Page 278 - Advances in Forensic Applications of Mass Spectrometry - Jehuda Yinon

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–
results, forming major [M + NO ] ions. Nitrate adduct ions were 6 to 40
3
times more intense than trifluoroacetate (TFA) or chloride (chloroacetoni-
15
trile) adduct ions. Experiments using labeled NO in the mobile phase did
3
not produce unlabeled nitrate adduct ions, indicating that self adduct ion
formation is excluded. When using ammonium fluoride and bifluoride, no
–
adduct ions were formed but [M – H] ions were promoted. Ammonium
chloride, bromide, and iodide formed adduct ions, the chloride and bromide
adduct ions being more stable than the iodide adduct ions. Chloride and
bromide adduct ions have the advantage of isotopic patterns, making iden-
tification easier.
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 RDX and HMX were prepared in a methanol–water
(50:50) solution. Figure 6.13 shows the APCI and ESI mass spectra of HMX.
–
–
The adduct ions [M + Cl] and [M + TFA] are formed from background
residues of TFA and a chlorinated compound.
A Finnigan LCQ ion trap was used for a study on ESI-MS and ESI-MS/MS
25
of a series of explosives including RDX and HMX. Samples were introduced
as methanol–water (50:50) solutions. No additive was used. The major ions
–
are at m/z 267 [M + 45] (base peak), at m/z 268 [M + 46] , and at m/z 281
–
[M + 59] . A smaller ion appears at m/z 221 [M – H] . The ions at m/z 267
–
–
and m/z 281 were erroneously interpreted as [M + NO –H] and [M + NNO
–
2 2
–
– H] , respectively, because it was believed that the adduct ions were formed
between RDX and its fragments, as in the gas phase ionization. 34–37
A Finnigan LCQ ion trap mass spectrometer was used to study the ESI
mass spectra of HMX, RDX and some of its degradation products, mononi-
troso-RDX (MNX) and trinitroso-RDX(TNX) (Scheme 6) in ground water. 38
15
Ring-labeled N -RDX was used as internal standard.
3
Samples were extracted and preconcentrated by Porapak RDX solid-phase
extraction (SPE) cartridges (500 mg Sep-Pak, Waters Corp., Milford, MA).
Chromatographic separation was achieved with a Kromasil C8 reversed-
phase column (250 ¥ 2 mm, Eka Nobel, Bohus, Sweden), using an isocratic
mobile phase of isopropanol–water–ammonium formate (20:78:2) at a flow
rate of 0.2 ml/min and a column temperature of either 30˚ or 32˚C. The
concentration of the ammonium formate solution was 0.5 M and was
adjusted to pH 8 with 10% ammonium hydroxide.
Several volatile buffers, including ammonium nitrate, ammonium ace-
tate, ammonium formate, formic acid, acetic acid, and trifluoroacetic acid,
were tested and evaluated for the ability to promote negative ion formation
in the electrospray ion source. Ammonium formate was found to produce
more stable negative ion adducts with the lowest background. All investigated
–
nitramine compounds produced major formate adduct ions, [M + CHOO] .

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