Biomedical Engineering Reference
In-Depth Information
quadrupole (QqQ), and ion trap (IT) systems belong to the most frequent mass analy-
sers also applied to TA analysis (Fig.
5c
). Statistical evaluation of the literature
referred to in this chapter (Table
5
-
8
) revealed a frequency of 4 % for offline double
focusing sector field mass spectrometer (EBQ
1
Q
2
), 19 % for IT, 21 % for SQ and
56 % for QqQ mass analysers (Fig.
5c
).
Early mass spectrometers were simply equipped with a SQ mass analyser merely
suitable for full scan MS mode and selected ion monitoring. Even though SQ spec-
trometers are highly superior to UV-detectors with respect to selectivity, they still
bear the risk of deterioration by matrix compounds of similar
m
/
z
values. Especially,
peptides and small proteins may cause a series of diverse
m
/
z
values due to their
multiple charge states after ES ionization potentially interfering with the analytes or
IS [
103,
105
] .
In contrast to SQ spectrometers, QqQ systems possess at least three quadrupoles
that allow full scan MS and SIM but additionally enable, e.g. product ion scan (MS/
MS) and multiple reaction monitoring (MRM) modes. Typically, four quadrupoles
are assembled in QqQ instruments. The first quadrupole (Q
0
) is part of the ion optics
that enables focusing of the ion beam originated from the ion source and is not con-
sidered or counted as part of the mass analyser. The second quadrupole (Q
1
) func-
tions as mass analyser (mass filter) that allows mass scanning for full scan MS mode
or selection of ions for SIM. Additionally, that assembly also enables selection of
precursor ions for subsequent fragmentation in the following third quadrupole (col-
lision cell, Q
2
). Fragmentation by collision-induced dissociation (CID, collision
with N
2
or noble gases) generates product ions valuable for structure elucidation and
selective quantification. The fourth and last quadrupole (Q
3
) again functions either
as mass analyser for the entire set of product ions (product ion scan) or as mass filter
to select a subset of product ions prior to detection. Monitoring preselected precur-
sor-product ion transitions is performed for MRM providing optimum selectivity
and best signal-to-noise ratios that enable optimum sensitivity. Detection of all
product ions is well suited for compound identification considering the complete
composition of MS/MS signals.
Ion trap machines allow to perform the same set of scan modes as described for
QqQ but additionally provide the opportunity for higher grade fragmentation of
product ions (MS
n
). This option expands the analytical possibilities for structure
elucidation considerably [
106
] .
In contrast to QqQ systems, IT mass spectrometers perform sequential ion trap-
ping, isolation of precursor ions and subsequent mass scanning in a single small
assembly (ion trap) making this system robust and valuable.
At last, the double focusing sector field mass spectrometer (EBQ
1
Q
2
) should
briefly be addressed. This mass analyser represents a highly sophisticated early
design that has rarely been used for routine analysis especially for quantification.
For detailed information on the functional principle the reader is referred to respec-
tive textbooks on mass spectrometry. We mention this technique as Kajbaf et al.
used EBQ
1
Q
2
for detection and identification of the QTA cimetropium and its
biotransformation products from liver microsomal mixtures in offline analysis of
HPLC fractions after FAB ionization [
23,
62
] (Table
7
).
