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interface. ESI sources can be used up to 1 ml/min, although this is at the top
of the flow range. ESI sources can also be used at flow rates down to <5
ml/min, thus allowing the interfacing of capillary columns.
Although a high temperature is applied to the APCI probe, most of the
heat is used in evaporating the solvent and heating the N gas, and the thermal
2
effect on the sample is much less than may be expected. However, with very
thermally labile compounds the heated probe may cause some thermal frag-
mentation. Since ESI uses a colder probe, it is sometimes possible to produce
+
–
[M + H] or [M – H] ions by ESI in compounds which fragment in APCI.
Many compounds can be analyzed by either APCI or ESI. There are no
definitive rules on when to use APCI over ESI, but a general observation is
that APCI tends to give better sensitivity than ESI for less polar compounds.
Unlike ESI, APCI does not produce multiple charged series and is there-
fore unsuitable for the analysis of high molecular weight compounds such
as proteins.

6.3 Analysis of Explosives

6.3.1 Nitroaromatic Explosives and Their Degradation Products

The most widely used military explosive is 2,4,6-trinitrotoluene (TNT). Its
main features include low melting point, stability, low sensitivity to impact,
friction, shock and electrostatic energy, and its relatively safe methods of
manufacture. 1,19,20 Its melting point is 80.65˚C. TNT is highly soluble in
acetone (132 g per 100 g acetone at 25˚C) but still soluble enough in water
–6
(10 mg per 100 g water at 25˚C). Its vapor pressure at 25˚C is 5.8 ¥ 10 Torr.
The addition of oxygen-rich products to TNT can form mixtures with
enhanced explosive power. Various explosive compounds have been added
to TNT to form binary explosives. Other compounds have been added to
decrease sensitivity and increase the mechanical strength of the cast.
Electrospray ionization coupled to tandem mass spectrometry was used
for the analysis of a series of explosives, including TNT. Analyses were
21
performed with a Perkin–Elmer Sciex API III-PLUS triple quadrupole mass
spectrometer with electrospray ionization. A standard solution of TNT was
made in acetonitrile–water (50:50), containing 2 mM of ammonium acetate.
The solution was introduced directly into the ion source by means of a
syringe pump at a flow rate of 5 ml/min. The major ion in the negative-ion
–
spectrum of TNT, at a concentration of 100 pg/ml, was the [M – H] ion at
m/z 226.
TNT, DNT, and 4-amino-2,6-dinitrotoluene (4-ADNT) were analyzed
by electrospray-MS with various additives in order to obtain ion intensity
enhancement. A Finnigan TSQ 700 mass spectrometer with an electrospray
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© 2004 by CRC Press LLC