Development of a Novel Mass Spectrometric ... - Jacobs University

Results and Discussion
After the confident identification of few PCBs within the waste extract, we
employed tandem-MS in order to obtain further structural information on the
components.
3.4.4 Tandem MS Measurements
Tandem MS measurements were carried out as direct infusion experiments in n-
heptane in the positive ion mode using an ion trap mass spectrometer with APCIionisation.
After injecting the waste sample into the iontrap, the lower mass
distribution of the sample, shown in figure 3-29, had ions with mostly odd mass
values (see figure 3-30). This indicated that the lower mass distribution mostly
consists of fragment ions formed from the molecular ions. This is as expected
because ionisation occurred via the high-energy process of electron abstraction by
N +• 2 . At first task (M-1) + series of standard n-alkanes ions were selected and
isolated with an isolation width of 1 Da and each n-alkane subjected to Collision
Induced Dissociation (CID). The (M-1) + ions of few analytes of n-alkanes within
the C7-C40 standard were fragmented as shown in figures 3-31 to 3-34. The
fragmentation pattern (MS 2 spectrum) seems to decrease by an increment of 14 Da
which reflects the CH 2 group that forms the structure of n-alkanes. This is further
emphasized in scheme 4 where a fragmentation route for pentacosane (C25) is
provided. Isolation of the molecular ions of the higher n-alkanes within the
standard C7-C40 like tetracosane (C40) was made possible. Figure 3-35 shows the
MS 2 spectrum of m/z 562, the molecular ion of C40. The advantage here is that this
ion could independantly evaporate into the gas phase, get ionised and further
isolated to be subjected to CID in Iontrap-MS. Further isolation of similar
molecular ions are demonstrated in figures 3-36 and 3-37. Access to fragmentation
patterns of molecular ions can make use of the GC library accummulated for many
years.
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