Biology Reference
In-Depth Information
(MS), and the resulting 2D data display greatly facilitates mass
mapping, amino acid sequence analysis, and determination of
site-specific protein modifications.
5
The Big Question: Choice of Mass Analyzer
As a result of its utility in (bio) chemical analysis, MS has become
a routinely used standard analytical tool. The emergence of post-
genomic disciplines, and efforts in the drug development industry,
has fuelled instrumental developments in mass spectrometry to
investigate the complexity of living organisms, which has led to a
wide variety of configurations. New configurations of mass spec-
trometry systems, as well as improvements of the current ones, will
become available in the near future. This raises the important ques-
tion of which MS configuration is better suited for addressing
one's research needs and expectations. The choice of mass analyzer
should be based upon the application, cost, and performance
desired. In addition, in assessing the suitability of different mass
analyzers, a range of instrument-specific factors must be taken into
account. The performance of a mass analyzer can be defined by the
following characteristics [
89
,
90
]: mass accuracy (the difference
between the true
m
/
z
and the measured
m
/
z
of a given ion,
divided by the true
m
/
z
of the ion, usually quoted in terms of parts
per million, ppm: [(
m
/
z
(exp) −
m
/
z
(theor))/
m
/
z
(theor)] × 10
6
)
[
91
]; mass resolving power (
M
/Δ
M
, the ratio of peak mass (
M
) to
the peak width at half maximum intensity, Δ
M
); mass range (the
range of
m
/
z
over which the mass analyzer can operate to record a
mass spectrum); linear dynamic range (the range over which the
ion signal is directly proportional to the analyte concentration);
tandem (MS/MS) and multi-stage (MS)
n
analysis capabilities;
abundance sensitivity (the ratio of the maximum ion current
recorded at an
m
/
z
of
M
to the signal level arising from the back-
ground at an adjacent
m
/
z
of
M
+ 1); and scan speed. Table
1
dis-
plays a nonexhaustive comparative overview of these parameters
from some major mass analyzer configurations (Fig.
3
) currently
used for proteomics research.
The most common mass analyzer for quantitative bioanalytical
analyses is the triple quadrupole due to its capability to operate
under selected-ion monitoring and scanning modes that are used
for identifying drug metabolites, biomarkers, and posttranslational
modifications such as glycosylation or phosphorylation. The
precursor ion scan essentially is the reverse of the product ion scan.
The third quadrupole is set to select a specific product ion formed
in Q2, and the first quadrupole is scanned for all precursor ions
forming the chosen fragment. Neutral loss scanning is also achieved
in triple quadrupoles. Neutral loss scans are used routinely to iden-
tify common functional groups present in a set of molecules.
