Page 203 - Advances in Forensic Applications of Mass Spectrometry - Jehuda Yinon

1522_book.fm Page 186 Thursday, November 13, 2003 9:58 AM

While chemical ionization (CI) is also possible, 12,36 the following
describes only EI followed by MS/MS in the analysis of ILRs. Ion traps have
an inherent advantage in sensitivity over quadrupole mass spectrometers in
that the target analytes can be stored in the trap during the analysis while
maintaining a high efficiency of detection. The daughter-ion mass spectra
obtained during an MS/MS experiment differ from EI mass spectra, requiring
a new library of MS/MS spectra to be created by the user.
The sample stream exits the chromatographic column and enters the trap
through a hole between one of the quartz spacers and one of the end caps
(see Figure 5.1). The internal ionization ion trap experiment first creates the
ions by sending a pulse of electrons into the trap. In MS/MS, a predetermined
ion (called parent or precursor ion) formed in the first MS stage can be
separated from all other ions, trapped, and subjected to collision induced
dissociation (CID) through collision with an inert gas such as helium, argon,
or nitrogen. The precursor ion can be either the molecular ion or a fragment
ion but needs to be unique to the component of interest. Ions formed by
CID, referred to as daughter ions, are ejected and detected. Figure 5.2 illus-
trates the different steps in an MS/MS experiment. The true value of an
MS/MS experiment is the ability to isolate ignitable liquid residue target
components from coeluting peaks of interfering compounds. This helps to
improve the signal-to-noise ratios of the target compounds, thereby improv-
ing the level of detection while simultaneously improving the selectivity of
the target compounds.

A C
B
D E
Ions created during the ionization process

MS 1

Mass Selection (1)
A
C
B
C
Collision C 1
C C 1 C 2 MS 2
Chamber
C 3 C 2
D C 3
E Fragmentation by CID Mass Selection (2)
and Detection
Figure 5.2 The different steps in an MS/MS experiment.

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