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The MSQ
™
Plus Mass Detector was employed for the
detection of the explosive compounds. Full scan mode
with a mass range of 50-400 amu was employed for the
compound identification and confirmation, while SIM
mode was used for the sensitivity and quantitation studies.
The mass spectra for some explosive compounds are
difficult to be predicted because of their reactivity. An array
of the ions, such as additive adducts and decomposing ions,
is observed in the LC/MS analyses of explosives.
4
The
observed ion signals vary depending on many factors, for
example, the ionization sources, analytes concentrations,
additive concentrations, impurities in the mobile phases
and the contaminations of the LC/MS system.
APCI was used in the MS detection of the explosives
because it gave better sensitivities than ESI. Nitroaromatics,
nitroamines and nitrate esters were detected using APCI
negative mode, while peroxides were detected using APCI
positive ionization (Figure 2). Some explosive standards,
including TNB, 1,3-DNB, TNT, 2,6-DNT, 2,4-DNT, 4-A-
2,6-DNT and 2-A-4,6-DNT, showed both molecular ion
signals ([M]
-
or [M-H]
-
) and decomposing ions ([M-30]
-
and/or [M-17]
-
) in their MS spectra. Other explosive
standards showed only decomposing ions: the nitrate
esters, including EGDN, DEGDN, NG, TMETN and
PETN, showed decomposing ions of [NO
3
]
-
at
m/z
61.95;
the nitroamines, including RDX and HMX, showed
decomposing ions at
m/z
102.05 and 129.16. TATP
formed adduct ions with its decomposing ions and
ammonium, [M+NH
4
+H(OOC(CH
3
)
2
OOH]
+
at
m/z
of
348.08. In this case, the addition of 1 mM ammonium
acetate in the mobile phase A was critical, providing the
sources of ammonium ions to facilitate the formation of
the ammonium adduct.
The two isomer pairs, 2,6-DNT and 2,4-DNT, 4-A-
2,6-DNT and 2-A-4,6-DNT, demonstrated significant
differences in their fragmentation MS spectra with the
source induced fragmentation (SID) of the MSQ Plus
Mass Detector. The spectrum of the 2,6-DNT showed one
major fragmentation ion [M-30]
-
at
m/z
152.10, while
2,4-DNT gave two major fragmentation ions [M-30]
-
at
m/z
152.11 and [M-17]
-
at
m/z
165.15. 4-A-2,6-DNT
showed one major fragmentation ion [M-30]
-
at
m/z
167.09, while 2-A-4,6-DNT gave two major fragmentation
ions [M-30]
-
at
m/z
167.10 and [M-17]
-
at
m/z
180.16.
Thus, the identification of these isomers was strengthened
with the single quadrupole MS detector.
The identification of the explosive compounds with
EPA 8330 method is based solely on the retention times of
LC separations. The interference of the sample matrices
alters the retention times of target compounds and causes
false identifications. With the current UHPLC/MS method,
target compounds are identified and confirmed by
matching the APCI mass spectra against the MS spectra
library. Figure 3A showed a total ion chromatogram (TIC)
of a customer sample collected by this method. TNT and
2,4-DNT were easily identified by library spectra search
against more than 20 explosive compounds (Figure 3).
The Thermo Scientific Xcalibur software displayed the
searching result with a list of compounds ranked by their
matching scores. The implementation of the MS spectra
library in compound identification provided more
confirmative results compared to EPA 8330 method.
Detection Linearity and Sensitivity
The detection linearity of the UHPLC/MS system was
investigated using the explosives standard. Calibration
curves of seventeen standards were constructed over a
concentration range of 10-100,000 ng/mL (ppb). Correlation
coefficients of 0.999 or better were achieved for most of
the standards (Table 1). The calibration curves for TNB,
TNT, 2,6-DNT, 2,4-DNT and TETRYL showed linearity
over four orders of magnitude working ranges (Table 1).
Improved sensitivities were observed by high
throughput UHPLC because of the sharper and taller
peaks produced by the sub-2 µm particle columns. The
SIM mode of the MSQ Plus Mass Detector further
extended the detection sensitivity compared to the
traditional UV detector. The limit of quantitation (LOQ)
and the limit of detection (LOD) for seventeen standard
explosive compounds were examined. The sensitivities
were achieved at ppb level for TNB, 1,3-DNB, TNT,
2,6-DNT, 2,4-DNT, TATP and TETRYL (Table 1). This
represents a thirty-five times improvement in the detection
sensitivity for TATP relative to the detection sensitivity of
the Agilent instrument and method. The detection
sensitivities obtained by the UHPLC/MS method with
library matching of APCI mass spectra was more than
tenfold versus the EPA 8330 method.
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