Page 293 - Advances in Forensic Applications of Mass Spectrometry - Jehuda Yinon

1522_book.fm Page 264 Thursday, November 13, 2003 9:58 AM

sodium chlorate (NaClO ), potassium chlorate (KClO ), ammonium per-
3 3
chlorate (NH ClO ), and sodium perchlorate (NaClO ). Isotopically labeled
4 4 4
15
18
15
15
compounds [ NH NO , NH NO , NH N O , Na NO , and ( NH ) SO ]
15
4 3 4 3 4 3 3 4 2 4
were used to confirm ion characterization. Solutions of ammonium nitrate
were prepared in methanol–water (50:50) and in pure methanol. Solutions
of other oxidizers were prepared in water and diluted in methanol–water
(50:50). Solutions were introduced into the mass spectrometer by syringe
pump infusion at a flow rate of 5 ml/min. Heated capillary temperatures were
o
in the range of 55 to 250 C.
It was found that in the ESI mass spectrum of AN, at heated capillary
temperatures of 55 to 150 C, in the positive-ion mode, cluster ions of the
o
+
type [(NH NO ) NH ] (n = 1–3) were dominant, which enabled character-
4 3 n 4
ization and identification of the integral ammonium nitrate molecule. Figure
47
6.19 shows the full-scan positive-ion ESI mass spectra at heated capillary
15
15
of 100 C, of (a) 1 mM NH NO , (b) 1 mM NH NO , (c) 1 mM NH NO ,
o
4 3 4 3 4 3
and (d) 1 mM NH N O in methanol–water (50:50) introduced by syringe
18
4 3
pump infusion.
Characteristic ESI mass spectra of other oxidizers could be obtained in
both positive- and negative-ion mode, and over a wider range of tempera-
48
tures of the heated capillary. An exception is ammonium sulfate for which
only the positive-ion mass spectrum produced characteristic cluster ions.
Figure 6.20 48 shows the full-scan positive-(upper trace) and negative-ion
(lower trace) ESI mass spectra, at heated capillary of 220 C, of 1 mM ammo-
o
nium perchlorate in methanol–water (50:50). Table 6.4 shows the character-
istic ESI ions, in both positive- and negative-ion modes of explosive oxidizers.
6.3.5 Military, Commercial, and Other Explosive Mixtures
ESI and APCI in the negative-ion mode were compared for the analysis of a
mixture of explosives, using a Hewlett–Packard 5989B LC/MS mass spec-
trometer with a HP 1090L HPLC. 24 The mixture contained 14 standard
compounds (Table 6.5). HPLC column was HP Hypersil BDS, 100 mm ¥ 4
mm ID, 3 mm particle size. Mobile phase consisted of methanol–water with
a gradient of 26 to 40% methanol at 10 min to 53% methanol at 20 min, at
a flow rate of 0.72 ml/min. Figure 6.21 shows the total ion chromatograms
obtained in APCI and ESI. As can be seen, not all the components could be
detected in either mode of ionization.
Table 6.5 shows the base peak ions in ESI and APCI. In ESI of RDX,
HMX and tetryl, the major ion is the adduct ion [M + TFA] , the TFA
–
probably originating from residue background.
–
In APCI of tetryl, the major ions are at m/z 242, [M* – H] and m/z 212,
–
[M* – NO] , where M* represents N-methylpicramide, a hydrolysis product
of tetryl.
© 2004 by CRC Press LLC