrum of
2,6-DNT is shown in Figure 6.4. In 2,4-DNT the ion at m/z 181 has a higher
abundance than the one at m/z 182.
The major ions in the APCI mass spectrum of 1,3,5-TNB were at m/z
–
213,[M] and at m/z 183, [M – NO] . The major ions in the APCI spectrum
–
–
of 1,3-DNB were at m/z 168, [M] , at m/z 138, [M – NO] , and at m/z 108,
–
[M – 2NO] . Both the ESI and APCI mass spectra of 2- and 4-ADNT contain
–
–
a major ion at m/z 196, [M – H] .
The ESI mass spectrum of TNT without any additive was recorded with
25
a Finnigan LCQ ion trap mass spectrometer. The mass spectrum contained
a larger number of fragment ions than ESI mass spectra with additives.
Lowest detection limit in the SIM mode, at a signal-to-noise ratio of 3, was
5 pg.
A typical group of byproducts of industrial 2,4,6-trinitrotoluene (TNT)
include isomers of trinitrotoluene, dinitrotoluene, trinitrobenzene, and dini-
trobenzene. These compounds were investigated using APCI-LC/MS in the
negative-ion mode and MS/MS using a Thermo–Finnigan LCQ ion trap
DUO
mass spectrometer. . HPLC separations were done with a Restek reversed-
26
phase Allure C18 column (150 ¥ 3.2 mm, 5 mm particle size) at a flow rate
of 0.4 ml/min with sample injection volume of 10 ml. Three mobile phase
systems consisting of methanol–isopropanol–water or methanol–water were
used for different groups of standard mixtures. The separation of the six
standard TNT isomers (2,3,4-, 2,3,5-, 2,3,6-, 2,4,5-, 3,4,5-, and 2,4,6-TNT)
(Scheme 1) was carried out with a linear gradient elution of methanol–iso-

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