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was interfaced to an Extrel C50-Q quadrupole mass spectrometer. The elec-
trospray needle was cooled by a flow of water-cooled nitrogen, in order to
eliminate solvent evaporation inside the needle prior to electrospray. The
mobile phase consisted of methanol–water (90:10) with an addition of 2 mM
–
sodium chloride. Major ions in RDX and HMX were [M + Cl] , at m/z 257
and m/z 331, respectively.
Chloride adduct ion formation for a series of explosives, including RDX
41
and HMX, was investigated in negative-ion APCI-MS. It is important to
maximize the number of gas-phase chloride ions formed under APCI con-
–
ditions because their major reaction is to form the adduct ion [M + Cl] .
The higher the electron attachment rate constant of a solvent, the faster the
production of chloride ions and the higher the efficiency of formation of [M
–
+ Cl] adduct ions. Carbon tetrachloride was found to produce the greatest
–
increase in intensities of the [M + Cl] ion of RDX and HMX, followed by
chloroform and methylene chloride. These results were in agreement with
the electron attachment rate constants of these chlorinated solvents.
A series of explosives, including RDX, were analyzed by negative-ion
APCI mass spectrometry, using a Finnigan LCQ ion trap mass spectrometer. 42
Samples were introduced by infusion with a syringe pump into the LC stream,
which consisted of methanol–water (50:50). Without any additive the mass
spectrum contained weak ions at m/z 222, [M] , m/z 221, [M – H] , and
–
–
–
prominent ions at m/z 267 and m/z 268, interpreted as [M + NO – H] and
2
[M + NO ] , respectively. Addition of ammonium chloride gave an improve-
–
2
ment in response by a factor of 4, producing abundant [M + Cl] ions at m/z
–
257 and 259.
A series of explosives, including RDX and HMX, were studied using
negative-ion electrospray ionization Fourier transform ion cyclotron reso-
43
nance (ESI-FTICR) mass spectrometry. Samples were dissolved in acetoni-
trile and spiked with 0.5% ammonium acetate. The major ions in the mass
–
–
spectra of RDX and HMX were [M + 45] and [M + 59] . As a result of
accurate mass measurements with the FTICR mass spectrometer, these ions
–
–
were assigned as [M + HCOO] and [M + CH COO] , respectively. ESI mass
3
spectra of C4, a plasticized explosive based on RDX, contained adduct ions
–
identified as [RDX + NO ] , [RDX + H CO + HCOO] , and [RDX + H CO
–
2
2
3
– 43
+ CH COO] . These ions were also observed in the mass spectrum of pure
3
RDX but were more abundant in the mass spectrum of C4.
Table 6.2 shows a summary of the major adduct ions in ESI and APCI
mass spectrometry of RDX. Both ESI and APCI seem to produce rather
inconsistent mass spectra of RDX, resulting in the production of an array of
adduct ions. Furthermore, relative abundances of the ions largely fluctuate,
depending on analyte concentration, the presence of impurities in the mobile
phase, and contamination of the LC/MS system. Consequently, a study was
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