Page 259 - Advances in Forensic Applications of Mass Spectrometry - Jehuda Yinon

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The analysis of postexplosion residues is very difficult because it is based
on finding and identifying residues from the original explosive, rather than
products formed during the explosion. The amount of residual unexploded
explosive is usually very small and has to be isolated from large amounts of
debris which could cover wide areas. A scientifically sound method has not
yet been established to determine the debris on which there is a greater
probability to find residual explosives.
Several comprehensive schemes for screening of the debris, clean-up
1
procedures, extraction, and analysis have been described. The combination
of the separation power of high performance liquid chromatography (HPLC)
with the identification capability of mass spectrometry (MS) has resulted in
a powerful analytical system, LC/MS, which is being widely used in a large
2–4
variety of analytical applications, including forensic. 5–7
A great deal of progress has been made in the technology of LC/MS during
the last 20 years. The first LC/MS systems used for the analysis of explosives
8,9
were the direct liquid introduction (DLI)-LC/MS systems, which had a low
sensitivity because of effluent splitting. Thermospray (TS)-LC/MS interface
was the first to combine interface and ionization capabilities, in which ions
in the gas phase were produced directly from solution without the need of
10
an additional means of ionization. Analysis of explosives by TS-LC/MS with
a quadrupole mass spectrometer was found to have best sensitivity in the
negative-ion mode with filament on. Detection limits, in the full scan mode,
11
were 200 pg for TNT, and 1 ng for RDX, HMX, and PETN. Results obtained
with a triple-stage quadrupole mass spectrometer, in the single-ion monitor-
ing (SIM) mode, yielded a detection limit of 2.5 pg for PETN. 12
Particle beam (PB)-LC/MS, a solute transport-enrichment technique,
13
was also used for the analysis of explosives. The mode of ionization was
negative-ion chemical ionization, with methane as moderator gas, at a pres-
sure of 1.8 Torr in the ion source. Detection limits in the SIM mode were 60
pg for NG, 120 pg for TNT, and 200 pg for RDX and PETN.
The main drawbacks of thermospray and particle beam ionization for
analysis of explosives were the difficulty in reproducing results and in using
these techniques on a routine basis.

6.2 Electrospray Ionization (ESI) and Atmospheric Pressure
Chemical Ionization (APCI)

6.2.1 Principle of Operation of ESI

Electrospray ionization (ESI) is a soft ionization technique that allows the
analysis of large biomolecules, as well as a wide range of smaller polar mol-
ecules. 14–17 For molecules up to 1000 Da in molecular mass, either an [M +

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