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Table 6.3 LC/MS-ESI Limits of Detection (pg/mm mmL) of Nitrate Ester
Explosives
Additive NH 4 NO 3 NaNO 2 Propionic acid NH 4 Cl
Adduct ion [M + NO 3 ] – [M + NO 2 ] – [M + CH 3 CH 2 CO 2 ] – [M + Cl] –
PETN 5 10 10 10
NG 5 20 25 10
EGDN 2000 10000 ND* 2500
* ND: not detected
Source: Reprinted from Zhao, X. and Yinon, J., Identification of nitrate ester explosives by
liquid chromatography-electrospray ionization and atmospheric pressure chemical ionization
mass spectrometry, J. Chromatogr. A, 977, 59, Copyright 2002, with permission from Elsevier.

2 mM of ammonium acetate. The solutions were introduced directly into the
ion source by means of a syringe pump at a flow rate of 5 ml/min.
The base peak in the mass spectrum of PETN was at m/z 281, interpreted
as [M + CH COO] , the additive originating from the ammonium acetate.
–
3
–
Additional ions observed were [M – H] at m/z 315 and a formate adduct
ion, [M + HCOO] , which originated from formic acid background, used in
–
previous experiments.
A comprehensive study on LC/MS of nitrate esters, including EGDN,
NG, and PETN, was carried out using a Thermo–Finnigan LCQ DUO ion trap
mass spectrometer, with ESI and APCI in the negative-ion mode. Postcol-
45
umn additives were investigated for the promotion of characteristic adduct
ions. In ESI, additives were ammonium nitrate, sodium nitrite, propionic
acid, and ammonium chloride. Additives were dissolved in methanol–water
(70:30) and introduced postcolumn by a syringe pump through a T union
into the LC flow before entering the ion source of the mass spectrometer.
–
The characteristic adduct ions observed were [M + NO ] , [M + NO ] , [M
–
2
3
–
–
+ CH CH CO ] , and [M + Cl] , respectively. Table 6.3 shows the limits of
2
2
3
detection of the three explosives for each one of the additives.
Without additive, the major ions in the ESI mass spectrum of PETN are
–
–
–
[M + 45] at m/z 361, [M + 59] at m/z 375, and [M + 75] at m/z 391. In NG
–
the major ions observed without additive are [M + 45] at m/z 272 [M + 75] –
–
at m/z 302, and [2M + 75] at m/z 529. These adduct ions can probably be
interpreted as originating from impurities in the solvent, as proven with RDX. 44
Adduct ion formation in APCI of nitrate ester explosives was investigated
using dichloromethane, chloroform, carbon tetrachloride, and ammonium
chloride as additives. In APCI, both vaporizer and heated capillary temper-
45
atures have great impact on the adduct ion formation. Except for dichlo-
romethane, which required a vaporizer temperature of 330˚C, the other
–
additives produced intense [M + Cl] ions at vaporizer temperature of 150˚C.
These adduct ions are easily recognizable by their isotopic composition:
–
[M + Cl] and [M + Cl] . Heated capillary temperatures were in the range
35
–
37
© 2004 by CRC Press LLC