Introducing the MSQ
The Source–An Introduction to API Techniques ________________________________________________
Source Fragmentation
Both electrospray and APCI are regarded as soft ionization techniques.
Ionization generally results in spectra dominated by either the protonated
molecule [M+H]
+
(positive ion mode) or deprotonated molecule [M-H]
-
(negative ion mode), depending on whether positive or negative ionization
mode has been selected. Choice of ionization mode is governed by the
functional chemistry of the molecule under investigation.
Source fragmentation can be induced to give additional information on a
compound, such as diagnostic fragment ions for structural determination or
an increased response on a particular confirmatory ion for peak targeting.
Formation of Diagnostic Fragment Ions
The MSQ allows the simultaneous acquisition of MS data at a number of
different source voltages. For example, the MSQ can be programmed to
acquire data at source voltages of 20, 40, and 60 V on an alternating scan
basis within a single acquisition. The benefits of setting up acquisitions in
this way are:
The optimum source voltage for a particular ion can be determined in
one acquisition for compounds where sample volume is at a premium.
•
•
The intensity of fragment ions can be maximized to gain structural
information.
Fragmentation at increased source voltages is useful for most compounds.
For example, using source fragmentation of salbutamol in electrospray
ionization, a number of confirmatory fragment ions can be generated and
their intensity maximized (see Figure 1-14).
The mechanism for the formation of the fragment ions is characteristic for
not only salbutamol, but also for related
β
-agonists such as clenbuterol,
terbutaline, and metaproterenol. It involves loss of water (-18 amu; resulting
in the fragment ion at m/z 222 (middle trace)) and an additional loss of the
tert-butyl group (-56 amu; resulting in the fragment at m/z 166 (lower
trace)).
Note.
The MSQ uses the term “cone voltage” to represent source voltage.
1-16
__________________________ MSQ Hardware Manual ____________________________
