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

Results and Discussion
Intens.
[%]
50
505.6 521.6
+MS
40
30
20
449.5 465.5
561.6 577.6
10
0
535.5
479.5
407.4 421.4 435.5
493.3
549.6
591.6
617.7
425 450 475 500 525 550 575 600 m/z
Figure 3-5 APCI mass spectrum in positive ion mode of dotriacontane (C 32 H 64 ),
hexatricontane (C 36 H 74 ) and tetracontane (C 40 H 82 )
These reported measurement were my starting steps before I tried to optimise
solvent. These measurements were absolutely considered novel hits because it is
well known that n-alkanes are difficult to be ionised. As far as the effect of other
solvents over the ionisation process in APCI was concerned, two further
experiments were performed. Two solutions of model n-alkane mixture of the
higher mass range (C20 to C40) were prepared and injected as direct infusions in
n-pentane and chloroform as the APCI reagent. None of the two solvents afforded
better results compared to those with n-heptane. Figure 3-6 shows the mass spectra
of few analytes in the aforementioned range in n-pentane. n-Pentane was seen to
enhance fragmentation potential of alkanes by possibly increasing the efficiency of
collisions of reagent ions with the neutral n-alkane analytes inside the APCI
source. To this end, the appearance of such n-alkane species is in agreement with
the results summarized in the introduction in particular the study performed by
Bell et al.. 78 The latter’s study of a set of low mass n- alkanes produced (M-1) + ,
(M-3) + and (M-3) + H 2 O. The results of individual model n-alkanes were
summarised in table 3.1.
40